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  • i

    SOIL ORGANIC CARBON SEQUESTRATION

    POTENTIALS IN AGGREGATE FRACTIONS OF

    CULTIVATED AND UNCULTIVATED SOILS OF

    SOUTHEASTERN NIGERIA

    BY:

    OKEBALAMA CHINYERE BLESSING (REG. NO: PG/M.Sc./06/41040)

    A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF

    THE REQUIREMENTS FOR THE AWARD OF THE

    DEGREE OF MASTER OF SCIENCE

    (M.Sc.) IN SOIL SCIENCE

    DEPARTMENT OF SOIL SCIENCE

    UNIVERSITY OF NIGERIA

    NSUKKA

    DECEMBER, 2009

  • ii

    CERTIFICATION

    Okebalama, Chinyere B., a postgraduate student in the Department of Soil Science, with

    the Reg. No: PG/M.Sc./06/41040 has satisfactorily completed the requirements for course and

    research work for the degree of Master of Science (M.Sc.) in Soil Science.

    The work embodied in this dissertation is original and has not been published or

    submitted in part or full for any other diploma or degree of this or any other University.

    ---------------------------------- ----------------------------------

    Prof. C. A. Igwe Prof. C. L. A. Asadu

    Supervisor Head of Department

  • iii

    DEDICATION

    This work is dedicated to Almighty God who in His infinite grace and mercy sustained

    and supported me all through, having endowed me with the intellect to complete it successfully.

    I also dedicate this work to my wonderful parents, Elder and Mrs S.U. Okebalama who

    set the standard I strive to reach.

  • iv

    ACKNOWLEDGEMENT

    My sincere gratitude goes to God Almighty for the knowledge, understanding, sustenance

    and grace granted to me for the successful completion of this research work.

    I am very grateful to my project supervisor, Prof. C.A. Igwe for his intellectual

    inspiration, guidance, encouragement, fatherly advice and provision of valuable materials used in

    the course of this work. Indeed, his intuition is incisive and I am greatly privileged to have been

    mentored by him thus far.

    My special gratitude goes to all members (past and present) of the Department; late Prof.

    J.S.C. Mbagwu of blessed memory, Prof. M.E. Obi, Prof. F.O.R. Akamigbo, Prof. C.L.A. Asadu,

    Prof. N.N. Agbim, Prof. C.C. Mba, Mr M.C. Jidere, Miss I.M. Okpara, Mr S.E. Obalum and all

    the laboratory staff for their directives, advice, suggestions, encouragement and knowledgeable

    contributions in making this work a better one.

    I also appreciate Prof. I.U. Obi, Prof. K.P. Baiyeri, and Dr P.N. Ogbonna whose

    guidance, advice and encouragement were so helpful in the project facilitation. I am thankful to

    Prof. O.A. Opara-Nadi of Abia State University, Uturu who allowed me source for relevant and

    valuable literature from his collections.

    I am particularly grateful to the members of my family, especially my parents whose

    moral and financial assistance made it possible for me to enroll in this programme. Their advice

    has greatly benefited me in my career development.

    Finally, I appreciate all my friends: Mr Funso Fasipe, Gideon Umoh, Darlington Achonu,

    Christopher Nwokocha, Chinyelu Kanemeh, Adaobi Ikeanyi, Mr and Mrs Tayo Ajayi, Emeka

    Muora, Chinedu Okoro, Chimezie Nwaimo, Ozoagu Afamefuna, and Christian Nwosu for all

    their inestimable support and involvement in seeing to the production of this work. May the good

    Lord reward and bless you all exceedingly, Amen.

  • v

    TABLE OF CONTENTS

    PAGE

    TITLE PAGE ........................................................................................................................ i

    CERTIFICATION ................................................................................................................ ii

    DEDICATION ...................................................................................................................... iii

    ACKNOWLEDGEMENT .................................................................................................... iv

    TABLE OF CONTENTS ...................................................................................................... v

    LIST OF TABLES ................................................................................................................ vii

    LIST OF FIGURES .............................................................................................................. viii

    LIST OF APPENDIX........................................................................................................... ix

    ABSTRACT .......................................................................................................................... x

    CHAPTER ONE

    1.0 INTRODUCTION .................................................................................................... 1

    CHAPTER TWO

    2.0 LITERATURE REVIEW ......................................................................................... 4

    2.1 An Overview of Global Carbon ................................................................................ 4

    2.2 Soil Organic Carbon Dynamics ................................................................................ 6

    2.3 Soil Organic Matter and Total Soil Nitrogen in Water Stable Aggregates ............... 10

    2.4 The Concept of Soil Organic Carbon Sequestration ................................................. 12

    2.4.1 Importance of Carbon Sequestration ........................................................................ 14

    2.5 Influence of Soil Carbon and Total Nitrogen on Soil Properties .............................. 15

    2.6 Effects of Land Use Management on Soil Carbon and Nitrogen ............................. 17

    2.7 Factors Affecting Soil Organic Carbon and Total Soil Nitrogen

    Pool in the Tropics ..................................................................................................... 20

    CHAPTER THREE

    3.0 MATERIALS AND METHODS .............................................................................. 26

    3.1 Site Description ......................................................................................................... 26

    3.2 Methods ..................................................................................................................... 27

    3.2.1 Field Study ................................................................................................................. 27

    3.2.2 Laboratory Study ....................................................................................................... 27

    3.2.2.1 Physical Analysis ....................................................................................................... 28

    3.2.2.2 Chemical Analysis ..................................................................................................... 29

    3.3 Statistical Analysis ..................................................................................................... 30

    CHAPTER FOUR

    4.0 RESULTS AND DISCUSSION .................................................................................. 31

    4.1 Results .......................................................................................................................... 31

    4.1.1 Soil Characteristics ...................................................................................................... 31

  • vi

    4.1.2 Microaggregate Stability Indices of the Soils .............................................................. 35

    4.1.3 Water-Stable Aggregates, Mean Weight Diameter and Geometric Mean Weight

    Diameter of the Soils ................................................................................................... 38

    4.1.4 Soil Carbon and Nitrogen Content, and C/N Ratio in the Water-Stable Aggregates .. 40

    4.1.5 Soil Organic Carbon Pool ............................................................................................ 43

    4.1.6 Land Use Effect on Aggregates Size Distribution, C, and N

    Contents of the Soils .................................................................................................... 46

    4.1.7 Soil Texture in Relation to Soil Organic Carbon Pool ................................................ 46

    4.1.8 Relationship between Aggregate Stability Indices and Soil Properties ....................... 56

    4.2 Discussion .................................................................................................................... 64

    CHAPTER FIVE

    5.0 SUMMARY, CONCLUSION AND RECOMMENDATION .................................... 70

    REFERENCES

  • vii

    LIST OF TABLES

    TABLE TITLE PAGE

    1 Global carbon pool and fluxes among them ............................................................. 5

    2 Global soil organic carbon pool ................................................................................ 7

    3 Particle size distribution and textural classification of the soils .............................. 32

    4 Chemical Properties of the soils under cultivated and uncultivated land uses ........ 33

    5 Microaggregate stability indices and water content at field

    capacity (FC) of the soils .......................................................................................... 36

    6 Soil moisture, bulk density, porosity, coefficient of linear extensibility (COLE) and volumetric shrinkage (VS) of the soils ............................ 37

    7 Water stable aggregates (WSA), mean weight diameter (MWD) and geometric mean weight diameter (GMWD) of the soils. ......................................... 39

    8 Carbon and nitrogen content (g kg-1), and C/N ratio in the various water- stable aggregate (WSA) fractions of cultivated and uncultivated soils ................... 41

    9 Soil organic carbon (SOC) pool of cultivated and uncultivated soils

    at the various soil depths. .......................................................................................... 44

    10 Land use effects on water-stable aggregate (WSA) concentration at the various soil depths, and at 0-30 cm depth. ......................................................... 47

    11 Land use effects on soil organic carbon (SOC) content in water-stable aggregates (WSA) at the various soil depths ........................................ 48

    12 Land use effect on total soil nitrogen (TSN) content in water-stable aggregates (WSA) at the various soil depths ............................................................ 49

    13 Land use effect on soil organic carbon (SOC) content in water-stable aggregates (WSA) at 0-30 cm depth .................................................... 50

    14 Land use effects on total soil nitrogen (TSN) content in water-stable aggregates (WSA) at 0-30 cm depth. ....................................................................... 51

    15 Land use effects on C/N ratio of water-stable aggregates (WSA)

    at 0-30 cm depth........................................................................................................ 52

    16 Land use effects on bulk density, > 2.00 mm water stable- aggregate (WSA), mean weight diameter (MWD) and geometric

    mean weight diameter (GMWD) of the soils. .......................................................... 53

    17 Soil texture effects on soil organic carbon (SOC) pool at the various sampling depths ........................................................................................... 54

    18 Correlation coefficient (r) among particles sizes, aggregate indices and some selected properties of the soil under cultivated

    and uncultivated land uses ....................................................................................... 59

    19 Correlation coefficients (r) of the soils chemical properties and soil organic carbon (SOC) pool of cultivated and uncultivated land uses. ....... 62

    20 Relationship between soil organic carbon (X) and structural stability indices (Y) in cultivated and uncultivated land uses (n = 12) ..................... 63

  • viii

    LIST OF FIGURES

    FIGURE TITLE PAGE

    1 Processes affecting soil organic carbon dynamics ................................................... 9

    2 Soil depth effects on soil organic carbon (SOC) pool of cultivated and uncultivated soils ............................................................................................... 45

    3 Soil texture effects on soil organic carbon (SOC) pool at

    the various soil depths .............................................................................................. 55

  • ix

    LIST OF APPENDIX

    APPENDIX TITLE PAGE

    1 Final analysis of variance of a 4 x 2 x 3 factorial experiment in

    CRD showing sources of variation and degree of freedom only ...................... 88

  • x

    ABSTRACT

    A study was carried out on soils sampled at 0-10, 10-20, and 20-30 cm depths from both

    cultivated and uncultivated soils at four different locations (Awgu, Okigwe, Nsukka I, and

    Nsukka II), to evaluate the potentials of various aggregate size fractions of varying soil textures

    and depths to sequester carbon under different land uses. A 4 x 2 x 3 factorial experiment was

    conducted in a completely randomized design (CRD). Factor A was location at four levels, while

    factor B (land use) had two levels. Factor C (soil depth) comprised of three levels. Results

    showed that in both land uses, soil texture varied with depth in each location and included clay,

    loam, clay loam, sandy loam and sandy clay loam. Generally, all the soil properties varied with

    soil depth across the locations and land uses. Land use significantly (P = 0.05) affected pH in

    KCl, Ca2+

    , Al3+

    , CEC, 0.50-1.00 mm water stable aggregates (WSA), total soil nitrogen (TSN) in

    1.00-2.00 mm WSA, and soil organic carbon (SOC) in 1.00-2.00 mm and < 0.25 mm WSA.

    Cultivation at 0-30 cm depth significantly reduced SOC in 1.00-2.00 mm WSA by 19.30 %, and

    TSN in 1.00-2.00 mm WSA by 2.50 %. Land use effects on SOC in WSA at 0-30 cm depth of

    the various locations followed no consistent trend, except that SOC was higher in cultivated than

    in uncultivated soils of Nsukka II location. The SOC pool significantly decreased with soil depth.

    The SOC pool at 0-10 cm, 10-20 cm, and 20-30 cm depths averaged 17.62, 16.40 and 13.05 Mg

    C ha-1

    respectively, in cultivated soils; and 19.59, 17.86 and 12.03 Mg C ha-1

    respectively, in

    uncultivated soils. The SOC pool to the depth of 30 cm differed distinctly amongst the study sites

    in both land uses; however, cultivation had no significant effect on SOC pool. The effect due to

    soil texture on SOC pool indicated that C sequestration was significantly greater in clay loam >

    clay > sandy loam > loam > sandy clay loam. In all, SOC pool was most secluded at 10-20 cm

    depth, and least at 20-30 cm depth. Whereas SOC pool significantly correlated with dispersion

    ratio (DR), aggregated silt and clay (ASC), water dispersible clay (WDC), microporosity (Pmi),

    0.50-1.00 mm WSA, mean weight diameter (MWD), soil pH, K+, and C/N ratio in cultivated

    soils; it correlated significantly with ASC, Na+, and CEC in uncultivated soils. Apart from Pmi,

    whose variability was largely due to the effect of SOC that significantly predicted up to 76 %,

    SOC significantly accounted between 34 % and 54 % of the variability in MWD, WDC, and

    WSA classes of > 2.00 mm, 1.00-2.00 mm and 1.00-0.50 mm of the cultivated soils.

  • 1

    CHAPTER ONE

    INTRODUCTION

    The fundamental basis of carbon (C) sequestration and its effect on global climate change

    and agriculture have become a major concern in recent years. Emissions of greenhouse gases

    (water vapour, carbon dioxide (CO2), methane, and nitrous oxide) as a result of human activities

    continue to alter the atmosphere in ways that are expected to affect change in climate.

    Anthropogenic activities produce CO2, which is the primary greenhouse gas that contributes to

    climate change to be released to the atmosphere at rates much faster than the earths natural

    processes can cycle. To help alleviate or possibly reverse the trend, a variety of means of

    enhancing natural sequestration processes are being explored. Increasing CO2 sink (C

    sequestration) has been acknowledged and accepted as a major possible mitigation to these

    effects. This is buttressed by the report of Rice and McVay (2002) indicating that through C

    sequestration, atmospheric CO2 levels are reduced as soil organic carbon (SOC) levels are

    increased.

    Among the three natural sinks for C (ocean, forest and soil), soils contain more C than is

    contained in vegetation and the atmosphere combined (Swift, 2001). The SOC pool which forms

    the largest sink after sedimentary rocks and fossil deposits however is the most vulnerable to

    disturbance (Schlamadinger and Marland, 2000) especially because of the competition between

    the various types of land use. Six et al. (2000) reported that tillage operations promote the loss of

    SOC through macroaggregate disruption and exposure of soil organic matter (SOM) to microbial

    decomposition. Also, Blum (1997) indicated that the decomposition and alteration

    (mineralization and metabolization) of organic compounds produces trace gases which can be

    harmful to the global atmospheric cycle.

    The impact of organic carbon (OC) losses in soils may have a variety of serious

    environmental consequences. Lal (2004) reported that several depletion of SOC degrades soil

    quality, reduces biomass productivity, and adversely impacts water quality. Lal et al. (1998)

    observed that organic matter (OM) losses from soil worldwide contribute to increased

    atmospheric CO2 concentration. Lugo and Brown (1993) indicated that the net losses of SOC due

    to land use changes may occur as a result of decreased organic residue inputs and changes in

    litter composition, and increased rates of soil organic decomposition and soil erosion. The

    contribution of soil erosion to global C emission has also been recognized by Tans et al. (1990)

    as equally important to that of deforestation and fossil fuel burning. Lal (1995) estimated that the

    total SOC displaced by water erosion globally as 57 Pg yr-1

    [Pg = Petagram. Where, 1 Pg = 1 Gt

    (Gigaton) = 1015

    g = 1 billion tons]. Houghton et al. (1996) predicted that CO2 emission to the

  • 2

    atmosphere would increase from 7.4 Gt C yr-1

    in 1997 to approximately 26 Gt C yr-1

    by 2010.

    Furthermore, the annual CO2 flux from the soil to the atmosphere (68 Pg yr-1

    ) is 11.3 times the

    emissions from fossil fuel combustions (6 Pg yr-1

    ) (Raich and Schlesinger, 1992). However, the

    Inter-Government Panel on Climate Change (IPCC) recognised three main options for the

    mitigation of atmospheric CO2 concentrations by the agricultural sector: (i) reduction of

    agriculture-related emissions, (ii) creation and strengthening of C sinks in the soil, and (iii)

    production of bio-fuels to replace fossil fuels (Batjes, 1998). Hence, the need to evaluate the role

    of soil as one of the natural C sinks that secludes organic C as stable humus for enhancing soil

    fertility and stability of soil microaggregates. Therefore, soil C pool and its dynamics play vital

    role and the knowledge of their spatial distribution is important for understanding the pedosphere

    in the global C cycle for the overall management of C. It is with this background that several

    attempts have been made to access the potential of cropland (Lal et al., 1999; Lal and Bruce,

    1999), grazing systems (Follet et al., 2000), and forest ecosystem (Birdsey et al., 1993) to

    sequester C as possible strategies to curtail the rate of increase of atmospheric concentration of

    CO2.

    Carbon sequestration refers to the removal of C, from the atmosphere through

    photosynthesis and dissolution, and storage in soil as OM or secondary carbonates (Lal, 2001).

    Through this process, C storage in soil is enhanced and its loss minimized, thereby reducing the

    chances of global warming by the reduction of atmospheric concentration of CO2. Recognizing

    the soil as one of the important potential sinks for C requires understanding of the processes that

    influence C sequestration. Soil aggregation has been observed as an important process of C

    sequestration and hence a useful strategy for mitigating increase in concentration of atmospheric

    CO2 (Shrestha et al., 2007). Igwe et al. (2006) stressed the importance of the study of the role of

    SOC in restoration of soil fertility and stability of soil microaggregates.

    The impact of C sequestration on greenhouse gases and agricultural sustainability has not

    been well elucidated at regional, national or global scales. Some available statistics are generally

    based on extrapolation. Lal (2004) reported that the rates of SOC sequestration in agricultural

    and restored ecosystems range from 0 to 150 kg C ha-1

    yr-1

    in dry and warm regions, and 100-

    1000 kg C ha-1

    yr-1

    in humid and cool climates. He also estimated the total potential of C

    sequestration in world soils as 0.4-1.2 Gt C yr-1

    , all of which were derived from national resource

    inventory. Improvement in the data base on the concentration of SOC needed to be validated with

    ground truth measurement/assessment, as the use of reliable data is essential for developing

    techniques of soil management and identifying policy options needed for promoting appropriate

    measures. Despite several studies carried out on the quantification of soil sequestered C in

    different geographical regions of the world (Cruz-Rodriguez, 2004; Denef et al., 2004; Lal et al.,

    1998; Lal, 2001; Shrestha et al., 2007), there are limited knowledge about SOC pool dynamics in

  • 3

    the tropical humid agroecosystem of southeastern Nigeria. Quantification of SOC within

    aggregate size classes permits evaluation of aggregation under different soil management

    systems and its contribution to the accumulation and loss of OM (Sotomayor-Ramrez et al.,

    2006). The relevance of this study is to generate reliable information which is essential for

    developing techniques of soil/land management systems and for recommendation of agricultural

    practices that promote C sequestration for sustainable agriculture leading to advancement in food

    security and consequently, mitigate global warming. The hypothesis is that SOC sequestration is

    a function of soil texture and soil aggregation; and that SOC is similar between soil phases

    (cultivated and uncultivated) of the same soil series. Therefore, the main objective of the study

    was to assess the potentials of various aggregate size fractions of varying soil textures and depths

    to sequester C in cultivated and uncultivated soils. The specific objectives included to;

    (i) Determine the soil physico-chemical properties of cultivated and uncultivated soils.

    (ii) Quantify SOC and total soil nitrogen (TSN) stocks and assess their distribution across

    aggregate size fractions as stratified by location, land-use, soil texture and soil depth.

    (iii) Determine the effect of SOC and TSN on soil aggregation and other soil properties.

    (iv) Understand the SOC pool dynamics among different soil textures and depths, and

    between cultivated and uncultivated soils.

  • 4

    CHAPTER TWO

    LITERATURE REVIEW

    2.1 An Overview of Global Carbon

    Understanding the concept of carbon (C) sequestration seems to be the most reasonable point

    to start a lesson, but C sequestration which is an aspect of C management cannot be absolutely

    understood and appreciated as one of the identified mitigation options against global warming

    and climatic change, without a critical survey of global C as documented by most researchers.

    Three main reservoirs regulate the C cycle on earth (IPCC, 1990): the oceans 39000 x

    1015

    g (or Pg) of C; the atmosphere ( 750 Pg C), and terrestrial systems ( 2200 Pg C). The

    fourth reservoir which is a permanent sink - the geological reservoir, is estimated at 65.5 x 106 Pg

    (Kempe, 1979). Table 1 shows the principal global C pools comprising of oceanic, geologic,

    pedological, atmospheric, and biotic. These pools are interconnected with sizeable fluxes among

    them. For example, the atmospheric pool is increasing at the rate of 3.3 Gt C yr-1

    . The oceanic

    pool is absorbing about 92 Gt C yr-1

    and emitting 90 Gt yr-1

    , with a net gain of 2 Gt yr-1

    . The

    biotic pool photosynthesizes 120 Gt C plant respiration and the remaining 60 Gt C yr-1

    by soil

    respiration (Lal et al., 2007). Although the soil-vegetation C pool is small compared with that of

    the oceans, potentially it is much more labile in the short term (Batjes, 1996). On average, the

    soil contains about 2.5 times more organic carbon (OC) than the vegetation ( 650 Pg C) and

    about twice as much C as is present in the atmosphere ( 750 Pg C) (Batjes, 1998). The soil is the

    largest terrestrial pool of OC, with global estimates ranging from 1115 to 2200 Pg C (Batjes,

    1992), 1576 Pg C (Eswaran et al., 1995), 1400 Pg C (Falloon et al., 1998) and 1220 Pg C

    (Sombroek et al., 1993). Estimates of global soil C content have also been made by several

    researchers including, Kimble et al. (1990), Buringh (1984), Bohn (1982), Batjes (1998), Lal

    (2002), Post et al.(1990), Johnson and Kerns (1991).

    World soils or the pedological pool comprises two distinct components: soil organic carbon

    (SOC) and soil inorganic carbon (SIC) pools estimated at 1576 Gt and 938 Gt respectively, at 1

    m depth (Post et al., 1982; Schlesinger, 1995; Eswaran et al., 1993). The SOC pool is

    concentrated in soils of arctic, boreal, and temperate regions which includes highly active humus

    and relatively inert charcoal C, while the SIC pool includes elemental C and carbonate minerals,

    such as calcite, dolomite, and gypsum and those of arid and semiarid climates (Schnitzer, 1991;

    Stevenson, 1994; Lal et al., 2000; Wagner, 1981; Paul and Clark, 1996). Lal (2004) recognized

    two types of carbonates in soils: primary or lithogenic carbonates and secondary or pedogenic

    carbonates. Of the global SOC pool (2500 Gt), which includes about 1550 Gt of SOC

  • 5

    Table 1: Global carbon pool and fluxes among them

    Pool Reservoir (Gt) Flux (Gt C yr-1

    )

    Oceanic 38000 90-92 months the atmospheric

    Geologic 2500 7.4 to the atmospheric pool

    Coal 1550

    Oil 950

    Gas 4130

    Others 3510

    Pedologic 250 60 months biotic and 1 month the oceanic

    SOC 140

    SIC 250

    Atmospheric 760 120 months the biotic pool

    Biotic 560 60 months the pedological pool

    _________________________________________________________________________

    (Adapted from Lal et al., 2007)

  • 6

    and 950 Gt of SIC therefore, the total soil C including both SOC and SIC pools in the active soil

    layer of 1 m depth constitutes about 23000 Pg (Lal, 2002b) which is 3.3 times the size of the

    atmospheric pool (760 Gt) and 4.5 times the size of the biotic pool (560 Gt) (Lal, 2004). Table 2

    shows the global mass of SOC in the upper 30 cm, 1 m, 2 m, and 3 m of soil. The SOC pool in

    the top 1 m depth of world soils ranges between 1462 and 1600 Pg, which is nearly three times

    that in the aboveground biomass and approximately double that in the atmosphere; 32 % (or 506

    Pg) of this is contributed by soils in the tropics (Eswaran et al., 1993; Lal et al., 1995; Batjes

    1996). According to Batjes (1996) total soil C pools for the entire land area of the world,

    excluding carbon held in the litter layer and charcoal, amounts to 2157 to 2293 Pg of C in the

    upper 100 cm. Owning to the problem of making accurate global estimates of C, Eswaran et al.

    (1993) suggested that employing the coefficient of variation (CV) which is an expression of the

    variability will aid in understanding and accepting the reliability of most generalization.

    Lal et al. (2007) documented the estimates of global carbon pool up to 1 m depth of the

    various USDA soil orders. Another study that provided global mean of SOC estimates in soils of

    the tropics reported values of 8.3 for Ultisols, 9.7 for Oxisols, and 10.4 kg m-2

    for Inceptisols

    among other soil orders (Lal, 2002a). These differences between soil orders in the tropics are

    mainly in relation to temperature, rainfall, soil texture and land use (Batjes, 2000). Similarly,

    Kimble et al. (1990) reported SOC density of principal soil orders of the world, where among

    others the mean SOC density is 9.7 kg m-2

    with CV of 42 % for tropical Oxisols and 8.3 kg m-2

    with CV of 70 % for tropical Ultisols. About 52 % of this C pool is held in the top 30 cm of the

    soil profile, the layer most susceptible to land use changes and responsive to management

    practices (Lal, 2002b).

    2.2 Soil Organic Carbon Dynamics

    Lal (2001) noted that the SOC pool which is a function of soil characteristics and climatic

    factors is a highly variable and dynamic entity. It is variable over space because its density

    differs widely among soils and ecoregions; and variable over time because it changes with

    change in land use and management. Lal (2002b) observed that SOC pool is in a dynamic

    equilibrium with its environment, with a balance of input and output at a steady state level. Thus,

    the SOC pool represents a dynamic equilibrium of gains and losses (Fig. 1). Follett (2001)

    pointed out that temporal variation in SOC level results from the balance between plant biomass

    input and decomposition and OC losses via leaching, oxidative, and erosional processes.

    Substrate quality is one of the main factors affecting decomposition and has been linked to the

    relative abundance of specific compounds such as nitrogen, lignin (Melillo et al., 1982; Tian et

    al., 1993), and phenolic acids (Martens, 2000). Lal (1999) hypothesized that lack of nutrients,

    especially N, could explain the low C conversion efficiency. Knops and Tilman (2000) observed

  • 7

    Table 2: Global soil organic carbon pool

    -----------------------------Soil depths---------------------------

    Reference 0-30 cm 0-1 m 0-2 m 0-3 m

    --------------------------SOC pool (Pg) -------------------------

    Batjes (1996) 684-724 1462-1548 2376-2456 -

    Eswaran et al. (1993; 2000) - 1526-1555 - -

    Jobbagy and Jackson (2000) - 1502 1993 2344

    Batjes and Sombrock (1997) - 1200-1600 - -

  • 8

    that the rate of carbon accumulation in agricultural abandoned fields was controlled by the rate of

    nitrogen accumulation, which in turn depended on atmospheric nitrogen deposition and

    symbiotic nitrogen fixation by legumes. More so, the turnover time of organic matter (OM)

    increases with depth in the soil, ranging from several years for litter to 15-40 years in the upper

    10 cm and over 100 years below a depth of about 25 cm (Harrison et al., 1990; Lobo et al.,

    1990). In soils of the tropics, particle size fractionation techniques have been used to characterize

    relationships between SOC and aggregation at the macro and microaggregate scale (Feller et al.,

    1996). The concept is that soil organic fractions associated with different sized particles differ in

    structure and function, and therefore play different roles in SOC turnover (Christensen, 1992).

    The SOC pool consists of a mixture of plant and animal residues at various stages of

    decomposition, of substances synthesized microbiologically and/or chemically from the

    breakdown products, and of the bodies of live microorganisms and soil animals and their

    decomposing products (Schnitzer, 1991). The different C pools existing in the soils have been

    described in terms of the different mean residence times, ranging from years (active fraction), to

    decade to hundreds of years (passive), to thousands of years (stable) (Carter et al., 2002). The C

    pools are relative concepts based on the rate of decomposition of particular constituents and are

    more related to biological function than to particular soil chemical C constituents. For example,

    the active fraction consists of live microorganisms (microbial biomass), microbial products, and

    unprotected chemical constituents such as proteins and polysaccharides with a turnover time of a

    few weeks or months. The slow fractions are more resistant to decomposition due to partial

    physical and chemical protection with a longer turnover time (Theng et al., 1989). The passive

    organic constituents include humic substances and other macromolecules that are intrinsically

    resistant against microbial attack due to chemical recalcitrance, physical protection by adsorption

    on mineral surfaces, or entrapment within soil aggregates (Gregorich et al., 1997). Lal (2001)

    pointed out that formation of stable microaggregates in the subsoil takes C out of circulation by

    encapsulating it (physical and chemical protection from microbial activity) and is thus

    sequestered. The long-term stabilization of C in temperate and tropical soils is mediated by soil

    biota (e.g. fungi, bacteria, roots and earthworms), soil structure (e.g. aggregation) and their

    interactions, and is influenced by agricultural management (Six et al., 2002).

    In most soils, C is organic and constituents approximately 57 % of the soil organic matter

    (SOM) that includes a wide spectrum of organic compounds, from labile components, such as

    relatively fresh plants material and microbial biomass, to refractory components such as charcoal,

    which accumulates slowly over thousands of years (Trumbore, 1993). Piccolo (1996) indicated

    that the process of turning agricultural soil into sink for OC sequestration would be complete if

    the stored OM were transformed into stable and recalcitrant humic substances. Accordingly,

    humified OC, humic acids and humin in particular, represent the most persistent pool for SOC

  • 9

    Fig. 1: Processes affecting soil organic carbon dynamics.

    Arrows pointed upward indicate emissions of Co2 into the atmosphere (Adapted from Lal, 2004).

  • 10

    accumulation with mean residence time of several hundreds of years. Spaccini et al., (2002) is of

    the view that hydrophobic protection provided by humified matter may substantially reduce

    decomposition of labile organic compounds in soils, thus they reported that the higher the

    hydrophobicity of a humic material, the larger the sequestration of OC in soil. Bayer et al.

    (2000) reported that in southern Brazil, SOC associated with sand and silt fractions was less

    humified than that associated with finer-sized fractions. Nevertheless, Oades et al. (1987) and Six

    et al. (2000) demonstrated that the most humified or oldest fraction is associated with silt

    particles. Most of the C losses following soil disturbance such as tillage originate from the active

    and slow pools, which comprise the biologically defined SOM pools described as active (labile),

    slow (partially labile), and passive (stable) (Jenkinson and Rayner, 1977; Jenkinson, 1990;

    Duxbury and Nkambule, 1994). Biological separation of SOC empirically separates labile from

    recalcitrant forms by allowing microbes to mineralise C under controlled conditions with the

    most labile C mineralised first and recalcitrant C mineralised later.

    Physical fractionation of the soil according to aggregate size has been used to study the

    portioning of OC in the soil (Buyanovsky et al., 1994). Fractionation using physically based

    models is possible because OC is protected within and between aggregates (Cambardella and

    Elliot, 1992; Six et al., 2000; Snyder and Vazquez, 2004). In its simplest case, there is a free

    light fraction (LF) of labile C between the aggregates and intra-aggregate particulate OM (iPOM)

    within macroaggregates (Cambardella and Elliot, 1992; Six et al., 1998). The LF may be more

    related to residue input rates and soil environmental conditions and the iPOM more related to

    aggregate turnover, which is strongly affected by tillage management (Six et al., 1998).

    Aggregate hierarchy levels of formation occur in which the intra-macroaggregate POM

    (particulate organic matter) facilitates the binding of microaggregates into macroaggregates,

    which in turn affects the variation in the accessibility of soil microorganisms to SOC that leads to

    pools which differ in stability and dynamics. For example, in relatively undisrupted systems such

    as no-tilled agricultural and native systems, the greatest C concentration is usually found in the

    small macroaggregate size class (2502000 m), with C in this fraction being most affected by

    cultivation (Beare et al., 1994; Cambardella and Elliott 1994). Furthermore, a greater proportion

    of the SOC pool in large microaggregates implies greater C losses to the atmosphere if

    macroaggregates are broken by soil management practices (Cruz-Rodrguez, 2004). The losses of

    SOC are in turn associated with losses in the POM fraction and therefore the amount of

    aggregation and aggregate turnover (Six et al., 1999).

    2.3 Soil Organic Matter (SOM) and Total Soil Nitrogen (TSN) in Water-Stable

    Aggregates (WSA)

    Much interest has been shown in the role of OM in the formation and stabilization of both

  • 11

    macro- and micro- aggregates. Organic matter influences soil structure and compactibility by

    binding soil mineral particles, reducing aggregate wettability, and influencing the mechanical

    strength of soil aggregates, which is the measure of coherence of inter-particle bonds

    (Onweremadu et al., 2007). According to Hamza and Anderson (2005), OM retains soil water

    thus helping soil to rebound against compaction. Maintaining an adequate amount of OM in the

    soil stabilizes soil structure and makes it more resistant to degradation (Cochrane and Aylmore,

    1994; Thomas et al., 1996), and decreases bulk density and soil strength (Sparovek et al., 1999;

    Carter, 2002). The following mechanisms have been identified as the most common means by

    which OM influences soil structure and compactibility: (a) binding soil mineral particles (Zhang,

    1994), (b) reduction of aggregate wettability (Zhang and Hartge, 1992), and (c) influencing the

    mechanical strength of soil aggregates, which is the measure of coherence of inter-particle bonds

    (Quirk and Panabokke, 1962). In the research carried out in Northern Appalachian, Blanco-

    Canqui et al. (2007) found that SOC concentration explained 48 % of the variability in aggregate

    disintegration and 86 % in aggregate wetting.

    Castro et al. (2002) recognized the important role OM plays in soil particle aggregation and

    consequently, reported that part of the aggregate-size variation and aggregation indices in

    tropical soils can be attributed to the variation in the OM content. They further observed that a

    soil that is low in OC will be poorly aggregated since the weight and the content of OC in the >

    2.00 mm size class is lower than the weight and the content of OC of other classes.

    Microaggregates have a lower OM concentration than macroaggregates (Dormaar, 1983) and this

    OM is less labile than that associated with macroaggregates (Elliott, 1986). Moreover, reductions

    in aggregate stability after cultivation are most pronounced in soil macroaggregates, while the

    stability of soil microaggregates remains unchanged (Tisdall and Oades 1982; Oades, 1984).

    Nwadialo and Mbagwu (1991) reported that OC does not influence microaggregate stability

    greatly when the values of SOC are low and do not reach critical limits. Christensen (1985)

    observed that the OM contents of macroaggregates obtained by dry-sieving were closely related

    to their clay and silt contents. The findings by Christensen (1985) and Christensen and Sorensen

    (1985) showed that C and N are associated with finer soil particles, and these particles may vary

    among different aggregate fractions. Pikul et al. (2007) reported a significant positive

    relationship (r2 = 0.79) between WSA and fPOM/SOM. Mbagwu and Piccolo (1990) reported

    that on some north central Italian soils, in terms of total content, C, N and P were preferentially

    concentrated in the macroaggregates. Zibilske and Bradford (2007) reported that aggregation was

    significantly greater at 0-5 cm depth with no-tillage (NT) and ridge tillage, especially in the >

    4750 and 500-212 m size classes, where aggregate C and N contents were as much as 60 % and

    > 100 % respectively, more C and N were retained in the > 4750 and 500-212 m size classes at

    0-5 cm, and most C and N were detected in the > 4750 m size fraction at the 10-15 cm depth.

  • 12

    Shrestha et al. (2007) showed that macroaggregates in the surface layers of Nepal soil contained

    14.9 to 24.8 and 5.5 to 20.7 g kg-1

    SOC in cultivated and forest soils respectively, while

    microaggregates contained 12.5 to 30.8 and 11.9 to 25.4 g kg-1

    SOC, respectively.

    2.4 The Concept of Soil Organic Carbon Sequestration

    A number of definitions of C sequestration have been proposed by many researchers.

    Microsoft Encarta dictionary (2008) defined sequestration as the act of going into or being in

    isolation. Dick et al. (1998) described sequestration as seclusion or temporarily setting aside of

    something. Carbon sequestration therefore is the term describing processes that remove CO2

    from the atmosphere (Wikipedia, 2007). It is defined as the removal of CO2 from the atmosphere

    to the soil where it is secluded (stored) as part of the SOM (Dick et al., 1998). Jacinthe et al.

    (2002) referred to it as a process of conversion of plant-derived carbonaceous compounds into

    stable SOC pools in amounts in excess of soil respiration. According to Lal (2004), soil C

    sequestration means increasing SOC and SIC stocks through judicious land use and

    recommended management practices (RMPs). Lal (2001) defined C sequestration as the removal

    of C, as CO2, from the atmosphere through photosynthesis and dissolution, and the storage of C

    in soil as OM or secondary carbonates. The Department of Energy (DOE) (2006) referred C

    sequestration as the provision of long-term storage of C in the terrestrial biosphere, underground,

    or the oceans so that the buildup of CO2 (the principal greenhouse gases (GHG)) concentration in

    the atmosphere will reduce or slow. In contrast to SOC sequestration, however, SIC sequestration

    is the immobilization of C in the form of pedogenic (secondary) carbonates, and leaching of

    carbonates and bicarbonates into the ground water (Lal et al., 2000). The SIC includes elemental

    C and carbonate minerals of primary and secondary origin. Primary carbonates are derived from

    the parent material and secondary carbonates are found through the reaction of atmospheric CO2

    with Ca2+

    and Mg2+

    (Lal and Kimble, 2000). Lal et al. (2000) is of the view that SIC

    sequestration may be the significant pathway of C sequestration in arid and semi-arid regions.

    Soil C sequestration is one of the important mechanisms wherein C storage in soil is

    enhanced and its loss minimized, thereby reducing the rate of increase of atmospheric

    concentration of CO2. Consequently, C sequestration as SOC is based on the proportion of net

    primary production (NPP) returned to the soil and the conversion of biomass into humus,

    whereby NPP is the difference between gross primary production by the plant and the respiration

    of the plant (Macyk and Richens, 2002). Soil organic carbon sequestration depends on factors

    and processes that determine the net primary productivity and its addition to the soil body, and

    those that affect SOM accretion and decomposition in the soil: where changes in SOC content

    reflect the net result of C input (via plant litter) and C loss (via decomposition) Akala and Lal

    (2000). According to Lal et al. (2007), C sequestration process may be naturally or

  • 13

    anthropogenic-driven; the natural process includes terrestrial sequestration in soil (humification

    and formation of secondary carbonates) and trees (biomass production and storage in

    aboveground and below ground components). However, soil sink capacity and permanence are

    related to clay content and mineralogy, structural stability, landscape position, moisture and

    temperature regimes, and ability to form and retain stable microaggregates (Lal, 2004). For

    instance, both the rate and magnitude of SOC sequestration are more in heavy-textured than

    light-textured soils, on poorly drained than well-drained landscapes, in cool than warm climates,

    and in humid than dry ecoregions (Lal et al., 2007). Lal (2001) identified some of the processes

    leading to SOC sequestration, which include humification (conversion of biomass into humus),

    aggregation (formation of organomineral complexes as secondary particles), translocation of

    biomass into subsoil by deep roots and bioturbation, and by SIC leaching into groundwater as

    bicarbonates.

    Furthermore, the rate and magnitude of soil C sequestration also depend on the nature of

    tree species; nitrogen-fixing trees usually increase soil C pool by as much as 20-50 % more than

    non-N-fixers (Boring et al., 1988; Johnson and Henderson, 1995). Gerding (1991) reported a

    strong correlation between SOC pool in the surface layer (30 cm depth) and the N and Ca2+

    content of soils under 20-yr-old Pinus radiate D. Mkip (1995) reported an appreciable

    increase of C due to fertilization from 1900 kg ha-1

    (14 %) to 5530 kg ha-1

    (87 %). Carbon

    sequestration in humus is strongly related to soil management practices and especially to soil-

    conserving practices. And the practices that increase SOM include: leaving crop residues in the

    field, choosing crop rotations that include high-residue plants, using optimal nutrient and water

    management practices to grow healthy plants with large amounts of roots and residues, growing

    cover crops, applying manure or compost, using low-or no-till systems, and mulching to help

    conserve the soil. Accordingly, addition of manure enhances SOC pool and improves soil

    physical fertility (Blair et al., 2006). In addition, SOC sequestration is also caused by those

    management systems that add high amounts of biomass to the soil, cause minimal soil

    disturbance, conserve soil and water, improve soil structure, enhance activity and species

    diversity of soil fauna, and strengthen mechanisms of elemental cycling (Lal, 2004).

    2.4.1 Importance of Carbon Sequestration

    In southeastern Nigeria, as in most parts of the tropics characterized by low-input

    agriculture, the rate of SOM losses due to high-mineralization rate and nutrient losses to soil

    erosion and leaching losses is high (Igwe, 2000). However, SOC plays an important role in

    enhancing crop production (Stevenson and Cole, 1999) and also, in sustainable use of

    agricultural soils (Mulongoy and Merckx, 1993). Large areas of world soils are being degraded

    (Oldeman, 1994), therefore, desertification control and restoration of degraded soils is an

  • 14

    important strategy to sequester C in the biosphere. Because SOM is made up of the residues of

    plants and microorganisms which require many elements, sequestration of C requires other

    nutrients (Lal et al., 2007). For example, Hines (1997) observed on a weight basis, that the C/N

    ratio of SOM was 12/1, C/P was 50/1, and C/S was 70/1. Therefore, to sequester 1 t of C, 83 kg

    of N, 20 kg of P, and 14 kg of S are required. Thus, increasing SOC pool of degraded soils

    increases crop yields by improving soil quality through: (i) increasing available water capacity,

    (ii) improving supply of nutrients, and (iii) enhancing soil structure and other physical properties.

    Blair et al. (2006) observed a strong relationship between the SOC pool and soil physical

    fertility. Lal (2006) reported that increase in SOC pool by 1 t ha-1

    yr-1

    would lead to a total

    increase in food grain production of 30-50 Mt yr-1

    in the developing countries. Of the total

    increase, 24-39 Mt yr-1

    would occur in the tropics and subtropics, composed of 4-6 Mt yr-1

    in

    Africa, 6-10 Mt yr-1

    in Latin America, and 14-23 Mt yr-1

    in Asia. Therefore, the importance of

    SOC sequestration in achieving global food security cannot be overemphasized. In addition to

    improving the quantity of food, the attendant improvement in soil quality will also alleviate the

    hidden hunger affecting billions of people. Lal et al. (2007) was of the view that an increase in

    the SOC pool within the root zone by 1 t C ha-1

    yr-1

    can enhance food production in developing

    countries by 30-50 Mt yr-1

    including 24-40 Mt yr-1

    of cereal and legumes, and 6-10 Mt yr-1

    of

    roots and tubers.

    Soil organic carbon sequestration also mitigates global warming (Lal et al., 1995; Post

    and Kwon, 2000) by offsetting fossil fuel emissions. No doubt, atmospheric CO2 can be

    transferred into long-lived pedological pools (Lal, 2005), and once SOC is sequestered, it

    remains in the soil as long as restorative land use or RMPs are followed, and sequestration rates

    can continue for 30 and up to 50 yrs (Lal, 2004). The common RMPs that lead to SOC

    sequestration are mulch farming, conservation tillage, agroforestry and diverse cropping systems,

    cover crops (Lal, 2004). Moreover, there is a large potential for C sequestration in biomass in

    forest plantations, short rotation woody perennials, and so on (Lal et al., 2007).

    There are numerous benefits of sequestering SOC in soil, besides the potential of

    mitigating the greenhouse effect (Rosenberg and Izaurralde, 2000). Land stewardship (Lord and

    Lord, 1950) is a major benefit, because increase in SOC content enhances soil quality and

    sustainability. Increase in soil quality, due to improvement in SOC content, enhances crop yield

    and productivity. Lal (2002b) documented the importance of soil C sequestration in improvement

    of soil quality, improvement of quality of natural water, air quality, and in reduction of silt load

    and sedimentation. In their study in Thialand, Petchawee and Chaitep (1995) observed a linear

    relationship between SOC and grain yield of maize (Zea mays L.). Improvements in soil

    structure, in terms of both the degree and the stability of aggregation, led to a decrease in the soil

    erodibility and a reduction in the risks of non-point-source pollution. The benefits in

  • 15

    improvements of water quality were also due to increases in soil buffering, detoxification and

    biodegradation of pollutants, and increases in the retention capacity of soil to absorb ions on the

    exchange complex.

    2.5 Influence of Soil Carbon and Total Nitrogen on Soil Properties

    Soil organic matter is a complex mixture of organic compounds with different turnover

    times (Christensen, 2001) and contains 50 to 58 % C (Nelson and Sommers, 1982). Different

    pools of SOM have been conceptualized for modeling SOC dynamics (Skjemstad et al., 1998).

    Particulate organic matter (Cambardella and Elliott, 1992) is an uncomplexed fraction of SOM

    composed of particulate (> 0.05 mm), partially decomposed plant and animal residues, fungal

    hyphae, spores, root fragments, and seeds. This fraction provides a substrate for microbial

    activity and is an important agent in the formation of macroaggregates. It is a fraction that lies

    intermediate between litter (fast turnover) and mineral-associated SOM (slow turnover). Soil

    organic matter mediates many chemical and physical soil properties (Carter, 2002). Indirectly,

    SOC and TSN affect several physical, chemical and biological processes (Bauer and Black,

    1994). In the highly weathered Ultisols and Oxisols, OM is a major determinant of soil CEC, and

    its reduction leads to a decrease in the nutrient and water retention capability, and lowers soil

    fertility (Sotomayor-Ramrez et al., 2006). The SOM stored in the top soil contributes most

    actively to nutrient cycling in the soil-water-plant system and to gaseous exchanges with the

    atmosphere, but the subsoil can also be important (Nepstad et al., 1991; Davidson et al., 1993).

    Depletion of SOM reduces soil fertility and degrades soil physical properties such as infiltration

    rate, soil structure, and water-holding capacities (Houghton et al., 1983). Experiments have

    shown that the plant available water capacity increases by 1 to 10 g for every 1 g increase in

    SOM concentration (Emerson, 1995). The increase is small and usually happens only in medium-

    textured soils, but is critical to sustaining crop growth between rainless periods of 5 to 15 days.

    Boyle et al. (1989) reviewed the influence of SOM on soil aggregation and water infiltration and

    concluded that SOM had a disproportionate effect on soil physical properties. The quantity and

    quality of SOM affects WSA (Tisdall and Oades, 1982). Soils high in SOM generally have

    greater available water-holding capacity than soils of similar texture with less SOM (Hudson,

    1994); although Bauer and Black (1992) found that a decline in SOM did not change the

    available water-holding capacity of moderately coarse-textured soils. An increase in phytomass

    input to a loamy sand improved aggregate stability and water infiltration (Bruce et al., 1992).

    Maintenance of SOM therefore, is a key component in sustainability of the soil resource and crop

    productivity (Doran et al., 1998).

    Saggar et al. (2001) found that percentage C loss differed with soil type, with Marton silt

    loam (260 g kg-1

    , clay) soil losing one and half times as much percentage C as Kairanga silty clay

  • 16

    loam (420 g kg-1

    , clay) soil. Soil C accounted for about 70 to 90 % of the variability in soil

    aggregate stability of a clay loam soil (Mbagwu and Bazzoffi, 1989). In long-term tillage, residue

    management, and N-fertility plots, Pikul and Zuzel (1994) reported that an increase in SOM

    increased the porosity of surface crusts in a silt loam soil, while on a Naff silt loam, Mulla et al.

    (1992) were not able to establish a relation between SOM and physical properties of conventional

    and alternatively managed farms. The alternative farm studied by Mulla et al. (1992) used a

    cropping system that was more diverse than the conventional farm; however, tillage was used

    on both farms.

    Depletion of SOC concentration leads to reduction in aggregation, and soil structure with

    attendant susceptibility to crusting, hard-setting, compaction, accelerated erosion, anaerobiosis,

    drought and salt imbalance (Lal, 1991; Paustian et al., 1997; Grace et al., 1998). Generally, soil

    compaction decreases with increasing SOM (Soane, 1990; Hudson, 1994). Lal (2002b) remarked

    that SOC affects soil physical quality through change in soil structure, aggregation, total- and

    macroporosity, susceptibility to crusting and compaction, and ease of root system development.

    Degens (1997) provided a review on the function of labile organic bonding and binding agents

    related to soil aggregation. Labile compounds, in the context used by Degens (1997), were

    considered to be components of SOM that were readily decomposable and recently deposited by

    roots and microorganisms.

    2.6 Effects of Land Use Management on Soil Carbon and Nitrogen

    Soils respond differently to management depending on the inherent properties of the soil

    and the surrounding landscape (Andrews et al., 2006). Management choices affect the amount of

    SOM, soil structure, soil depth, and water- and nutrient-holding capacities (Lal et al., 2007).

    Under undisturbed natural conditions, the soil C pool is in equilibrium and the input of C (litter

    fall, root biomass, C brought in by run-off, dust) is balanced by output (erosion, decomposition,

    and leaching) (Lal, 2004). The reduction in C input is caused by a decline in biomass production

    and reduction in the fraction returned to the soil. The increase in C output is attributed to increase

    in oxidation of SOM because of change in soil moisture and temperature regimes, and increase in

    losses caused by soil erosion and leaching (Lal et al., 2007). Conversion of natural ecosystems to

    agriculture leads to depletion of SOC through increase in decomposition and mineralization of

    biomass caused by aeration, erosion and preferential removal of SOC by erosional processes

    (Lal, 1984, 1989; Tisdall, 1996), which typically reduces the amount of C input and increases the

    magnitude of output. These C and N losses are caused by decreased plant OM inputs and by

    increased decomposition and erosion associated with agriculture. Decomposition increases

  • 17

    because of a change in aggregate structure of the soil due to cultivation (Coote and Ramsey,

    1983).

    Anthropogenic disturbance of terrestrial biosphere can deplete the SOC pool (Schlesinger,

    1993). Emission of CO2 from soil-related processes began with the onset of settled agriculture

    about 10,000 years ago, and that of CH4 with the cultivation of rice paddies and domestication of

    animals about 5000 years ago (Ruddiman, 2003, 2005). Tropical deforestation continues to be a

    major source of CO2 (Wallace, 2007). Soil tillage, extractive farming practices, and accelerated

    soil erosion cause a rapid depletion of the SOC pool (Lal et al., 2007). Many cultivated soils have

    lost 50 to 75 % of their antecedent SOC pool (Lal, 2004). According to Knops and Tilman (2000)

    on the average, agricultural practices resulted in a 75 % loss of soil N and an 89 % loss of soil C

    at the time of abandonment, whereas recovery to 95 % of the preagricultural levels is predicted to

    require 180 yr for N and 230 yr for C. Other studies suggest that losses of SOM caused by

    agricultural practices range from 16-77 % (Mann, 1986), with a mean of 29 % (Schlesinger,

    1986). Estimates of the historic loss of SOC pool from world soils vary widely. The loss of C

    pool from world soils was estimated at 40 Gt by Houghton (1995), 55 Gt by IPCC (1995) and

    Schimel (1995), and 150 Gt by Bohn (1978). The rate of C loss from world soils caused by

    current land use change and soil cultivation is also variable and estimated at 0.6 to 2.6 Gt yr-1

    by

    Lashof and Hare (1999) and 1 to 2 Gt yr-1

    by Bohn (1978).

    Agricultural practices can cause the loss of a large fraction of SOM (Mann, 1986;

    Schlesinger, 1986). Losses are influenced by soil texture with higher losses on sandy soils (Bauer

    and Black, 1981; Campbell and Souster, 1982). Coote and Ramsey (1983) and Nichols (1984)

    obtained a positive relationship between percentage clay and SOM. It is generally agreed that

    cultivation causes a decrease of SOC content (Alvarez et al., 1998). Ellert and Gregorich (1996)

    estimated that 30 to 35 % of SOC originally present in the A and B horizons of the native

    temperate forest soils was lost after cultivation for 30 yr or more. In the study of the effect of

    cultivation on SOC, and soil structure in soils of contrasting mineralogy, Shepherd et al. (2001)

    found that soil susceptibility to structural degradation increased with years of cultivation, and

    from light textured to heavier textured soils. Cultivation generally results in reduced stability and

    amount of macroaggregates but does not affect microaggregate stability (Tisdall and Oades,

    1982). Therefore, the SOM that binds microaggregates into macroaggregates has been suggested

    to be the primary source of OM lost upon cultivation (Elliott, 1986). When soils under pasture

    are brought into cultivation there is a rapid loss of OM during the first few years, reflecting, in

    part, exposure of more surfaces due to soil disturbance and dry matter removed during harvesting

    (Sparling et al., 1992; Shepherd et al., 2001). Farage et al. (2003) found that stocks of soil C in

    Nigeria were 8-23 t ha-1

    before cultivation, 6-12 t ha-1

    after cultivation, and 0.05-0.01 t ha-1

    y-1

    with effect of conventional tillage (CT) practices.

  • 18

    The SOC content in cropland is strongly correlated to crop and soil management practices

    which include crop species and rotation, tillage methods, fertilizer rate, manure application,

    pesticide use, irrigation and drainage, and soil and water conservation (NRC, 1989; Paustian et

    al., 1997). Tillage and especially bare soil conditions are the most important management

    practices leading to the depletion of SOM and SOC (Lal et al., 2007). No tillage has been

    proposed as an alternative to conventional cropping systems for reducing soil degradation. It

    generally leads to an increase of C in the top 5-10 cm of the profile, relative to plow layer (Kern

    and Johnson, 1993). Generally, organic C and N concentrations in the surface 15 cm of non-tilled

    soils is greater than for tilled soils (Doran, 1980). Conversion of plow tillage to NT can increase

    SOC pool by up to 10 Mg ha-1

    during 5-20 yr (Paustian et al., 1997). However, CT inverts the

    OM; consequently, the distribution is more uniform throughout the plow layer than in a no-till

    system (Fox and Bandel, 1986). Hulugalle (2000) showed that minimum tillage decreased SOC

    content in Vertisols in New South Wales, Australia. Method of tillage also influence the degree

    of change in OM, [moldboard plowed tillage decreases OM more than stubble mulch tillage

    which decreases it more than NT (Lamb et al., 1985)], soil texture [coarse and fine textured soils

    are likely to show greater changes than medium textured soils (Bauer and Black, 1981)] and

    weather conditions [(drought results in less crop residues and more soil erosion and may decrease

    OM (Campbell et al., 1995)]. Denef et al. (2004) showed that total SOC as well as

    microaggregate-associated C (mM-C) was greater with NT compared with CT. No tillage

    increased fine particulate organic matter (fPOM)/SOM by 19 and 37 % compared with tillage

    following 4 and 10 yr of NT, respectively. A 5 year diverse rotation increased fPOM/SOM by 36

    % compared with monoculture. Pikul et al., (2007) demonstrated that diversity of rotation or

    reduction of tillage increased fPOM and WSA and this may help to curb soil loss by maintaining

    surface conditions resistant to erosion.

    Adoption of RMPs has great potential for increasing the amount of C sequestered (Cole et

    al., 1993; Lal et al., 1997; 1999). Whilst Watson et al. (1996) estimated that 0.4-0.8 Pg C yr-1

    could be sequestered in agricultural soils globally by implementation of appropriate management

    practices. Lal et al. (1995) estimated that with improved land use, cultivated and (resilient)

    degraded soils can sequester 0.1-1.0 Pg C yr-1

    , depending on management. Pacala and Socolow

    (2004) estimated that conversion of all cropland to no-till has a potential C sink capacity of 1 Gt

    yr-1

    . Lal (1986) pointed out that land use practices and cropping systems that do not return

    residue to the soil can cause a significant decline in SOC concentration. Cropping systems to

    increase SOC include the use of rotations, planting winter cover crops, and placing land into the

    conservation reserve program (CRP) (Six et al., 2004). Carbon sequestering practices to enhance

    SOC sequestration on cropland include improved use of crop residues; increased use of CT

    (especially NT), crop residue, and other biomass management approaches; reduction of fallow;

  • 19

    use of organic materials and systems that enhance belowground root biomass including that from

    weeds (Follett, 2001). However, Six et al. (1999) found that the greater C stabilization with NT

    relative to CT is only partly explained by a greater amount of macroaggregates. Furthermore, a

    reduced rate of macroaggregate turnover under NT increases the formation of microaggregates in

    which C is stabilized and sequestered in the long term. Particulate organic matter is a labile

    intermediate in the SOM continuum from fresh organic materials to humified SOM (Paul et al.,

    2001), and is more sensitive to changes in management than total SOM (Cambardella and Elliott,

    1992 and 1993; Cambardella et al., 2001). Gale et al. (2000) reported that under NT practices,

    aggregate formation was directly related to root-residue decomposition and POM dynamics,

    while in undisturbed soils, POM is derived primarily from roots since new microaggregates

    probably form around decomposing pieces of root-derived POM inside macroaggregates. An

    aggregate life cycle was proposed by Six et al. (2000) in which aggregates form and stabilize

    around fPOM encrusted with microbial products, and eventually destabilize due to a cessation of

    microbial activity. Six et al. (2004) identified five factors (soil fauna, microorganisms, roots,

    inorganic soil components, and physical processes) that are important in the link between soil

    biotic activity, SOM decomposition and stabilization, and soil aggregate dynamics. Several

    reports have shown that crop residue mulch associated with NT management improves soil

    aggregation and increases SOC content (Havlin et al., 1990; Carter, 1992; Cambardella and

    Elliot, 1992; 1993). However, this increase is generally restricted to the surface soil (S et al.,

    2001). In tropical zones, Bayer et al. (2000); Resck (1998); and S (1993) observed a significant

    impact on SOC concentrations for 0-10 cm layer. Unger (1991) and Christensen et al. (1994)

    demonstrated that distribution of SOC and plant nutrients changed with no-till management. No-

    till resulted in increased SOC concentration near the surface even when comparing no-till with

    stubble-mulch tillage, which does not invert the soil. However, tillage mixes SOC in the surface

    layers, which alters the distribution and may increase decomposition (Potter et al., 1997). Other

    soil management practices such as application of N fertilizer and animal manure also increased

    SOC content (Paustian et al., 1997), and Potter et al. (1997) found that fallow limited C

    accumulation in a dryland cropping system. Findings documenting the effectiveness of land use

    and soil management practices for SOC sequestration in relation to soil and ecoregional

    characteristics abound in literature (Lal et al., 1998; Batjes, 1999; IPCC, 2000; Lal, 2000).

    2.7 Factors Affecting Soil Organic Carbon (SOC) and Total Soil Nitrogen (TSN) Pool in

    the Tropics

    The tropics cover 8.2 billion hectares or approximately 40 % of the world land area and are

    relatively characterized by high risk of soil and environmental degradation because of harsh

  • 20

    climate and resource-poor farmers, and rapid decomposition of SOM because of continuously

    high temperature (Lal, 2002a). The soil pool is highly reactive and dynamic, and the cultivation

    of virgin soils or conversion of natural to agricultural ecosystems leads to depletion of the soil C

    pool with attendant emission of GHGs into the atmosphere (Lal, 2002b). Agricultural practices

    can render a soil either a sink or a source of atmospheric CO2, with direct influence on the

    greenhouse effect (Lugo and Brown, 1993; Lal et al., 1995). Although cultivation of agricultural

    soils is not as obvious a source of GHGs as is direct fossil fuel combustion, however, soil misuse

    can lead to emissions of GHGs from soil to the atmosphere (Lal, 1999). It has been recognized

    that the CO2 contribution to radiative forcing is about 50 %, and 22.9 % of total CO2 emissions to

    the atmosphere is attributed to agriculture, deforestation, and land use (IPCC, 1996). The

    cumulative historic loss of C by cultivation since the dawn of settled agriculture is enormous

    (Lal, 2000). Total CO2-C emitted by anthropogenic activities between 1850 and 2000 is

    estimated at 270 30 Gt by fossil fuel combustion and 136 55 Gt by land use change and

    deforestation (IPCC, 1999; 2001). In fact, until 1970, more C came from soil cultivation and

    deforestation than fossil fuel combustion (Lal, 2002b). During the 1990s, about 25 % of global

    emissions came from land-use change and soil cultivation (IPCC, 1996). From 1850 to 1998, 270

    30 Pg C were emitted from fossil fuel combustion and cement production (Marland et al.,

    1999). During the same period, net cumulative CO2 emissions from land use change were

    estimated to have been 136 55 Pg (Houghton, 1997, 1999; Houghton et al., 1999, 2000). The

    emission from soil cultivation is estimated at 78 17 Pg C (Lal, 1999). Therefore, conversion to

    restorative land uses (e.g. afforestation, improved pastures) and adoption of RMP can enhance

    SOC and improve soil quality. Important RMP for enhancing SOC include CT, mulch farming,

    cover crops, and integrated nutrient management including use of manure and compost, and

    agroforestry (Lal et al., 2007). In agricultural land, techniques such as decreased tillage and

    efficient use of fertilizers and irrigation have been proposed as ways to increase SOC and

    decrease atmospheric CO2 (Lal et al., 1998; IPCC 2000).

    It has been ascertained that the C balance of the terrestrial ecosystems can be altered

    remarkably by the direct impact of human activities including deforestation, biomass burning,

    land use change, and environmental pollution which release trace gases that enhance the

    greenhouse effect (IPCC, 1990; Trabalka and Reichle, 1986). Six et al. (1999) stated that SOC

    and SON pools may be influenced by the magnitude and quality of C inputs, disruption of

    aggregates, or microclimatic changes within a relatively small area. More so, a drastic loss of

    SOC during mining has been recognized by Akala and Lal (2000), and it is thus conceivable that

    reclamation of mine lands can lead to recuperation and sequestration of SOC. Besides climatic

    factors, the following activities that influence pools and fluxes of C are widely recognized:

  • 21

    1. Deforestation: Conversion of natural to agricultural ecosystems causes depletion of the SOC

    pool by 75 % or more in cultivated soils of the tropics (Lal, 2004). The estimated rate of SOC

    loss is 90-219 Tg C yr-1

    (Tg = teragram = 1 x 1012

    g) because of tropical deforestation (Lal and

    Logan, 1995). In Malaysia, Andriesse and Schelhaas (1987) reported 28 Mg C ha-1

    yr-1

    rate of

    depletion of SOC pool by deforestation. Motavalli et al. (2000) showed that conversions of

    secondary forest to continuous cultivation can decrease the SOM carbon by 44 % within 5 years

    after cultivation. Lal and Kimble (2000) indicated that deforestation of the tropical rainforest

    (TRF) accounts for about 15 % of the total global warming potential, comprising 10 % due to

    CO2 and 5 % due to other trace gases (e.g., N2O, CH4 and NO), whereby estimates of CO2

    release range from 400 to 2500 Tg C yr-1

    . Because deforestation accounts for 20 % of the total

    global GHG emissions (more than all the worlds cars, trucks, trains and airplanes combined),

    world bank announced setting up a new fund (US$300 million) to subsidize developing countries

    to protect and replant tropical forests for carbon sequestration (ECOS, 2008). Extensive

    deforestation and intensive agricultural management have led to an increasing transfer of C from

    SOM to atmospheric CO2 (Schlesinger, 1997). However, afforestation of agricultural land has

    been shown by several researchers to sequester SOC in soil (Lugo and Sanchez, 1986; Johnson,

    1992; Huntington, 1995; Ellert and Gregorich, 1996; Post and Kwon, 2000). Schlesinger (1999)

    is of the view that a substantial sink for C in soils may derive from the application of

    conservation tillage and the regrowth of native vegetation on abandoned agricultural land.

    2. Fire: Lal and Kimble (2000) observed that fire has been used as a principal tool in removing

    the biomass in both tropical rainforest (TRF) and tropical savanna (TS) ecosystems, which leads

    to emissions of several radiatively-active gases, viz. CO2, CH4, N2O, NOx etc. Accordingly,

    biomass burning in all tropical regions may account for an annual release of about 3.4 Pg of C.

    Lal and Logan (1995) estimated an annual SOC loss of 112-276 Tg C yr-1

    by burning of

    grasslands. Both the direct effects of fire and also the overall changes to the ecosystem

    encountered in a post-fire situation can lead to short-, medium- and long-term changes in the soil

    which relate to soil functioning in the physical, biological and chemical sense (Chandler et al.,

    1983; Neary et al., 1999; Gonzlez-Prez et al., 2004). Doerr and Cerd (2005) indicated that fire

    affects the entire ecosystems their flora, fauna, the atmosphere and soil. Consequently, burning

    and resulting post-fire environmental conditions can alter the functioning of soils physically (e.g.

    aggregate stability, pore size, distribution, water repellency and runoff response), chemically

    (e.g. nutrient availability, mineralogy, pH and C/N ratios) and biologically (e.g. biomass

    productivity, microbial composition and carbon sequestration). More so, the effect of fire

    decreasing the ability of soil to recover and lowering soil resilience has been recognized (Lal,

    1998). However, the effects of a fire on soil characteristics will depend on the fire intensity, soil

  • 22

    properties and type of vegetation. On the one hand the enrichment of available nutrients in

    burned soils may lead to an increase in nutrient content in the recovering plant cover, thus

    enhancing primary productivity (Carter and Foster, 2004). The fire-induced increase in soil

    nutrient content may also positively influence decomposition rates and nutrient cycling (De

    Marco et al., 2004). On the other hand, a fire of high intensity may cause changes in soil

    properties that accelerate erosion and nutrient losses (Neary et al., 1999; Thomas et al., 1999;

    Gonzlez-Prez et al., 2004). For example, the complete destruction of the organic layer severely

    limits soil functions, reducing infiltration and water storage (Imeson et al., 1992; Giovannini and

    Lucchesi, 1997). In addition, the destruction of the vegetation cover leaves the soil exposed to

    raindrop impact and water erosion. Fire, directly and indirectly, affects microorganisms

    inhabiting soil surface layers. The functioning of the soil microbial community is central to

    sustaining ecosystem-level processes such as decomposition of OM and nutrient cycling, which

    are essential in the maintenance of soil fertility (De Marco et al., 2005). The heat generated in the

    soil during a fire may kill a large or a small part of the soil microbial biomass, depending on

    length of exposure to high temperatures (Neary et al., 1999).

    3. Land use change and management: Cambardella and Elliot (1993, 1994) demonstrated that

    land use and management practices can alter the distribution of organic C and N among labile

    and stable pools with kinetically different turnover rates. For example, intensive cultivation of

    grassland reduces the SOC and SON because of the destruction of soil macroaggregates by

    different levels of tillage (Cambardella and Elliot, 1993). Lal and Kimble (2000) remarked that

    biomass production which is an important determinant of C pool and fluxes depends on land use

    and management. Consequently, conversion of natural (TRF and TS) ecosystems to agricultural

    land use (plantation, pastoral and cropland) has a drastic impact on C pool and emissions of

    GHGs to the atmosphere. Tillage also results in partial aggregate destruction and concomitant

    OM loss (Six et al., 1999; Wright and Hons, 2005). Furthermore, a general increase in C levels

    ( 325 113 kg C ha-1

    yr-1

    ) was observed under NT compared with CT in tropical and temperate

    soils (Six et al., 2002). However, adoption of CT practices could result in lower costs and in

    improved soil quality by decreasing SOC losses (Sotomayor-Ramrez et al., 2006). Estimates of

    historic SOC loss included 3.8-9.2 Tg C yr-1

    by shifting cultivation and 38-92 Tg C yr-1

    by

    plowing of cropland (Lal and Logan, 1995). The rate of depletion of SOC pool was 27-38 Mg C

    ha-1

    yr-1

    by shifting cultivation in eastern Africa (Woomer et al., 1994); in Australia, Dalal and

    Mayer (1986) and Chan (1997) reported a decrease of 19-67 % of SOC concentration in the 0-10

    cm depth by cultivation over 20-70 yrs.

    Applications of agricultural input such as fertilizer also affect SOC pool. It is still observed

    that addition of manure enhances SOC (Blair et al., 2006). Farage et al. (2003) reported that

  • 23

    additions of green manure (10 t ha-1

    crop-1

    ) or farmyard manure (1.5 t ha-1

    each cropping year) to

    the no-till system, lead to increases in C sequestration (0.029-0.034 t C ha-1

    y-1

    ). On the other

    hand, additions of farmyard manure and green manure both lead to marked increases in C

    sequestration rates in the region of 0.18-0.25 t C ha-1

    y-1

    ; therefore replacing inorganic fertilizer

    with farmyard manure promotes C sequestration (0.22 t ha-1

    y-1

    ) while combining the green

    manure and farmyard manure applications gave the best C accrual rate, 0.29 t ha-1

    y-1

    in Santa

    Mara river valley, Catamarca province of Argentina. More so, soil C (0.1 t C ha-1

    y-1

    ) was lost

    when only inorganic fertilizers were used to maintain soil fertility in the intense systems of the

    Kano closed-settlement zone in Nigeria. Replacing the manure input to this system with

    inorganic fertilizer (urea 100 kg ha-1

    ) resulted in a large reduction in soil C, with stocks falling by

    > 0.1 t C ha-1

    y-1

    . It was thus conclusively noted that maintaining crop yields through the

    application of inorganic fertilizers alone will most likely result in substantial losses of SOM.

    4. Soil factors and processes: Batjes (1998) showed that moisture status, soil temperature, O2

    supply (drainage), soil acidity, soil nutrient supply, clay content and mineralogy are the major

    environmental factors which control the behaviour of OM in the soil. Lal et al. (2007) reported

    that change in soil moisture and temperature regimes affects oxidation of SOM. Batjes (1998)

    also remarked that the soil forming factors, notably climate as well local biological activity in

    which humans are often a predominating factor, control the amount of SOM that corresponds

    with equilibrium conditions in a certain natural ecosystem or agro-ecosystem. Different factors

    influence different SOC pools. Free OM particles and microbial biomass in soils are controlled

    by residue inputs (management of crop residue and mulching) and climate, for example, the

    microbial populations and activities in pasture tends to be higher compared to the corresponding

    agricultural soils due to the positive imparts of the surface cover, vegetation, belowground C

    allocation via roots, and lack of tillage of pasture (Acosta-Martnez et al., 2004). More so, soil

    aggregation, texture and mineralogy control OM in macroaggregates. The other pools are less

    influenced by agronomic factors but mainly by pedological factors - micro-aggregates, clay

    composition (Feller and Beare, 1997). Furthermore, the turnover and cycling of SOC is more

    rapid in the tropics than in temperate regions (Trumbore et al., 1995). Six et al. (2002) found on

    the average that soil C turnover was twice as fast in tropical compared with temperate regions -

    one possible reason being the continuous inputs of fresh OM in tropical soil; but no major

    differences were observed in SOM quality between the two regions.

    Lal and Kimble (2000) pointed out that the important soil processes influencing C pool and

    dynamic include soil nutrient reserves and cycling, and soil aggregation and C sequestration.

    Carbon sequestration by natural TRF was attributed to (i) CO2 fertilization effect, (ii) tree

    mortality due to catastrophic events, and (iii) effect of El Nio in exacerbating tree mortality.

  • 24

    Other factors that have been classified as immediate causes of a decline in SOC include residue

    removal, soil erosion, intensive tillage, and bare fallowing (Lal and Kimble, 2000; Paustina et al.,

    2000). The various important soil factors affecting the SOC pool have been widely documented

    and include texture, clay minerals, soil fertility, and plant available water capacity of the root

    zone (Lal, 2001). It is generally agreed that the SOC pool increases with increase in clay content;

    for the same clay content, however, soils with high activity clay have more SOC content than

    those with low activity clay.

    5. Landscape position: Novel understanding of the nature of the environment indicates that the

    exogenous factors of climate, landscape, texture, and clay mineralogy which cannot be readily

    altered are among important determinants of soil and biomass C pool (Lal, 2001). The SOC

    content also depends on landscape position due to soil erosion and leaching, which are

    predominant on sloping landscape. The effects of cropland management practices and landscape

    position on SOC pool depend on climate, soil type and landscape morphology, and are thus site

    specific (Hao et al., 2002). Increasing water content and soil deposition at lower slope position

    affect SOC decomposition and crop biomass production where the SOC content would often be

    higher (Pennock et al., 1994; Fahnestock et al., 1995; Gregorich et al., 1998). Hall (1983)

    reported that steeper slopes contribute to greater runoff, as well as greater translocation of surface

    materials downslope through surface erosion and downhill movement of soil mass.

    6. Soil erosion: Decrease in the SOC pools may be caused by three often simultaneous

    processes: mineralization, transport by soil erosion processes, and leaching into the ground water

    or subsoil (Lal, 2000). Mokma and Sietz (1992) reported a significant reduction in SOC

    concentration of the surface layer as a result of erosion on a cultivated soil. The rate of SOC

    depletion by erosion was reported to be 0.4 Mg C ha-1

    yr-1

    in Nigeria (Lal, 1976), and 0.5-1.1 Mg

    C ha-1

    yr-1

    in Zimbabwe (Tagwira, 1992). Lal (2001) indicated that the removal of SOC pool by

    soil erosion may constitute half of the total loss of SOC pool from agricultural soils, the other

    half being due to an increase in mineralization caused by climatic conditions (change in soil

    temperature and moisture regimes) and properties of SOC. Accordingly, the increased loss of

    SOC by accelerated erosion due to water or wind occurs because it is concentrated in the surface

    area, has low density (1.2-1.5 Mg m-3

    for OM compared with 2.6-2.7 Mg m-3

    for mineral

    fraction) and the labile particulate fraction is relatively unconsolidated. Thus, soils under

    agricultural crops or forest cover conserve C in greater magnitude than eroded areas (Lal, 2002b).

    In conclusion therefore, three main factors namely climatic factors, soil factors

    physical, chemical, biological and processes therein, and agronomic factors (land use and

    management) could be responsible for losses of SOC and SON in the tropics.

  • 25

    CHAPTER THREE

    MATERIALS AND METHODS

    3.1 Site Description

    In this research study, soils from four different locations within two agro-ecological zones

    (derived savanna and secondary forest) in southeastern Nigeria were considered. Within each

    sampling location, soils of similar textures were collected from both cultivated and uncultivated

    soils at 0-10, 10-20, and 20-30 cm depths. The study locations included University of Nigeria

    Nsukka Research Farm beside the meteorological station designated as Nsukka I (Nkpologu

    series soil), and Uvuru series soil at the hills behind Miriam Babangida Auditorium designated as

    Nsukka II, both in Nsukka Local Government Area of Enugu state; Awgu in Awgu Local

    Government Area of Enugu state; and Ugwaku in Okigwe Local Government Area of Imo state.

    Location/Climate

    Nsukka I and II are located between latitudes 6o30 and 6

    o41N, and longitudes 7

    o10 and

    7oI4E and is 400 m above sea level. On the other hand, Awgu and Ugwaku are located between

    latitudes 5o43 and 6

    o04N, and longitudes 7

    o21 and 7

    o28E. The climate is characterized by

    tropical wet and dry seasons with most of the rains falling between April and October at very

    high intensity, followed by the dry seasons which last from November to March (Igwe and

    Okebalama, 2006). The mean annual rainfall is about 1500-1600 mm, and the relative humidity

    of 70 % prevails in the study area. Temperature is uniformly high throughout the year with mean

    minimum and maximum values of 26 oC and 31

    oC, respectively.

    Vegetation/Landuse

    The vegetation of Nsukka location is mainly derived savanna of which 60-70 % of the

    vegetation is covered with grasses (Igbozurike, 1975), while that of Awgu and Okigwe fall

    within the secondary forest of the humid tropics. The predominant tree species in these four

    locations are oil palm (Elaeis guineensis), sweet orange (Citrus sinensis), mango (Mangifera

    indica), cashew (Anacardium occidentale) and oil bean (Pentaclethra macrophllum). The major

    grass species are guinea grass (Panicum maximum), elephant grass (Pennisetum purpureum),

    Calapogonium species, Andropogon species, Chromolena odorantum.

    Agriculture is the predominant occupation of the indigenous land owners, who are mostly

    operating at the subsistence level. The common arable crops grown are cassava (Manihot

    esculenta), maize (Zea mays), yam (Dioscorea species), cocoyam (Colocasia specie), local beans

    (Phaseolus species), vegetables including telfairia, garden egg (Solanum species), Amaranthus

  • 26

    species, and plantation crops such as plantain (Musa paradisiaca), banana (Musa sapientum), and

    oil palm (Elaeis guineensis).

    Geology

    Soils of Nsukka I and II originated from similar parent material mainly weathered

    Sandstone (Ajali formation), and were classified in the early 1960s during a soil survey of the

    Anambra Do River valley areas, as Nkpologu series and Uvuru series, respectively (Jungerius,

    1964). The soils are deep, permeable and well-drained. The underlying geology of Awgu and

    Okigwe soils is mainly Shale. The topography of the four locations is of hilly terrain.

    3.2 Methods

    This research work encompassed two main stages namely the field, and the laboratory

    studies. The field study involved collection of samples from the four locations - Awgu, Okigwe,

    Nsukka I and II. The laboratory study involved soil handling and sample preparation, including

    laboratory analyses of the samples.

    3.2.1 Field Study

    The soils used for this study were sampled at 0-10, 10-20, and 20-30 cm depths of the

    above listed locations for each of the manually-tilled cultivated and uncultivated soils of 4-5

    years. The information about the land was obtained from the indigenous land owners. Additional

    undisturbed core samples were collected from each site and from corresponding depths for the

    determination of bulk density (BD), total porosity, macroporosity (Pma), microporosity (Pmi),

    water content at field capacity (FC), and soil moisture content. A total of twenty-four samples

    were collected, put in plastic bags, labeled and brought to the laboratory for proper analyses.

    3.2.2 Laboratory Study

    Soil handling and sample preparation

    Sampled soils were air-dried and divided into two equal parts. One portion was passed

    through a 4.75 mm sieve to remove rock fragments after which the fractions that passed through

    the mesh were used for aggregate size separation for the determination of aggregate size

    distribution of soil organic carbon (SOC) and total soil nitrogen (TSN). The second portion

    passed through 2 mm sieve and was used for the determination of particle size distribution in

    H2O and in calgon, soil pH, SOC, total nitrogen, available phosphorus, cation exchange capacity

    (CEC), exchangeable acidity (H+, Al

    3+), exchangeable bases (Na

    +, K

    +, Ca

    2+, Mg

    2+) and

    coefficient of linear extensibility (COLE), according to standard analytical procedures as

    described below.

  • 27

    3.2.2.1 Physical analysis

    Particle size analysis of < 2 mm fine earth fractions were carried out to determine the soil

    texture by the hydrometer method as described by Gee and Bauder (1986), using sodium

    hydroxide as the dispersing agent, and with deionized water alone for the determination of water

    dispersible clay and silt. The microaggregate stability indices of the soils including clay

    dispersion index (CDI), dispersion ratio (DR), aggregated silt and clay (ASC) and clay

    flocculation index (CFI) were all calculated as follows according to Igwe et al. (1999):

    CDI = [% clay (water) / % clay (calgon)] x 100

    DR = [% silt + % clay (water)] / [% silt + % clay (calgon)]

    ASC = [% clay + % silt (calgon)] [% clay + % silt (water)]

    CFI = [% clay (calgon) % clay (water)] / [% clay (calgon)] x 100

    Soil moisture content was measured, and the soil water content at field capacity (FC) was

    determined by using the same core sample used for bulk density, macroporosity, microporosity

    and total porosity. The core samples were weighed after 24 h of complete saturation in water and

    then, placed on a tension table adjusted to 60 cm of tension for 24 h, after which they were

    reweighed and oven dried until constant weight was obtained. Bulk density measurement was

    obtained by the cylindrical core method as described by Blake and Hartge (1986), which was

    used to calculate SOC on an area basis. Total porosity was determined in undisturbed water

    saturated core samples. The above parameters were calculated as follows:

    Soil moisture content = [(wet weight oven dry weight) (g) / oven dry weight (g)] x 100

    Water content at FC = [(60 cm tension weight oven dry weight) (g) / oven dry weight (g)] x BD

    Bulk density = mass of oven dry soil (g) / volume of bulk soil (cm3)

    Total porosity = volume of H2O in soil at saturation (cm3) / volume of bulk soil (cm

    3)

    Macroporosity = volume of H2O drained out at 60 cm tension (cm3) / volume of bulk soil (cm

    3)

    Microporosity = volume of H2O retained at 60 cm tension (cm3) / volume of bulk soil (cm

    3)

    Coefficient of linear extensibility (COLE) was determined using the method described by

    Schafer and Singer (1976), thus:

    COLE rod = (Lm Ld)/ Ld

    Where; Lm = length of moist soil

    Ld = length of dry soil.

  • 28

    The volumetric shrinkage (VS) was also calculated using the relationship stated below:

    VS = [(COLE + 1)3 1] x 100

    Aggregate size separation was performed on 100 g of 4.75 mm sieved soil by wet sieving

    air-dried soil through a series of sieves (Elliot 1986), after submerging the soil sample in water at

    room temperature on top of 4.75 mm sieve for 5 minutes. A series of four sieves were used to

    obtain five different water-stable aggregate (WSA) fractions as follows: > 2.00 mm, 1.00-2.00

    mm, 0.50-1.00 mm, 0.25-0.50 mm, and < 0.25 mm. Materials retained on each sieve (WSA) were

    oven dried at 40 oC to constant weight.

    The mass of aggregates > 0.25 mm were calculated by subtracting the sum of the oven-

    dried weights of materials retained on each sieve from the air-dried weight of the original sample.

    The proportion of each class to the total sample weight was computed thus:

    Wi = Mi / Mt

    Where Wi = proportion of the total sample weight occurring in the corresponding size fraction.

    Mi = weight of the oven-dry aggregates (uncorrected for sand) in the size class

    fractions after sieving.

    Mt = total weight of the initial material (100 g) before sieving.

    The mean weight diameter (MWD) of each WSA fraction was calculated from the equation:

    MWD = xi wi

    Where xi = mean diameter of each size fraction (mm)

    wi = proportion of the total sample weight occurring in the corresponding size fraction

    Geometric mean weight diameter (GMWD) was calculated according to the equation:

    GMWD = exp [( wi log xi )/ wi]

    Where wi = the weight of aggregates

    xi = average diameter

    wi = total weight of the sample

    3.2.2.2 Chemical analysis

    Soil pH was determined in distilled water and potassium chloride solution at ratios of

    1:2.5 soil/water suspension using pH meter (McLean, 1982).

  • 29

    Soil organic carbon content of the whole soil (< 2.00 mm) and that associated with the

    various water-stable aggregate (WSA) fractions (2.00-4.75 mm, 1.00-2.00 mm, 0.50-1.00 mm,

    0.25-0.50 mm, and < 0.25 mm) were quantified by Walkley and Black wet oxidation method as

    described by Nelson and Sommers (1982). The SOC in WSA was expressed in g C/kg (gram

    carbon per kilogram) of soil using the appropriate conversion factors. The SOM was computed

    by multiplying the % OC with the conventional Van Bernmeller factor of 1.724. The SOC pool

    content was calculated using the following equation (Lal et al., 1998).

    Mg C ha-1

    = [%C x corrected b x d x 10

    4 m

    2 ha

    -1] / 100

    Where Mg C ha-1

    = mega gram carbon per hectare (1 Mg = 106g)

    %C = percentage of C given by laboratory results

    b (Mg m

    -3) = soil bulk density (Megagram per cubic meter)

    d = depth in meters

    Total nitrogen content of the bulk soil (< 2.00 mm) and that associated with water-stable

    ggregate fractions (> 2.00, 1.00-2.00, 0.50-1.00, 0.25-0.50, and < 0.25 mm), were determined by

    the macro kjeldahl digestion method using CuSO4 and Na2SO4 catalyst mixture (Bremner and

    Mulvaney, 1982).

    Cation exchange capacity (CEC) was determined by the NH4OAC (ammonium acetate)

    pH 7 method (Thomas, 1982). Exchangeable bases were determined in 1N NH4OAC (ammonium

    acetate) at pH 7 leachate using the flame photometer for sodium and potassium while

    complexometric EDTA titration method was used for calcium and magnesium (Thomas, 1982).

    Exchangeable acidity was determined by titration method using 1N KCI extract as described by

    Mclean (1982).

    3.3 Statistical Analysis

    The experiment was designed as 4 x 2 x 3 factorial in completely randomized design

    (CRD). Factor A was location at four levels namely Awgu, Okigwe, Nsukka I, and Nsukka II.

    The levels of factor B (land use) were cultivated and uncultivated soils, while factor C (soil

    depth) comprised of three levels viz.: 0-10, 10-20 and 20-30 cm. The final analysis of variance

    (ANOVA) table is shown in Appendix 1. The data collected on the various parameters were

    analyzed using ANOVA. Separation of treatment means for statistical significance was

    performed by the F-LSD procedure according to Obi (1986) to determine land use and soil type

    with the highest potential of sequestering SOC. Correlation and regression analysis were

    performed among WSA, SOC, TSN, and soil properties to show their relationships.

  • 30

    CHAPTER FOUR

    RESULTS AND DISCUSSION

    4.1 Results

    4.1.1 Soil Characteristics

    The particle size distribution and textural classification of the soils are presented in Table

    3. The particle size fractions among the locations at 0-30 cm depth were significantly different at

    P = 0.05 in both land uses. Okigwe and Awgu soils contained the highest amount of clay and silt

    size particles respectively, while Nsukka I and II soils were more of sand particles than clay and

    silt. The particle sizes differed with soil depth, however, the effect due to land use was not

    statistically different. In both land uses, the textural classes varied with depth in each location

    and included clay, loam, clay loam, sandy loam and sandy clay loam.

    Table 4 presents the chemical properties of the soils. All the chemical properties differed

    significantly (P = 0.05) among the locations in both land uses. Generally, the pH values of the

    soils in KCl indicated that they are acid and decreased slightly with increase in soil depth. The

    mean value of soil pH in KCl at 0-30 cm depth ranged from 3.87 to 4.20 and 4.00 to 4.80 in

    cultivated and uncultivated land uses respectively. The OC content of the soils were generally

    low and concentrated mostly at 0-10 cm depth. The mean value at 0-30 cm depth among the

    locations ranged from 0.65 % to 1.47 % and 0.63 % to 1.74 % in cultivated and uncultivated land

    uses respectively, with Okigwe soil having the highest value while Nsukka I had the least value

    in both land uses. Available phosphorus (P) and total nitrogen (N) of the soils varied with depth

    and were generally low. The mean value at 0-30 cm depth ranged from trace to 0.93 cmol/kg (P)

    and 0.11-0.15 % (N) in cultivated soils, and trace to 1.73 cmol/kg (P) and 0.11 to 0.17 % (N) in

    uncultivated land use. Of all the exchangeable cations (Na+, K

    +, Ca

    2+, Mg

    2+, Al

    3+), Na

    + content

    was mostly the same within the various soil depths among the locations, and the mean value at 0-

    30 cm depth ranged from 0.10 to 0.19 cmol/kg among the locations in both land uses. The rest of

    the exchangeable cations varied with soil depth except Al3+

    which concentrated more at 20-30

    cm depth than at 0-10 cm depth. More so, among all the exchangeable cations, Al3+

    has the

    highest mean value at 0-30 cm depth in almost all the locations ranging from 2.27 to 10.53

    cmol/kg and from 2.40 to 7.80 cmol/kg in both cultivated and uncultivated soils, respectively.

    Following Al3+

    is Ca2+

    with mean ranging from 0.80 to 2.93 cmol/kg and from 0.60 to 10.40

    cmol/kg at 0-30 cm depth among the locations in cultivated and uncultivated soils, respectively.

    Exchangeable Mg2+

    mean ranged from 0.27 to 3.00 cmol/kg and from 0.27 to 4.47 cmol/kg at 0-

    30 cm depth among the locations in cultivated and uncultivated soils, respectively. Exchangeable

    K+ mean value ranged from 0.09 to 0.18 cmol/kg, and from 0.08 to 0.22 cmol/kg at 0-30 cm

    depth among the locations in cultivated and uncultivated land uses respectively. The CEC of the

  • 31

    Table 3: Particle size distribution and textural classification of the soils

    Depth Clay Silt Fine Coarse Total Textural

    Land use Location (cm) sand sand sand class --------------------------------- % ------------ ------------------

    Cultivated Awgu 0-10 17 49 13 21 34 L

    10-20 27 39 17 17 34 L

    20-30 31 23 27 19 46 SCL

    Mean 25 37 19 19 38

    Okigwe 0-10 27 23 24 27 50 SCL

    10-20 43 15 19 23 42 C

    20-30 47 9 20 25 44 C

    Mean 39 16 21 25 45 Nsukka

    I 0-10 16 9 27 49 76 SL

    10-20 22 7 27 44 72 SCL

    20-30 22 7 25 47 72 SCL

    Mean 20 8 26 47 73 Nsukka

    II 0-10 26 7 34 34 68 SCL

    10-20 26 9 33 32 66 SCL

    20-30 30 5 44 31 66 SCL

    Mean 27 7 37 32 67

    LSD (0.05) 5.35* 8.20* 4.86* 6.31* 9.05*

    Uncultivated Awgu 0-10 27 39 33 2 34 L

    10-20 31 25 38 6 44 CL

    20-30 27 29 35 9 44 L

    Mean 28 31 35 6 41

    Okigwe 0-10 17 25 29 29 58 SL

    10-20 31 25 20 24 44 CL

    20-30 43 13 20 24 44 C

    Mean 30 21 23 26 49

    Nsukka

    I 0-10 14 11 25 51 76 SL

    10-20 21 8 23 48 72 SCL

    20-30 25 10 31 35 66 SCL

    Mean 20 10 26 45 71

    Nsukka

    II 0-10 26 3 36 36 72 SCL

    10-20 28 7 29 37 66 SCL

    20-30 22 7 24 47 72 SCL

    Mean 25 6 30 40 70

    LSD (0.05) 4.31* 6.51* 3.58* 9.58* 8.53 *

  • 32

    Table 4: Chemical properties of the soils under cultivated land use

    Depth pH pH SOC SOM Total N Avail P Na+ K

    + Ca

    2+ Mg

    2+ Al

    3+ CEC C/N

    Land use Location (cm) H2O KCl ------------%-------------- ppm -------------------------cmol/kg-------------------------

    Cultivated

    Awgu

    0-10 5.70 4.30 0.88 1.52 0.11 1.19 0.19 0.15 3.00 2.20 8.80 13.60 8.00

    10-20 5.60 4.10 1.03 1.78 0.14 0.40 0.19 0.10 2.80 2.40 9.20 19.60 7.36

    20-30 5.50 4.10 0.96 1.66 0.11 1.19 0.19 0.15 3.00 3.60 13.60 27.60 8.73

    Mean 5.60 4.17 0.96 1.65 0.12 0.93 0.19 0.13 2.93 2.73 10.53 20.27 8.03

    Okigwe 0-10 5.40 4.30 1.80 3.10 0.17 trace 0.10 0.25 3.00 2.60 5.60 25.20 10.59

    10-20 5.20 4.20 1.57 2.71 0.15 trace 0.10 0.20 2.00 3.00 8.20 23.60 10.47

    20-30 5.20 4.10 1.03 1.78 0.14 trace 0.19 0.10 1.20 3.40 11.00 18.40 7.36

    Mean 5.27 4.20 1.47 2.53 0.15 trace 0.13 0.18 2.07 3.00 8.27 22.40 9.47

    Nsukka I

    0-10 4.70 3.80 0.83 1.43 0.14 2.79 0.10 0.14 1.00 0.20 4.80 19.60 5.93

    10-20 4.90 3.90 0.54 0.93 0.18 trace 0.10 0.08 0.60 0.40 3.60 21.20 3.00

    20-30 4.50 3.90 0.58 1.00 0.13 trace 0.10 0.06 0.80 0.20 4.80 25.60 4.46

    Mean 4.70 3.87 0.65 1.12 0.15 - 0.10 0.09 0.80 0.27 4.40 22.13 4.46

    Nsukka

    II

    0-10 5.20 4.20 1.41 2.43 0.11 trace 0.10 0.14 1.40 0.20 2.00 19.20 12.82

    10-20 5.40 4.10 1.41 2.43 0.13 trace 0.10 0.11 1.00 0.60 2.00 19.20 10.85

    20-30 5.50 4.10 0.93 1.60 0.10 trace 0.10 0.11 0.80 0.20 2.80 21.60 9.30

    Mean 5.37 4.13 1.25 2.15 0.11 trace 0.10 0.12 1.07 0.33 2.27 20.00 10.99 Grand mean 5.23 4.09 1.08 1.86 0.13 nil 0.13 0.13 1.72 1.58 6.37 21.20 8.24

    LSD (0.05) 0.21* 0.09* 0.22* 0.39* 0.01* nil 0.03* 0.03* 0.57* 0.81* 2.17* 2.21* 1.64*

  • 33

    Table 4 continuation: Chemical properties of the soils under uncultivated land use

    Depth pH pH SOC SOM Total N Avail P Na+ K

    + Ca

    2+ Mg

    2+ Al

    3+ CEC C/N

    Land use Location cm H2O KCl ------------%-------------- ppm ----------------------- -cmol/kg------------------------

    Uncultivated

    Awgu

    0-10 5.90 5.30 1.34 2.31 0.13 0.80 0.19 0.25 12.40 5.60 3.60 8.00 10.31

    10-20 5.60 4.70 0.84 1.45 0.11 trace 0.10 0.20 9.60 3.80 4.80 10.00 7.64

    20-30 5.50 4.40 0.96 1.66 0.10 trace 0.19 0.20 9.20 4.00 4.80 8.40 9.60

    Mean 5.67 4.80 1.05 1.81 0.11 - 0.16 0.22 10.40 4.47 4.40 8.80 9.18

    Okigwe 0-10 5.30 4.50 2.45 4.22 0.24 trace 0.19 0.15 2.80 3.20 4.80 8.40 10.21

    10-20 5.20 4.20 1.69 2.91 0.15 0.80 0.19 0.20 1.60 1.80 7.60 8.40 11.27

    20-30 5.10 4.10 1.07 1.85 0.11 trace 0.19 0.10 1.20 1.60 11.00 7.20 9.73

    Mean 5.20 4.27 1.74 2.99 0.17 - 0.19 0.15 1.87 2.20 7.80 8.00 10.40

    Nsukka I

    0-10 5.40 4.20 0.97 1.67 0.15 3.98 0.10 0.11 1.00 0.40 2.00 11.60 6.47

    10-20 5.00 4.00 0.54 0.93 0.08 0.40 0.10 0.08 0.60 0.60 2.40 11.20 6.75

    20-30 4.90 3.90 0.39 0.67 0.14 0.80 0.10 0.06 0.80 0.40 2.80 13.20 2.79

    Mean 5.10 4.03 0.63 1.09 0.12 1.73 0.10 0.08 0.80 0.47 2.40 12.00 5.34

    Nsukka

    II

    0-10 5.00 4.00 1.17 2.02 0.07 0.80 0.10 0.11 0.60 0.40 2.40 8.00 16.71

    10-20 4.90 4.00 0.93 1.60 0.17 2.79 0.10 0.06 0.60 0.20 3.20 8.00 5.47

    20-30 5.20 4.00 0.78 1.35 0.10 1.59 0.10 0.06 0.60 0.20 2.80 11.60 7.80

    Mean 5.03 4.00 0.96 1.66 0.11 1.73 0.10 0.08 0.60 0.27 2.80 9.20 9.99 Grand mean 5.25 4.28 1.09 1.89 0.13 nil 0.14 0.13 3.43 1.83 4.35 9.50 8.73

    LSD (0.05) 0.18* 0.23* 0.32* 0.54* 0.03* nil 0.03* 0.04* 2.52* 1.05* 1.51* 1.12* 2.01*

    Grand LSD (0.05) NS 0.13* NS NS NS nil NS NS 1.31* NS 1.36* 2.72* NS

  • 34

    soils varied with depth and was between 13.60 and 27.60 cmol/kg in cultivated soils with

    mean ranging from 20.00 to 22.40 cmol/kg at 0-30 cm depth among the locations. In uncultivated

    soils, the CEC values at the various soil depths were between 7.20 and 13.20 cmol/kg with mean

    ranging from 8.00 to 12.00 cmol/kg at 0-30 cm depth among the locations. The C/N ratios of the

    soils also varied with depth. The mean value at 0-30 cm depth ranged from 4.46 to 10.99 and

    5.34 to 10.40 among the locations in cultivated and uncultivated land uses respectively, with

    Nsukka I having the least value in both land uses. Land use had no significant effect on all the

    chemical properties considered except on pH in KCl, Ca2+

    , Al3+

    , and CEC. Generally, the soils of

    the four locations were low in basic cations and soil available nutrients. They suffer severe

    leaching and interrill erosion as a result of high rainfall intensity (Igwe, 2005).

    4.1.2 Microaggregate Stability Indices of the Soils

    Microaggregate stability indices, including water content at field capacity of the soils are

    presented in Table 5. All the parameters at 0-30 cm depth varied significantly (P = 0.05) among

    the four locations in each of the land uses, expect that in cultivated soils, clay dispersion index

    (CDI) and clay flocculation index (CFI) were not significantly affected by location. The mean

    value of CDI at 0-30 cm depth among the locations ranged from 42.75 to 46.28 in cultivated and

    from 30.90 to 53.72 in uncultivated land use. Dispersion ratio (DR) mean value at 0-30 cm depth

    among the locations ranged from 0.56 to 0.78 and 0.61 to 0.81 in cultivated and uncultivated land

    uses, respectively. Aggregated silt and clay (ASC) mean value at 0-30 cm depth ranged from

    7.28 to 24.00 and 7.28 to 20.00 among the locations in cultivated and uncultivated soils,

    respectively, with Okigwe soil having the highest value while Nsukka I had the least in both land

    uses. The uncultivated soils had moderate CFI values with the mean range of 46.28 to 69.11.

    Water dispersible clay (WDC) significantly (P = 0.05) gave higher value of 15.15 for Okigwe

    soil at 0-30 cm depth in both land uses. This could be attributed to the higher value of clay

    content in Okigwe soil when compared with other locations. Water dispersible silt (WDSi)

    content among the locations at 0-30 cm depth mean value ranged from 11.23 to 36.61 in

    cultivated soils and 11.89 to 35.95 in uncultivated soils, with Awgu soil having the highest value

    while Nsukka I soil had the least value in each of the land uses. It thus follows this trend; Awgu

    > Okigwe > Nsukka II > Nsukka I. Water content at field capacity of the soils ranged between 28

    and 39 % among the locations in cultivated soils, and from 26 to 36 % in uncultivated soils with

    Nsukka I having the least value under both land uses. Land use showed no significant difference

    among all the parameters considered in Table 5.

    Table 6 presents soil moisture, bulk density, porosity, COLE and VS of the soils. All the

    parameters measured showed significant differences (P = 0.05) among the locations at 0-30 cm

  • 35

    Table 5: Microaggregate stability indices and water content at field capacity (FC) of the

    soils

    Land use Location Depth CDI DR ASC CFI WDC WDSi FC

    (cm)

    Cultivated Awgu 0-10 51.46 0.76 16.00 48.54 8.48 41.28 0.27 10-20 32.02 0.76 16.00 67.98 8.48 41.28 0.36 20-30 47.51 0.78 12.00 52.49 14.48 27.28 0.37 Mean 43.66 0.77 14.67 56.34 10.48 36.61 0.33 Okigwe 0-10 77.34 0.60 20.00 22.66 20.48 9.28 0.51 10-20 29.38 0.52 28.00 70.62 12.48 17.28 0.35 20-30 26.85 0.57 24.00 73.15 12.48 19.28 0.30 Mean 44.52 0.56 24.00 55.48 15.15 15.28 0.39

    Nsukka I 0-10 41.12 0.70 7.28 58.88 6.48 10.56 0.30

    10-20 48.16 0.74 7.28 51.84 10.48 10.56 0.28 20-30 38.97 0.74 7.28 61.03 8.48 12.56 0.25 Mean 42.75 0.73 7.28 57.25 8.48 11.23 0.28

    Nsukka II 0-10 40.68 0.77 7.28 59.32 10.48 14.56 0.34

    10-20 56.21 0.73 9.28 43.79 14.48 10.56 0.41 20-30 41.94 0.85 5.28 58.07 12.48 16.56 0.32 Mean 46.28 0.78 7.28 53.73 12.48 13.89 0.36 Grand mean 44.30 0.71 13.31 55.70 11.65 19.25 0.34 LSD (0.05) NS 0.06* 4.33* NS 2.17* 6.60* 0.04*

    Uncultiv-

    ated Awgu 0-10 54.68 0.79 14.00 45.32 14.48 37.28 0.39 10-20 34.38 0.82 10.00 65.62 10.48 35.28 0.35 20-30 39.58 0.82 10.00 60.42 10.48 35.28 0.33 Mean 42.88 0.81 11.33 57.12 11.81 35.95 0.36 Okigwe 0-10 63.59 0.62 16.00 36.41 10.48 15.28 0.41 10-20 54.07 0.64 20.00 45.93 16.48 19.28 0.23 20-30 43.50 0.57 24.00 56.50 18.48 13.28 0.25 Mean 53.72 0.61 20.00 46.28 15.15 15.95 0.30

    Nsukka I 0-10 47.09 0.70 7.28 52.91 6.48 10.56 0.33

    10-20 48.16 0.81 5.28 51.84 10.48 12.56 0.25 20-30 48.45 0.73 9.28 51.55 12.48 12.56 0.20 Mean 47.90 0.75 7.28 52.10 9.81 11.89 0.26

    Nsukka II 0-10 25.16 0.74 7.28 74.85 6.48 14.56 0.35

    10-20 37.75 0.67 11.28 62.25 10.48 12.56 0.32 20-30 29.78 0.60 11.28 70.22 6.48 10.56 0.36 Mean 30.90 0.67 9.95 69.11 7.81 12.56 0.34 Grand mean 43.85 0.17 12.14 56.15 11.15 19.09 0.31 LSD (0.05) 6.41* 0.05* 3.18* 6.41* 2.22* 6.02* 0.04* Grand LSD (0.05) NS NS NS NS NS NS NS

    CDI = clay dispersion index; DR = dispersion ratio; ASC = aggregate silt and clay; CFI = Clay

    flocculation index; WDC = water dispersible clay; WDSi = water dispersible silt.

  • 36

    Table 6: Soil moisture, bulk density, porosity, coefficient of linear extensibility (COLE)

    and volumetric shrinkage (VS) of the soils

    Soil Bulk Macro Micro Total

    Depth moisture density porosity porosity porosity COLE VS

    Land use Location (cm) (%) (Mg m-3

    ) ------------ % ------------- (%)

    Cultivated Awgu 0-10 22.32 1.56 4.31 41.50 45.81 0.02 6.12 10-20 26.66 1.35 4.66 48.70 53.35 0.09 29.50 20-30 35.22 1.36 4.03 49.67 53.69 0.10 33.10 Mean 28.07 1.42 4.33 46.62 50.95 0.07 22.91 Okigwe 0-10 13.98 1.42 8.97 72.57 81.54 0.04 12.49 10-20 19.74 1.61 10.42 56.41 66.81 0.11 36.76 20-30 27.39 1.61 11.00 48.28 59.28 0.11 36.76

    Mean 20.37 1.55 10.13 59.09 69.21 0.09 28.67

    Nsukka I 0-10 4.25 1.36 6.65 41.30 47.95 0.01 3.03

    10-20 8.53 1.52 6.91 42.08 48.99 0.02 6.12 20-30 10.49 1.56 5.90 39.15 45.05 0.04 12.49 Mean 7.76 1.48 6.49 40.84 47.33 0.02 7.21 Nsukka

    II 0-10 11.59 1.41 7.95 47.38 55.33 0.03 9.27

    10-20 13.72 1.29 7.91 52.14 60.06 0.02 6.12 20-30 9.82 1.45 9.64 46.33 55.97 0.02 6.12 Mean 11.71 1.38 8.50 48.62 57.12 0.02 7.17 Grand mean 16.98 1.46 7.36 48.79 56.15 0.05 16.49

    LSD (0.05) 5.36* 0.06* 1.37* 5.20* 5.90* 0.02* 7.71*

    Uncultivat-

    ed Awgu 0-10 32.89 1.32 8.76 50.99 59.75 0.04 12.49 10-20 29.97 1.41 6.18 48.64 54.82 0.05 15.74 20-30 31.32 1.32 6.75 43.10 49.86 0.09 29.50 Mean 31.39 1.35 7.23 47.58 54.81 0.06 19.24 Okigwe 0-10 27.60 1.26 6.56 51.01 57.57 0.03 9.27 10-20 20.75 1.81 5.64 42.26 47.90 0.08 25.97 20-30 20.18 1.78 11.32 44.82 56.14 0.06 19.10 Mean 22.84 1.62 7.84 46.03 53.87 0.06 18.11

    Nsukka I 0-10 6.94 1.42 7.07 46.59 53.66 0.00 0.00

    10-20 8.45 1.58 7.25 39.75 47.00 0.01 3.03 20-30 9.64 1.63 1.62 32.93 34.54 0.02 6.12 Mean 8.34 1.54 5.31 39.76 45.07 0.01 3.05 Nsukka

    II 0-10 10.48 1.37 6.75 48.25 55.00 0.02 6.12

    10-20 10.27 1.45 6.60 46.85 53.46 0.03 9.27 20-30 12.44 1.32 6.40 47.19 53.58 0.02 6.12 Mean 11.06 1.38 6.58 47.43 54.01 0.02 7.17 Grand mean 18.41 1.47 6.74 45.20 51.94 0.04 11.89

    LSD (0.05) 5.71* 0.11* 1.27* 2.97* 3.83* 0.02* 5.25*

    Grand LSD (0.05) NS NS NS NS NS NS NS

  • 37

    depth of both land uses. Soil moisture content ranged from 7.76 to 28.07 % and 8.34 to 31.39

    % in cultivated and uncultivated soils, respectively. The trend was similar to that of WDSi

    content in both land uses. This could be attributed to the high proportion of total sand relative to

    clay and silt size fractions in the particle size distribution. In cultivated land use, soil moisture

    content increased with depth except in Nsukka II, which can be attributed to the topography of

    the location (hilly) and the inclination of the direction of sunlight and rainfall. Bulk density of the

    soils varied with depth. The mean ranged from 1.38 to 1.55 Mg m-3

    and from 1.35 to 1.62 Mg m-3

    at 0-30 cm depth in cultivated and uncultivated soils, respectively. In both land uses,

    microporosity was higher than macroporosity with a mean range of 40.84 to 59.09 % and 39.76

    to 47.58 % at 0-30 cm depth in cultivated and uncultivated land uses respectively, as compared to

    macroporosity with mean ranging from 4.33 to 10.13 % in cultivated soils, and from 5.31 to 7.84

    % in uncultivated soils. Unlike soil moisture and bulk density mean values, total porosity of the

    cultivated soils was generally higher when compared with the uncultivated soils. This may be

    attributed to the effect of cultivation on the physical properties of the soil as these relate to soil

    structure and aggregation (Six et al., 1999). Generally, COLE values increased with soil depth

    except in Nsukka II, where uniformity in the texture of the soil (Table 3) could be responsible for

    the deviation from the above mentioned trend. The COLE mean values at 0-30 cm depth among

    the locations ranged from 0.02 to 0.09 in cultivated soils and from 0.01 to 0.06 in uncultivated

    soils. According to Schaffer and Singer (1976) outlined categories of COLE and their

    corresponding shrink-swell harzard ratings, the soils of Nsukka I and II fell within the slight

    shrink-swell category, whereas the Awgu and Okigwe fell within the severe shrink-swell harzard

    category. The VS which reflects the absolute values of COLE followed the same trend as COLE.

    In general, land use had no significant effect on any of the soil properties considered in Table 6.

    4.1.3 Water-Stable Aggregates (WSA), Mean Weight Diameter (MWD) and Geometric

    Mean Weight Diameter (GMWD) of the Soils

    Data on WSA, MWD and GMWD of the soils are presented in Table 7. Generally, the

    various aggregate sizes varied greatly across the various soil depths. Pooled over the three

    depths, location significantly (P = 0.05) influenced the various aggregate size fractions in both

    land uses. In Okigwe and Nsukka II soils had > 50 % of WSA > 2.00 mm at 0-30 cm depth. The

    reverse was the case with Awgu and Nsukka I soils, which had about 7 to 21 % and 19 to 49 %

    WSA of > 2.00 mm respectively. In Awgu location, the mean under cultivated land use were

    16.80 and 27.12 % for WSA of 0.50-1.00 mm and < 0.25 mm respectively. Okigwe recorded

    means of 7.44 and 53.04 % for 0.25-0.50 mm and > 2.00 mm WSA. Means of 10.65 and 30.67 %

    were obtained for 0.25-0.50 mm and 1.00-2.00 mm WSA in Nsukka I. For Nsukka II, the highest

  • 38

    Table 7: Water-stable aggregates (WSA), mean weight diameter (MWD) and geometric

    mean weight diameter (GMWD) of the soils

    Depth ------------------WSA (mm) ---------------- MWD GMWD

    Land use Location (cm) A B C D E (mm) (mm)

    -------------------------%---------------------

    Cultivated Awgu 0-10 46.15 13.47 10.34 10.22 19.82 1.90 0.82 10-20 8.78 12.88 16.57 24.55 37.22 0.76 0.67 20-30 7.19 27.68 23.50 17.32 24.33 0.93 0.78 Mean 20.71 18.01 16.80 17.36 27.12 1.20 0.76 Okigwe 0-10 65.69 13.22 6.47 5.18 9.44 2.51 0.92 10-20 52.70 19.86 11.21 5.56 10.67 2.21 0.90 20-30 40.73 19.96 17.60 11.59 10.12 1.88 0.91 Mean 53.04 17.68 11.76 7.44 10.08 2.20 0.91 Nsukka

    I 0-10 19.67 30.52 21.83 11.41 16.57 1.37 0.85 10-20 18.80 32.20 26.17 10.39 12.44 1.37 0.89 20-30 18.21 29.28 29.06 10.15 13.30 1.31 0.88 Mean 18.89 30.67 25.69 10.65 14.10 1.35 0.87 Nsukka

    II 0-10 73.86 6.80 5.43 4.02 9.89 2.67 0.91 10-20 83.37 6.56 2.18 1.53 6.36 3.00 0.95 20-30 80.52 5.35 4.51 3.92 5.70 2.87 0.95 Mean 79.25 6.24 4.04 3.16 7.32 2.85 0.94

    LSD (0.05) 16.34* 5.70* 5.29* 3.72* 5.15* 0.44* 0.05*

    Uncultivat-

    ed Awgu 0-10 3.01 7.79 11.87 19.57 56.76 0.46 0.49 10-20 2.84 22.36 21.50 11.50 41.80 0.69 0.62 20-30 14.47 21.03 20.53 14.86 29.11 1.07 0.74 Mean 6.77 17.06 17.97 15.31 42.56 0.74 0.62 Okigwe 0-10 74.40 9.96 5.23 2.61 7.80 2.71 0.93 10-20 67.35 11.19 5.85 2.95 12.66 2.50 0.88 20-30 56.47 16.32 12.78 5.35 9.08 2.29 0.92 Mean 66.07 12.49 7.95 3.64 9.85 2.50 0.91 Nsukka

    I 0-10 84.11 4.46 3.65 3.41 4.37 2.96 0.96 10-20 32.89 25.26 22.56 12.59 6.70 1.72 0.94 20-30 31.15 37.58 17.21 7.67 6.39 1.78 0.95 Mean 49.38 22.43 14.47 7.89 5.82 2.15 0.95 Nsukka

    II 0-10 89.16 4.96 1.20 0.92 3.76 3.13 0.96 10-20 73.46 13.12 5.42 2.24 5.76 2.81 0.95 20-30 73.92 9.10 4.78 3.37 8.83 2.69 0.92 Mean 78.85 9.06 3.80 2.18 6.12 2.88 0.94

    LSD (0.05) 18.34* 5.64* 4.46* 3.46* 9.92* 0.53* 0.09*

    A, B, C, D and E represent > 2.00, 1.00-2.00, 0.50-1.00, 0.25-0.50 and < 0.25 respectively.

  • 39

    mean was 79.25 % for > 2.00 mm WSA while the lowest was 3.16 % for 0.25-0.50 mm WSA.

    For uncultivated land use, Awgu location had the highest and lowest mean of 6.77 and 42.56 %

    for aggregate sizes of > 2.00 mm and < 0.25 mm, respectively. Okigwe followed the same trend

    as that obtained in cultivated land use with mean values of 3.64 and 6.77 %. The lowest and

    highest mean values of 5.82 and 49.38 % for WSA of < 0.25 mm and > 2.00 mm sizes were

    obtained in Nsukka I. Nsukka II had a similar trend as that obtained in cultivated soils with the

    lowest and highest mean values of 2.18 and 78.85 %. Both MWD and GMWD of the soils varied

    with soil depth in both land uses. The MWD mean values at 0-30 cm depth ranged from 1.20 to

    2.85 mm and 0.74 to 2.88 mm for cultivated and uncultivated soils, respectively. The GMWD,

    which represents the absolute values of MWD, had mean values at 0-30 cm depth ranging from

    0.76 to 0.94 mm and 0.62 to 0.95 mm in cultivated and uncultivated land uses respectively.

    4.1.4 Soil Carbon and Nitrogen Content, and C/N ratio in the Water-Stable Aggregates

    The distribution of C and N, as well as C/N ratios of the aggregate size fractions are

    presented in Table 8. In all the parameters determined, effect due to location differed

    significantly (P = 0.05) at 0-30 cm depth in both land uses. Generally, C and N contents varied

    among the WSA sizes and across the soil depths. However, in Nsukka I, C content decreased

    with soil depth in each of the land uses, except in 0.50-1.00 mm and 0.25-0.50 mm WSA of

    uncultivated land use. The same trend applied to N content for the same location except in the

    0.50-1.00 mm WSA of the cultivated soils, and in the 1.00-2.00 mm and < 0.25 mm WSA of the

    uncultivated soils. With a few exceptions, C accumulated more in the topmost 0-10 cm depth

    than the subsurface soil in both land uses. More so, the C and N contents of the soils were

    generally higher in uncultivated as expected than in cultivated soils. However, the reverse was

    the case with Nsukka I and II. In cultivated land use, C and N content did not vary substantially

    in the various WSA sizes. Nevertheless, smaller aggregates (< 1.00 mm) contained more N than

    the larger aggregates (> 1.00 mm). Highest amount of C at 0-30 cm depth was recorded in the

    0.50-1.00 mm WSA for Awgu and Okigwe soils, and in the > 2.00 mm WSA for Nsukka I and II

    soils. Hence, it is possible to infer that the underlying parent material of the soil could be

    responsible for the differences obtained in the preferential accumulation of C in the WSA classes.

    Also in cultivated land use at 0-30 cm soil depth, the < 0.25 WSA generally accumulated more N

    than the rest of the aggregate fractions except in Nsukka II, where the topography of the site

    could be responsible for the deviation. In all, the lowest N content was observed in 1.00-2.00 mm

    and > 2.00 mm WSA. This supports other findings suggesting that N is sequestered in the

    microaggregates of the soil where they are protected by a mechanism of incorporation into the

  • 40

    Table 8: Carbon and nitrogen content (g kg-1

    ), and C/N ratios in the various water-stable aggregate (WSA) fractions of cultivated soils

    ----------------------------------------------------WSA (mm) ---------------------------------------------------

    > 2.00 1.00-2.00 0.50-1.00 0.25-0.50 < 0.25

    ----------------------- -------------------- ---------------------- ---------------------- ---------------------

    Land use Location Depth C N C/N C N C/N C N C/N C N C/N C N C/N

    Cultivated

    Awgu 0-10 7.40 1.30 5.69 5.00 1.50 3.33 8.10 1.70 4.77 6.70 1.70 3.94 6.00 3.40 1.77

    10-20 7.70 1.50 5.13 5.70 1.10 5.18 6.70 1.80 3.72 7.40 1.70 4.35 7.00 2.00 3.50

    20-30 5.70 1.70 3.35 7.30 1.80 4.06 7.40 1.50 4.93 7.60 2.00 3.80 6.30 3.40 1.85

    Mean 6.93 1.50 4.72 6.00 1.47 4.19 7.40 1.67 4.47 7.23 1.80 4.03 6.43 2.93 2.37

    Okigwe 0-10 15.30 1.80 8.50 12.80 1.40 9.14 13.90 2.20 6.32 12.50 3.10 4.03 11.70 3.40 3.44

    10-20 8.70 2.00 4.35 11.70 2.90 4.04 10.70 2.40 4.46 10.8 3.10 3.48 8.70 3.10 2.81

    20-30 5.90 1.50 3.93 7.60 2.00 3.80 9.00 2.40 3.75 9.40 2.10 4.48 7.60 2.50 3.04

    Mean 9.97 1.77 5.59 10.70 2.10 5.66 11.20 2.33 4.84 10.90 2.77 4.00 9.33 3.00 3.10

    Nsukka I 0-10 11.40 2.00 5.7 8.00 1.50 5.33 6.40 1.40 4.57 6.00 1.50 4.00 9.10 2.80 3.25

    10-20 5.70 1.40 4.07 6.40 1.30 4.92 5.70 2.10 2.71 4.60 1.40 3.29 7.60 2.50 3.04

    20-30 5.70 1.40 4.07 4.00 1.10 3.64 5.30 1.10 4.82 4.60 1.30 3.54 4.60 1.70 2.71

    Mean 7.60 1.60 4.75 6.13 1.30 4.63 5.80 1.53 4.03 5.07 1.40 3.61 7.10 2.33 3.00

    Nsukka II 0-10 12.90 1.50 8.60 11.00 1.50 7.33 10.40 2.50 4.16 10.60 2.80 3.79 11.00 2.50 4.40

    10-20 14.80 1.70 8.71 11.40 1.70 6.71 11.40 1.40 8.14 11.70 2.80 4.18 12.50 1.10 11.36

    20-30 10.30 1.30 7.92 7.60 1.10 6.91 8.40 2.50 3.36 9.90 2.50 3.96 9.90 2.80 3.54

    Mean 12.67 1.50 8.47 10.00 1.43 6.98 10.07 2.13 5.22 10.73 2.70 3.98 11.13 2.13 6.43

    LSD (0.05) 2.07* 0.14* 1.18* 1.66* 0.29* 1.04* 1.49* 0.29* 0.83* 1.56* 0.39* 0.20* 1.40* 0.42* 1.45*

  • 41

    Table 8 continuation: Carbon and nitrogen content (g kg-1

    ), and C/N ratios in the various water-stable aggregate (WSA) fractions of

    uncultivated soils

    ---------------------------------------------------WSA (mm) --------------------------------------------------

    > 2.00 1.00-2.00 0.50-1.00 0.25-0.50 < 0.25

    ---------------------- -------------------- ---------------------- ---------------------- ---------------------

    Land use Location Depth C N C/N C N C/N C N C/N C N C/N C N C/N

    Uncultivated Awgu 0-10 20.10 2.00 10.50 18.40 2.80 6.57 15.40 1.50 10.27 9.70 1.80 5.39 5.60 2.50 2.24

    10-20 14.40 2.00 7.20 9.70 2.10 4.62 8.10 1.70 4.77 6.40 1.80 3.56 4.40 2.20 2.00

    20-30 9.40 3.10 3.03 9.40 1.40 6.71 6.40 2.00 3.20 5.60 1.70 3.29 3.80 1.40 2.71

    Mean 14.63 2.37 6.91 12.50 2.10 5.97 9.97 1.73 6.08 7.23 1.77 4.08 4.60 2.03 2.32

    Okigwe 0-10 17.40 2.10 8.29 17.40 2.00 8.70 17.00 3.60 4.72 14.10 3.40 4.15 8.70 3.60 2.42

    10-20 12.20 1.70 7.18 10.40 2.10 4.95 11.80 3.10 3.81 9.50 3.40 2.79 8.10 4.80 1.69

    20-30 8.70 1.10 7.91 8.30 1.80 4.61 8.70 2.00 4.35 8.10 3.10 2.61 5.70 3.10 1.84

    Mean 12.77 1.63 7.79 12.03 1.97 6.09 12.50 2.90 4.29 10.57 3.30 3.18 7.50 3.83 1.98

    Nsukka I 0-10 9.10 1.50 6.07 9.90 2.80 3.54 5.70 2.80 2.04 5.70 1.40 4.07 8.70 1.80 4.83

    10-20 9.10 1.40 6.50 6.40 0.60 10.67 4.70 2.10 2.24 3.70 1.10 3.36 7.20 1.10 6.55

    20-30 3.70 1.10 3.36 5.70 1.40 4.07 5.30 1.40 3.79 4.20 1.10 3.82 5.00 2.80 1.79

    Mean 7.30 1.33 5.31 7.33 1.60 6.09 5.23 2.10 2.69 4.53 1.20 3.75 6.97 1.90 4.39

    Nsukka II 0-10 8.70 1.50 5.80 11.00 2.20 5.00 9.50 2.20 4.32 9.90 1.10 9.00 10.60 3.10 3.42

    10-20 8.40 1.40 6.00 6.70 1.40 4.79 6.80 2.20 3.09 8.00 3.60 2.22 10.60 3.10 3.42

    20-30 8.70 1.50 5.80 8.40 1.30 6.46 5.30 1.40 3.79 6.80 2.20 3.09 8.40 2.80 3.00

    Mean 8.60 1.47 5.87 8.70 1.63 5.42 7.20 1.93 3.73 8.23 2.30 4.77 9.87 3.00 3.28

    LSD (0.05) 2.59* 0.32* 1.17* 2.30* 0.37* 1.20* 2.35* 0.40* 1.22* 1.67* 0.56* 1.04* 1.34* 0.58* 0.83*

  • 42

    hydrophobic domains present in the finest fraction (Piccolo and Mbagwu, 1999). In uncultivated

    soils, the highest amount of C was observed in > 1.00 mm WSA for Awgu location at 0-30 cm

    depth, while Nsukka II recorded the highest amount of C in < 0.25 mm WSA. The < 1.00 mm

    WSA generally contained less C in all the locations under uncultivated land use. With the

    exception of Awgu location, the < 1.00 mm WSA at 0-30 cm depth accumulated more N than the

    > 1.00 mm WSA. In both land uses, the C/N ratios of the soils were generally low which explains

    why there was no substantial different between C and N content of the cultivated and

    uncultivated soils. The low C/N ratios may have resulted from rapid decomposition and

    mineralization of SOM.

    4.1.5 Soil Organic Carbon Pool

    Table 9 presents SOC pool of cultivated and uncultivated soils at the various soil depths.

    Soil depth significantly (P = 0.05) influenced SOC pool under both land uses. The mean value of

    SOC pool at 0-30 cm depth ranged from 9.52 to 22.47 Mg ha -1

    and 9.55 to 26.84 Mg ha -1

    in

    cultivated and uncultivated soils, respectively. In both land uses, SOC pool decreased with depth,

    thus 0-10 cm depth sequestered more SOC and the least being 20-30 cm depth. This implies

    higher accumulation of organic matter at the topmost surface layer of the soil (0-10 cm) than the

    rest of the soil depths. This is graphically represented in Fig. 2. Effects of land use on SOC pool

    at 0-30 cm depth showed no significant differences. Similarly, depth and land use interaction

    showed no significant effect on SOC pool. Location significantly (P = 0.05) influenced SOC pool

    at 0-30 cm depth in both land uses. Considering the sum of SOC at 0-30 cm depth of the

    locations, the result showed that Okigwe sequestered the highest C of 67.42 Mg ha-1

    , followed by

    Nsukka II (51.56 Mg ha-1

    ), Awgu (40.70 Mg ha-1

    ) and the least was Nsukka I (28.55 Mg ha-1

    )

    under cultivated land use. In uncultivated land use, SOC pool at 0-30 cm depth was highest in

    Okigwe (80.51 Mg ha-1

    ), followed by Awgu (49.20 Mg ha-1

    ), Nsukka II (39.82 Mg ha-1

    ) in the

    uncultivated soils as against Okigwe > Nsukka II > Awgu > Nsukka I in the cultivated soils.

    There were no significant differences in SOC pool content between the two land uses at 0-30 cm

    depth. However, land use significantly affected SOC pool content of Awgu and Nsukka II

    locations, and had no significant influence on Okigwe and Nsukka I locations.

    4.1.6 Land Use Effect on Aggregate Size Distribution, C, and N Contents of the Soils

    Table 10 presents the effects of land use on water-stable aggregate (WSA) concentration

    at the various soil depths, and at 0-30 cm depth. Generally, there was no significant effect on the

    concentration of WSA between the two land uses, except at 0-10 cm and 0-30 cm depths, where

  • 43

    Table 9: Soil organic carbon (SOC) pool of cultivated and uncultivated soils at the various

    soil depths

    -----------Depth (cm) ------------

    Land use Location 0-10 10-20 20-30 Total Mean

    --------------------------- (Mg ha-1

    ) ----------------------------

    Cultivated Awgu 13.73 13.91 13.06 40.70 13.57

    Okigwe 25.56 25.28 16.58 67.42

    22.47

    Nsukka I 11.29 8.21 9.05 28.55

    9.52

    Nsukka II 19.88 18.19 13.49 51.56

    17.19

    Mean 17.62 16.40 13.05 15.69

    Uncultivated Awgu 17.69 18.84 12.67 49.20 16.40

    Okigwe 30.87 30.59 19.05 80.51

    26.84

    Nsukka I 13.77 8.53 6.35 28.65

    9.55

    Nsukka II 16.03 13.49 10.03 39.82

    13.27

    Mean 19.59 17.86 12.03 16.52

    LSD (0.05) Location effect (cultivated) = 3.27*

    LSD (0.05) Location effect (uncultivated) = 4.46*

    LSD (0.05) Depth effect (cultivated) = 3.27*

    LSD (0.05) Depth effect (uncultivated) = 4.46*

    LSD (0.05) Land use effect at 0-30 cm depth = NS

    LSD (0.05) Land use effects at various depths = NS (at all depths)

    LSD (0.05) Land use effects across locations at 0-30 cm depth: Awgu = 2.13*

    Okigwe = NS

    Nsukka I = NS

    Nsukka II = 2.86*

    * = Significant at 5% alpha level; NS = Not significant at 5% alpha level.

  • 44

    0

    5

    10

    15

    20

    25

    0-10 0-20 20-30

    Soil depth (cm)

    SO

    C p

    oo

    l (M

    g/h

    a)

    Cultivated

    Uncultivated

    Fig. 2: Soil depth effects on soil organic carbon (SOC) pool of cultivated and uncultivated

    soils

  • 45

    only 0.50-1.00 mm WSA were significantly (P = 0.05) not affected by cultivation. Land use

    effects on SOC concentration in WSA at the various soil depths is shown in Table 11. At the

    topmost 0-10 cm depth, land use significantly (P = 0.05) influenced C concentration in 1.00-2.00

    mm WSA while at 20-30 cm depth, significant difference (P = 0.05) existed only in 1.00-2.00

    mm and 0.25-0.50 mm WSA. Land use effects on total nitrogen content in WSA at the various

    soil depths is given in Table 12. The WSA fractions of 1.00-2.00 and 0.50-1.00 mm at 0-10 cm

    depth were significantly reduced by cultivation. The influence of land use on SOC content of the

    various WSA at 0-30 cm depth (Table 13) showed that, only 0.50-1.00 mm and < 0.25 mm WSA

    fractions were significantly influenced by land use. Whereas cultivation reduced SOC associated

    with 1.00-2.00 mm WSA, the reverse was the case with < 0.25 mm WSA. Land use significantly

    affected TSN associated with 1.00-2.00 mm WSA at 0-30 cm depth (Table 14). Notably, also is

    the increasing trend of N with decrease in the WSA classes. As such, N accumulated more in the

    microaggregates than in macroaggregates. The C/N ratios associated with WSA at 0-30 cm depth

    are low and decreased with decrease in WSA classes. Land use had no significant effect on the

    C/N ratio of the WSA at 0-30 cm depth (Table 15). Result of the effect of land use on BD, > 2.00

    mm WSA, MWD and GMWD of the soils at the various soil depths in Table 16 showed no

    significant influence on the parameters considered

    4.1.7 Soil Texture in Relation to Soil Organic Carbon Pool

    Since SOC sequestration process is soil specific (Lal et al., 2007), the sampling depths

    relative to soil textures across the locations were also considered (Table 17). Significant

    difference (P = 0.05) existed among the five soil textures (clay, loam, sandy loam, clay loam, and

    sandy clay loam). Clay loam sequestered the highest amount of SOC, while sandy clay loam soil

    had the lowest amount. In fact, the trend follows thus; clay loam > clay > sandy loam > loam >

    sandy clay loam. The differences in SOC pool due to soil texture may be related to the clay

    content of the soil relative to silt and sand particle sizes (Table 4). Effects of soil texture on SOC

    pool at different sampling depths indicated that SOC pool was most secluded at 10-20 cm depth,

    followed by 0-10 cm and the least was at 20-30 cm depth as shown graphically in Fig. 3.

    4.1.8 Relationship between Aggregate Stability Indices and Soil Properties

    Correlation coefficients (r) among the various soil particle sizes, aggregate indices and

    some selected properties of the soil under cultivated and uncultivated land uses are presented in

    Table 18. In cultivated soils, there was no significant correlation between BD and any of the

    variables considered. The properties that correlated significantly (P 0.05) with MWD included

  • 46

    Table 10: Land use effects on water-stable aggregate (WSA) concentration at the various

    soil depths, and at 0-30 cm depth

    ------------------------------WSA (mm) ----------------------------

    Depth/land use > 2.00 1.00-2.00 0.50-1.00 0.25-0.50 < 0.25

    ------------------------------------%----------------------------------

    (A) 0-10 cm depth

    Cultivated 51.34 16.00 11.02 7.71 13.93

    Uncultivated 62.67 6.79 5.49 6.63 18.17

    LSD (0.05)

    NS NS 4.58* NS NS

    (B) 10-20 cm depth

    Cultivated 40.91 17.88 14.03 10.51 16.67

    Uncultivated 44.14 17.98 13.83 7.32 16.73

    LSD (0.05)

    NS NS NS NS NS

    (C) 20-30 cm depth

    Cultivated 36.66 20.57 18.67 10.75 13.36

    Uncultivated 44.00 21.01 13.83 7.81 13.35

    LSD (0.05) NS NS NS NS NS

    (D) 0-30 cm depth

    Cultivated 42.97 18.15 14.57 9.65 14.66

    Uncultivated 50.27 15.26 11.05 7.25 16.09

    LSD (0.05) NS NS 3.46* NS NS

  • 47

    Table 11: Land use effects on soil organic carbon (SOC) content in water-stable aggregates

    (WSA) at the various soil depths

    (mm) -------------------------WSA (mm) ------------------------

    Depth/land use whole SOC > 2.00 1.00-2.00 0.50-1.00 0.25-0.50 < 0.25

    ------------------------------------g kg-1

    -------------------------------------

    (A) 0-10 cm depth

    Cultivated 12.30 11.75 9.20 9.70 8.95 9.45

    Uncultivated 14.83 13.83 14.18 11.90 9.85 8.40

    LSD (0.05)

    NS NS 3.18* NS NS NS

    (B) 10-20 cm depth

    Cultivated 11.38 9.23 8.80 8.63 8.63 8.95

    Uncultivated 10.00 11.03 8.30 7.85 6.90 7.58

    LSD (0.05)

    NS NS NS NS NS NS

    (C) 20-30 cm depth

    Cultivated 8.75 6.90 6.63 7.53 7.88 7.10

    Uncultivated 8.00 7.63 7.95 6.43 6.18 5.73

    LSD (0.05)

    NS NS 1.20* NS 1.50* NS

  • 48

    Table 12: Land use effects on total soil nitrogen (TSN) content in water-stable aggregates

    (WSA) at the various soil depths

    (mm) ------------------------WSA (mm) -------------------------

    Depth/land use whole soil > 2.00 1.00-2.00 0.50-1.00 0.25-0.50 < 0.25

    ------------------------------------g kg-1

    ------------------------------------

    (A) 0-10 cm depth

    Cultivated 1.33 1.65 1.48 1.95 2.28 3.03

    Uncultivated 1.48 1.78 2.45 2.53 1.93 2.75

    LSD (0.05)

    NS NS 0.42* 0.52* NS NS

    (B) 10-20 cm depth

    Cultivated 1.50 1.65 1.75 1.93 2.25 2.18

    Uncultivated 1.28 1.63 1.55 2.28 2.48 2.80

    LSD (0.05)

    NS NS NS NS NS NS

    (C) 20-30 cm depth

    Cultivated 1.20 1.48 1.50 1.88 1.98 2.60

    Uncultivated 1.13 1.70 1.48 1.70 2.03 2.53

    LSD (0.05)

    NS NS NS NS NS NS

  • 49

    Table 13: Land use effects on soil organic carbon (SOC) content in water-stable aggregates

    (WSA) at 0-30 cm depth

    (mm) ----------------------------WSA (mm) --------------------------

    Land use whole soil > 2.00 1.00-2.00 0.50-1.00 0.25-0.50 < 0.25

    ------------------------------------g kg-1

    -----------------------------------------

    Cultivated 10.81 9.29 8.21 8.62 8.48 8.50

    Uncultivated

    10.94 10.83 10.14 8.73 7.64 7.23

    LSD (0.05)

    NS

    NS

    1.52*

    NS

    NS

    1.07*

  • 50

    Table 14: Land use effects on total soil nitrogen (TSN) content in water-stable aggregates

    (WSA) at 0-30 cm depth

    (mm) -------------------------- -WSA (mm) --------------------------

    Land use whole soil > 2.00 1.00-2.00 0.50-1.00 0.25-0.50 < 0.25

    ------------------------------------g kg-1

    -----------------------------------------

    Cultivated 1.34 1.59 1.58 1.92 2.17 2.60

    Uncultivated 1.29 1.70 1.83 2.17 2.14 2.69

    LSD (0.05)

    NS

    NS

    0.24*

    NS

    NS

    NS

  • 51

    Table 15: Land use effects on C/N ratio of water-stable aggregates (WSA) at 0-30 cm depth

    ---------------------------------WSA (mm) -------------------------------

    Land use > 2.00 1.00-2.00 0.50-1.00 0.25-0.50 < 0.25

    Cultivated 5.84 5.37 4.64 3.90 3.73

    Uncultivated 6.47 5.89 4.20 3.95 2.99

    LSD (0.05)

    NS

    NS

    NS

    NS

    NS

  • 52

    Table 16: Land use effects on bulk density, > 2.00 mm water stable aggregate (WSA), mean

    weight diameter (MWD) and geometric mean weight diameter (GMWD) of the soils

    Bulk density > 2.00 WSA

    Depth/land use (Mg m-3

    ) (mm) MWD GMWD

    (A) 0-10 cm

    Cultivated 1.44 51.34 2.11 0.88

    Uncultivated 1.34 62.67 2.32 0.84

    LSD (0.05)

    NS NS NS NS

    (B) 10-20 cm

    Cultivated 1.44 40.91 1.84 0.85

    Uncultivated 1.56 44.14 1.93 0.85

    LSD (0.05)

    NS NS NS NS

    (C) 20-30 cm

    Cultivated 1.50 36.66 1.75 0.88

    Uncultivated 1.51 44.00 1.96 0.88

    LSD (0.05)

    NS NS NS NS

  • 53

    Table 17: Soil texture effects on soil organic carbon (SOC) pool at the various sampling

    depths

    No. of location Depth occurrence

    Soil texture SOC pool (Mg ha-1

    ) occurrence (cm)

    Clay 20.30 1

    20-30

    Loam 14.50 1

    0-30

    Sandy loam 18.64 2

    0-10

    Clay loam 24.72 2

    10-20

    Sandy clay loam 13.51 4

    0-30

    LSD (0.05) 2.70*

  • 54

    0

    5

    10

    15

    20

    25

    30

    clay loam sandy

    loam

    clay loam sandy

    clay loam

    Soil texture

    SO

    C p

    oo

    l (M

    g/h

    a)

    0-10 cm

    0-20 cm

    20-30 cm

    Fig. 3: Soil texture effects on soil organic carbon (SOC) pool at the various soil depths.

  • 55

    macroporosity, WSA > 2.00, 1.00-2.00, 0.50-1.00, 0.25-0.50 and < 0.25 mm, GMWD, and SOC.

    Among all the variables correlated, only DR correlated significantly (P = 0.05) with TSN. The

    SOC positively correlated significantly (P 0.05) with WDC, microporosity, WSA > 2.00 mm,

    and MWD; while negative significant (P 0.05) correlation was obtained between SOC and

    1.00-2.00 mm, and 0.50-1.00 mm WSA. Soil organic carbon pool negatively correlated

    significantly (P = 0.05) with DR and 0.05-1.00 mm WSA, but positively correlated significantly

    with WDC, microporosity, > 2.00 mm WSA, MWD. The > 2.00 mm WSA negatively correlated

    significantly (P = 0.01) with all the other WSA classes; and positively with MWD, and GMWD.

    The 1.00-2.00 mm WSA negatively correlated significantly (P 0.05) with > 2.00 mm WSA,

    and MWD, and positively with 0.50-1.00 mm WSA. The WSA class of 0.50-1.00 mm correlated

    significantly with WSA classes of > 2.00 mm, 1.00-2.00 mm and 0.25- 0.50 mm, MWD, and

    SOC. The properties that significantly correlated with 0.25-0.50 mm WSA include WDSi, macro

    porosity, WSA classes of > 2.00, 0.50-1.00 and < 0.25 mm, MWD, and GMWD. The > 0.25 mm

    WSA that positively correlated significantly with silt, WDSi, and 0.25-0.50 mm WSA, also had a

    negative significant correlation with macro porosity, > 2.00 mm WSA, MWD, and GMWD.

    Notably, among the major three soil separates, only silt positively correlated significantly (P =

    0.05) with > 0.25 mm WSA. More so, all the WSA classes correlated significantly with either or

    both of their neighboring WSA classes.

    In uncultivated soils, whereas positive significant (P = 0.05) correlation was obtained

    between BD and clay, and WDC; BD negatively correlated significantly with fine sand and

    microporosity. The MWD correlated significantly (P 0.05) with silt, coarse sand, DR, WDSi,

    soil moisture, GMWD, and all WSA classes except 1.00-2.00 mm fraction. The CDI and CFI

    correlated significantly (P = 0.05) with TSN. SOC showed no significant correlation with any of

    the variables considered. However, SOC pool positively correlated significantly (P = 0.05) with

    ASC. The properties that significantly (P 0.05) correlated with > 2.00 mm WSA included silt,

    coarse sand, DR, WDSi, soil moisture, WSA classes of 0.50-1.00, 0.25-0.50 and < 0.25 mm,

    MWD, and GMWD. The 1.00-2.00 mm WSA significantly (P = 0.01) correlated negatively with

    microporosity and positively with WSA class 0.50-1.00 mm. The 0.50-1.00 mm WSA positively

    correlated significantly (P 0.05) with DR, WSA classes of 1.00-2.00 mm, and 0.25-0.50 mm, it

    significantly correlated negatively with > 2.00 mm WSA, and MWD. The 0.25-0.50 mm WSA

    correlated significantly (P 0.05) with silt, coarse sand, WDSi, GMWD, and > 2.00 mm, 0.50-

    1.00 mm and < 0.25 mm WSA; while < 0.25 mm WSA correlated significantly (P = 0.01) with

    silt, coarse sand, WDSi, soil moisture, MWD, GMWD, < 2.00 mm, and 0.25-0.50 mm WSA.

  • 56

    Table 18: Correlation coefficients (r) among particle sizes, aggregate indices and some

    selected properties of the soil under cultivated land use

    SOC -------------------WSA (%) ------------------

    Variables BD MWD TSN SOC pool A B C D E

    %clay 0.40 0.18 0.07 0.38 0.49 0.17 -0.13 -0.14 -0.05 -0.21

    %silt -0.02 -0.34 -0.11 0.07 0.07 -0.26 -0.16 -0.08 0.52 0.70*

    %Fsand -0.42 0.51 -0.33 -0.00 -0.10 0.49 -0.30 -0.31 -0.52 -0.54

    %Csand 0.00 0.00 0.24 -0.44 -0.46 -0.08 0.46 0.38 -0.31 -0.41

    CDI -0.38 0.33 0.22 0.35 0.24 0.32 -0.18 -0.32 -0.36 -0.23

    DR -0.46 -0.09 -0.59* -0.51 -0.63* -0.05 -0.18 0.01 0.15 0.23

    ASC 0.48 0.00 0.33 0.52 0.65* 0.01 -0.04 -0.14 0.11 0.09

    CFI 0.38 -0.33 -0.22 -0.35 -0.24 -0.32 0.18 0.32 0.36 0.23

    WDC -0.14 0.47 0.24 0.69* 0.65* 0.46 -0.31 -0.45 -0.38 -0.38

    WDSi 0.04 -0.44 -0.38 -0.17 -0.15 -0.35 -0.17 0.02 0.67* 0.78**

    Soilmoist 0.07 -0.38 -0.22 0.10 0.13 -0.32 -0.02 0.09 0.57 0.55

    BD 1.00 -0.03 0.17 -0.20 0.02 -0.07 0.27 0.25 -0.11 -0.25

    MacroP 0.35 0.65* 0.24 0.48 0.56 0.61 -0.28 -0.42 -0.62* -0.74**

    MicroP -0.16 0.39 0.38 0.87** 0.85* 0.41 -0.36 -0.50 -0.26 -0.19

    COLE 0.28 -0.36 0.07 0.22 0.31 -0.33 0.14 0.20 0.47 0.36

    VS 0.29 -0.35 0.07 0.21 0.31 -0.33 0.14 0.20 0.47 0.35

    WSA A -0.07 0.99** -0.23 0.63* 0.60* 1.00 -0.81** -0.91** -0.86** -0.73**

    WSA B 0.27 -0.73** 0.43 -0.58* -0.52 -0.81** 1.00 0.93** 0.40 0.21

    WSA C 0.25 -0.86** 0.28 -0.74** -0.68* -0.91** 0.93** 1.00 0.60* 0.40

    WSA D -0.11 -0.91** 0.05 -0.43 -0.44 -0.86** 0.40 0.60* 1.00 0.94**

    WSA E -0.25 -0.80** -0.06 -0.27 -0.31 -0.73** 0.21 0.40 0.94** 1.00

    MWD -0.03 1.00 -0.18 0.60* 0.58* 0.99** -0.73** -0.86** -0.91** -0.80**

    GMWD 0.23 0.81** 0.07 0.28 0.31 0.74** -0.22 -0.41 -0.93** -1.00**

    %Fsand = % fine sand, %Csand = % coarse sand, CDI = clay dispersion index; DR = dispersion

    ratio; ASC = aggregated silt and clay; CFI = Clay flocculation index; WDC = water dispersible

    clay; WDSi = water dispersible silt; Soilmoist = soil moisture; BD = bulk density; MacroP =

    macroporosity; MicroP = microporosity; TotalP = total porosity; COLE = coefficient of linear

    extensiblilty; VS = volumetric shrinkage; WSA A, B, C, D, E = water-stable aggregate > 2.00,

    1.00-2.00, 0.50-1.00, 0.25-0.50 and < 0.25 mm respectively; MWD = mean weight diameter;

    GMWD = geometric mean weight diameter; SOC = soil organic carbon, TSN = total soil

    nitrogen.

    *, ** = Significant at 0.01 and 0.05 alpha level (2-tailed), respectively.

  • 57

    Table 18 continuation: Correlation coefficients (r) among particle sizes, aggregate indices

    and some selected properties of the soil under uncultivated land use

    SOC ------------------WSA (%) --------------------

    Variables BD MWD TSN SOC pool A B C D E

    %clay 0.60* -0.26 -0.33 -0.15 0.11 -0.28 0.24 0.28 0.12 0.21

    %silt -0.20 -0.68* 0.28 0.49 0.52 -0.61* -0.08 0.27 0.63* 0.82**

    %Fsand -0.63* -0.45 -0.14 -0.09 -0.17 -0.44 0.13 0.24 0.36 0.50

    %Csand 0.11 0.76** -0.00 -0.24 -0.35 0.72** -0.10 -0.40 -0.63* -0.86**

    CDI 0.17 -0.14 0.69* 0.55 0.49 -0.13 0.07 0.07 0.19 0.10

    DR -0.30 -0.69** -0.42 -0.39 -0.38 -0.70* 0.36 0.64* 0.73** 0.55

    ASC 0.48 0.06 0.34 0.52 0.66* 0.09 -0.18 -0.18 -0.16 0.07

    CFI -0.17 0.14 -0.69* -0.55 -0.49 0.13 -0.07 -0.07 -0.19 -0.10

    WDC 0.69* -0.33 0.14 0.19 0.39 -0.34 0.26 0.28 0.26 0.26

    WDSi -0.31 -0.86** -0.14 0.09 0.17 -0.80** 0.07 0.49 0.78** 0.94**

    Soilmoist -0.27 -0.68* 0.18 0.48 0.51 -0.61* -0.04 0.32 0.57 0.80**

    BD 1.00 0.11 -0.13 -0.19 0.10 0.05 0.36 0.16 -0.18 -0.30

    MacroP 0.02 -0.03 -0.17 0.23 0.24 0.03 -0.50 -0.08 0.14 0.21

    MicroP -0.61* 0.05 0.22 0.58 0.48 0.17 -0.80** -0.46 -0.05 0.36

    COLE 0.26 -0.39 -0.04 0.26 0.43 -0.38 0.10 0.28 0.27 0.41

    VS 0.26 -0.39 -0.04 0.25 0.43 -0.37 0.10 0.29 0.27 0.40

    WSA A 0.05 0.99** 0.26 0.31 0.21 1.00 -0.57 -0.86** -0.93** -0.81**

    WSA B 0.36 -0.45 -0.16 -0.55 -0.45 -0.57 1.00 0.80** 0.35 0.01

    WSA C 0.16 -0.79** -0.35 -0.48 -0.36 -0.86** 0.80** 1.00 0.75** 0.43

    WSA D -0.18 -0.94** -0.30 -0.25 -0.25 -0.93** 0.35 0.75* 1.00 0.82**

    WSA E -0.31 -0.88** -0.12 0.05 0.11 -0.81** 0.01 0.43 0.82** 1.00

    MWD 0.11 1.00 0.24 0.23 0.14 0.99** -0.45 -0.79** -0.94** -0.88**

    GMWD 0.31 0.87** 0.13 -0.06 -0.12 0.80** 0.01 -0.42 -0.81** -1.00**

    %Fsand = % fine sand, %Csand = % coarse sand, CDI = clay dispersion index; DR = dispersion

    ratio; ASC = aggregated silt and clay; CFI = Clay flocculation index; WDC = water dispersible

    clay; WDSi = water dispersible silt; Soilmoist = soil moisture; BD = bulk density; MacroP =

    macroporosity; MicroP = microporosity; TotalP = total porosity; COLE = coefficient of linear

    extensiblilty; VS = volumetric shrinkage; WSA A, B, C, D, E = water-stable aggregate > 2.00,

    1.00-2.00, 0.50-1.00, 0.25-0.50 and < 0.25 mm respectively; MWD = mean weight diameter;

    GMWD = geometric mean weight diameter; SOC = soil organic carbon, TSN = total soil

    nitrogen.

    *, ** = Significant at 0.01 and 0.05 alpha level (2-tailed), respectively.

  • 58

    Table 19 shows the correlation coefficients (r) of the soils chemical properties and SOC

    pool under the two land uses. SOC content positively correlated significantly with soil pH,

    exchangeable K+, and C/N ratio of the cultivated soils; but positively correlated significantly (P =

    0.05) with total N, and exchangeable Na+

    in uncultivated land use. Also, a negative significant

    correlation was obtained between SOC and CEC in uncultivated soils. In cultivated soils,

    whereas SOC pool highly correlated significantly (P = 0.01) with soil pH, SOC, exchangeable

    K+, and C/N ratio; a positive significant (P 0.05) correlation was obtained with SOC, and

    exchangeable Na+, and negatively with CEC of uncultivated soils.

    In view of the notable contributions of some variables that correlated significantly with

    SOC, regression analysis was performed on the selected structural stability indices to show their

    relationship with SOC (Table 20). In cultivated land use, SOC explained significantly (P = 0.05)

    39 % and 34 % of the variation in WSA classes of > 2.00 mm and 1.00-2.00 mm, and 54 % in

    1.00-0.50 mm WSA. Whereas SOC significantly predicted 36 % of the variation in MWD

    content, it could only explain 48 % of WDC variability. The variability in micro porosity was

    largely due to the effect of SOC which predicted significantly up to 76 % of the variation. Unlike

    in cultivated land use, SOC content predicted non significant low values as a measure of the

    variability in the dependent variables of uncultivated soils.

  • 59

    Table 19: Correlation coefficients (r) of the soils chemical properties and soil organic

    carbon (SOC) pool of cultivated and uncultivated land uses

    ---------Cultivated--------- --------Uncultivated--------

    Independent variables SOC SOC pool SOC SOC pool

    --------------------Correlation coefficient (r) ---------------------

    Soil pH 0.70* 0.73** 0.38 0.36

    SOC (%) 1.00 0.97** 1.00 0.91**

    Total N (%) 0.12 0.15 0.68* 0.57

    Exchangeable Na+

    (cmol/kg) -0.20 -0.18 0.66* 0.65*

    Exchangeable K+

    (cmol/kg) 0.79** 0.80** 0.46 0.55

    Exchangeable Ca2+

    (cmol/kg) 0.40 0.39 0.11 0.13

    Exchangeable Mg2+

    (cmol/kg) 0.34 0.42 0.41 0.41

    Exchangeable Al3+

    (cmol/kg) -0.07 0.01 0.32 0.54

    CEC (cmol/kg) 0.11 0.10 -0.58* -0.60*

    C/N ratio 0.86** 0.81** 0.53 0.51

    *, ** = Significant at 0.01 and 0.05 alpha level (2-tailed) respectively.

  • 60

    Table 20: Relationship between soil organic carbon (X) and some structural stability indices

    (Y) in cultivated and uncultivated land uses (n = 12)

    ----------------------Land uses --------------------------

    ----------Cultivated ------------ -----------Uncultivated-----------

    Dependent variables (Y) Regression model R2 Regression model R

    2

    WSA: > 2.00 (mm) Y = -6.28 + 4.56X 0.39* Y = 30.76 + 1.78X 0.09ns

    WSA: 1.00-2.00 (mm) Y = 34.17 1.48X 0.34* Y = 26.07 0.99X 0.31ns

    WSA: 1.00-0.50 (mm) Y = 33.35 1.74X 0.54* Y = 18.51 0.68X 0.23ns

    WSA: 0.05-0.25 (mm) Y = 17.37 0.71X 0.19ns

    Y = 10.28 0.28X 0.06ns

    WSA: < 0.25 (mm) Y = 21.40 0.62X 0.07ns

    Y = 14.38 + 0.16X 0.00ns

    MWD Y = 0.63 + 0.12X 0.36* Y = 1.65 + 0.04X 0.05ns

    WDC Y = 4.42 + 0.67X 0.48* Y = 9.68 + 0.13X 0.04ns

    Micro porosity (%) Y = 26.97 + 2.02X 0.76* Y =39.28 + 0.54X 0.33ns

    * = significant at 0.05 alpha level, ns = non significant.

  • 61

    4.2 Discussion

    The clay content of the soils is moderately low when compared with the total sand

    fraction. The clay distribution with depth indicated some form of eluviation and illuviation.

    Among the particle size classes, sand fraction dominates with silt being the least which is more

    evident in soils of Nsukka locations. Akamigbo (1984) attributed this to the nature of the parent

    material and mineralogy of the soils. However, silt content of Awgu soil is relatively high when

    compared with similar soil (Okigwe) of the same agroecological zone. This could have resulted

    from flow of silt materials from the adjacent river because of its proximity to the sites where the

    soil was sampled. Igwe et al. (1995) showed that pedologically, high silt content may be related

    to mud flow and silt materials from river deposits and the underlying geological materials.

    The pH of the soils was slightly acidic and as such contributed significantly to the low

    OM content of the soils. This is in line with the findings of Hillel (1982), which indicated that

    acidic condition incapacitates soil microorganisms from producing different kinds of OM which

    enhances the aggregate stability of the soils. The low SOC contents may be attributed to leaching

    and high erodibility of the soils as a result of high rainfall intensity (Igwe, 2005). Evans (1980)

    pointed out that erodible soils are mainly sandy loams and loamy sands. The soil of Nsukka I,

    which fell within the sandy loam textural class at 0-10 cm depth (Table 3), also recorded the

    lowest SOC content showing that it is potentially erodible. Remarkably, SOC content increased

    with increase in clay content and WDC. Emerson (1971) indicated that low SOC causes severe

    dispersion of clay in water. Thus, low SOM enhances dispersion, and this affects the soil

    potential for interrill erosion and runoff during storms. The low exchangeable cations content of

    the soils could be due to excessive precipitation, which causes considerable loss of nutrient

    elements through erosion and leaching.

    All the parameters listed in Table 6 are estimates of soil structural stability indices.

    Whereas CDI, DR, WDC, WDSi estimate the rate of dispersibility (instability) of the soils, CFI

    and ASC are indices of stability for microaggregates, hence higher values of CFI and ASC imply

    higher or better stability (Igwe et al., 2009), and higher values of DR and CDI imply lower

    stability (Igwe et al., 1999). With CFI value of > 50 % and low DR, WDC and WDSi, the soils of

    the four locations are moderately stable and less dispersive which may be attributed to their

    underlying parent material. Igwe et al. (2009) reported similar observation and indicated that

    soils with low to moderate DR, and low WDC and WDSi are stable and less erodible. However,

    the moderate WDSi obtained in Awgu could be as a result of silt materials from river deposits. It

    is worthy to note the relative contribution of % clay to aggregate stability since it follows the

  • 62

    same trend with WDC. Therefore, it can be deduced that increase in clay content increases the

    rate of clay dispersion in water. Nikos et al. (2002) pointed out that clay is inherently more prone

    to dispersion than are silts and sands. Also, Boix-Fayos et al. (2001) observed a positive

    correlation between water stability of microaggregates and clay content. This is because clay

    upon wetting swell, slake or disperse. This supports the argument that swelling is a precursor to

    particle dispersion, which is dependent on the clay content of a soil (Emerson and Bakker, 1973).

    The variation in WSA fractions obtained among the locations, soil depths and across land

    uses could be due to differences in OC content (Castro et al., 2002) and the MWD of the soils.

    With the exception of Okigwe and Nsukka II soils, which had comparatively moderate to high

    concentration of > 2.00 mm WSA at 0-30 cm soil depth, the other two locations exhibited high

    rate of macroaggregate instability in water which is also evident in the low values of MWD

    obtained in both land uses. More so, a positive significant correlation was obtained between

    MWD and > 2.00 mm WSA as opposed to the negative correlation between MWD and with all

    other classes of WSA in both land uses (Table 18). In Table 10, only 0.50-1.00 mm WSA was

    significantly influenced by land use at 0-10 cm and 0-30 cm soil depth. This suggests that

    cultivation did not affect the stability of microaggregates since the amount is greater in cultivated

    than in uncultivated soils. This confirms the observation that reductions in aggregate stability

    after cultivation are most pronounced in soil macroaggregates, while the stability of

    microaggregates remains unchanged (Tisdall and Oades, 1982; Oades, 1984).

    Great variations exist in SOC and TSN associated with WSA classes and C/N ratios of

    the soils. In SOC associated with WSA, whereas 1.00-2.00 mm WSA at 0-10 cm and 20-30 cm,

    including the 0.25-0.50 mm WSA at 20-30 cm were significantly affected by land use, 1.00-2.00

    mm and < 0.25 mm were significantly influenced at 0-30 cm soil depth. The reduction of SOC in

    > 2.00 mm to 0.50-1.00 mm WSA of cultivated soils may be due to effect of tillage which

    disaggregates the soil particles and increases oxidation of SOM thereby leading to C loss through

    erosion, leaching or emission to the atmosphere. Research findings in confirmation to this

    abound in literature (Mokma and Sietz (1992); Six et al. (1999); Lal et al. (2007); Wright and

    Hons (2005); Lal and Kimble (2000); Paustina et al. (2000), and Follett (2001). The variation

    may also be attributed to the quantity and quality of SOC, including the differences in soil types

    (Tisdall and Oades, 1982; Saggar et al., 2001; Mbagwu and Bazzoffi, 1989). Feller and Beare

    (1997) were of the view that soil aggregation, texture and mineralogy control OM in

    macroaggregates. Notably, although the low C content in 1.00-2.00 mm WSA of the cultivated

    soils was expected, the reverse was the case with 0.25-0.50 mm and < 0.25 mm WSA. The SOC

    content generally increased in the fraction of 0.25-0.50 mm and < 0.25 mm WSA at all depths

  • 63

    and significantly at 20-30 cm and 0-30 cm soil depths which indicates associations such as

    organo-silt complexes and organo-clay fractions (Feller, 1994), and hence more stabilization of

    SOC. Although, it is generally agreed that cultivation causes a decrease of SOC (Alvarez et al.,

    1998); one is uncertain why this is the case, but, a positive significant correlation was obtained

    between % silt and 0.25-0.50 mm, and < 0.25 mm WSA (Table 18). Whereas Dormarr (1983)

    and Mbagwu and Piccolo (1990) reported higher concentration of C in macroaggregates than

    microaggregates, Christensen and Sorensen (1985) reported otherwise. Similarly, in TSN

    associated WSA, 1.00-2.00 mm and 0.50-1.00 mm WSA were significantly affected by land use

    at 0-10 cm soil depth, and only 1.00-2.00 mm was influenced at 0-30 cm soil depth. The results

    suggest that C and N associated with 1.00-2.00 mm WSA are mostly lost may be as a result of

    increased decomposition and erosion rate due to change in aggregate structure associated with

    cultivation. The earlier assertions by Beare et al. (1994), Coote and Ramsey (1983), and

    Cambardella and Elliott (1994) support this observation. Therefore, whereas > 1.00 mm WSA at

    0-30 cm soil depth contained about 8.20 to 9.30 g kg-1

    C / 1.58 to 1.59 g kg-1

    N and 10.10 to

    10.80 g kg-1

    C / 1.70 to 1.83 g kg-1

    N in cultivated and uncultivated soils respectively; < 1.00 mm

    WSA contained 8.50 to 8.60 g kg-1

    C / 1.92 to 2.60 g kg-1

    N and 7.20 to 8.70 g kg-1

    C / 2.14 to

    2.69 g kg-1

    N, in cultivated and uncultivated soils, respectively.

    The N content of the soils was far much lower when compared with C content. The C/N

    ratios of the soils were generally low, although higher at 0-10 cm when compared with the other

    depths. This suggests that the availability of N in the top most surface layers is not a key

    component for reducing C losses and increasing the SOC content. The decreasing trend observed

    in the C/N ratio associated with the WSA from > 2.00 mm to < 0.25 mm indicated that microbial

    alteration of soil OM was more associated with the microaggregates than the macroaggregates

    (Monrozier et al., 1991). S et al. (2001) observed lower C/N ratios in 2- to 20- m and < 2- m

    fractions than in larger sized fractions and suggested that 40 to 60 % of the microbial biomass

    may be associated with the microaggregates 2- to 20- m, depending on the amount and type of

    clay. More so, under tropical forest and savanna conditions, bacterial cell walls or colonies at

    different stages of decomposition can be observed in fractions < 20 m. Hence, the low C/N ratio

    observed in the finer-size fractions would be an indication that SOM was more stabilized and

    more aromatic (Bayer et al., 2000).

    The SOC pool significantly decreased with soil depth, as such, SOC pool was highest at

    the topmost soil surface (0-10 cm). This could be as a result of high C input, biomass activity at

    the soil surface and consequent fast C turnover rate in tropical soils. These results are in

    conformity with the earlier assertions by Lal (2004), Lal et al. (2007), Six et al. (2002) and Six et

  • 64

    al. (1999). Resck (1998) and S (1993) observed a significant impact on SOC concentrations for

    0-10 cm soil layer; also Carter (1992) reported increase in SOC content at the surface soil. From

    the observation, SOC pool at 20-30 cm soil depth was higher in cultivated than in uncultivated

    soils. This may be as a result of anthropogenic disturbance of soil structure during tillage in

    preparation of the land for cultivation, which inverts and mixes the soil hence altering the

    distribution of C by incorporating the organic materials on the soil surface to the subsoil where it

    would not be easily lost (Potter et al., 1997). The reason for the higher SOC pool obtained in

    cultivated soils of Nsukka II than in uncultivated land use could be attributed to the sloping

    landscape position of the location due to soil erosion and leaching (Hall, 1983). Thus, whereas

    cultivated soils contained an average of 15.69 Mg ha-1

    C, uncultivated soils had an average of

    16.52 Mg ha-1

    C. The non significant difference in SOC pool obtained between both land uses is

    thought to may have resulted from the following factors: (i) the low rate of biomass production,

    C input and mineralization (Lal and Kimble, 2000; Lal et al., 2007); (ii) the short period of time

    (4-5 yrs) the uncultivated land had been on fallow of which the rate of residue input did not

    compensate the high losses of organic matter due to mineralization (S et al., 2001; Harrison et

    al., 1990; Lobo et al., 1990); (iii) low C conversion efficiency due to low N content (Knops and

    Tilman (2000); Lal, 1999); and (iv) increased rate of C loss through erosion (Tans et al., 1990).

    Therefore, loss of C (Mg ha-1

    ) caused by cultivation amounted to 10 % and 9 % at 0-10 cm and

    10-20 cm soil depth respectively, with an average of 5 % at 0-30 cm.

    The results obtained from this study showed that across all the depths, cultivated soils as a

    result of tillage disintegrated the > 1.00 mm WSA, which contained the highest amount of C

    since SOC decreased with decrease in soil particle sizes (Table 13). Hence, SOC was

    significantly reduced in 1.00-2.00 mm WSA at 0-10 cm and 20-30 cm soil depths (Table 11).

    Therefore, the role of soil as a C sink in mitigating climatic change can only be enhanced if the

    rate of macro- and micro- aggregates disruption is minimized. The practice of soil tillage as is

    obtained in most parts of southeastern Nigeria should be replaced with adoption of recommended

    soil management practices such as no-till or reduced tillage, increased crop rotation intensity, use

    of cover crops, residue mulching, addition of organic manure, etc to promote C sequestration,

    thereby reducing agriculture related CO2 emissions and consequently mitigating global warming

    (Denef et al., 2004; Doran, 1980; Kern and Johnson, 1993). More so, selection of varieties and

    hybrids that store more C for cultivation will strengthen C sinks in the soil.

    In considering SOC pool in relation to soil texture, clay loam sequestered the highest

    SOC while sandy clay loam sequestered the least. The reason for this could be related to their

    clay content and possibly, the clay type which was not considered in this study. Since the

  • 65

    increase in SOC pool followed the same trend with clay content, it can be deduced that SOC is a

    function of clay content. Hence, increase in clay content increases the rate of SOC sequestration.

    Batjes (1998) showed that clay, soil acidity amidst others were among the major environmental

    factors that control the behaviour of OM in the soil. Also in Table 19, soil pH positively

    correlated significantly with SOC pool where r = 0.73 in cultivated soils. Coote and Ramsey

    (1983), and Nichols (1984) obtained a positive relationship between % clay and SOM.

    The significant correlation of SOC with WDC, micro porosity, WSA > 2.00 mm, and

    MWD indicated the relative importance of SOC to these properties; hence increase in SOC

    content resulted in improved rate of aggregation. Also, the significant correlation between SOC

    pool and these properties, including DR, ASC, 0.50-1.00 mm WSA, and SOC confirms their

    contributions to the improvement of SOC pool accumulation and stabilization. Thus, the positive

    significant correlation between SOC/SOC pool and > 2.00 mm WSA is an indication that more

    than 60 % SOC may be associated with > 2.00 mm WSA in cultivated soils. The SOC accounted

    for about 70 to 90 % of the variability in soil aggregate stability of a clay loam soil (Mbagwu and

    Bazzoffi, 1989). Therefore, decreased proportion of WSA classes of < 1.00 mm enhanced SOC

    accumulation and possible stabilization in > 2.00 mm within the top soil (0-30 cm). Similarly, the

    highly significant correlation between MWD and macroporosity and with all the WSA classes

    indicates the relative importance of MWD to the improvement of the structural stability of the

    soil. The relation between SOC and soil chemical properties indicates that, whereas soil pH,

    exchangeable K+ and C/N ratio appear to be the most contributing factors to SOC improvement

    in cultivated land use, total N, exchangeable Na+ and CEC could be responsible for SOC

    accumulation in uncultivated land use. Gerding (1991) reported a strong correlation between

    SOC pool in the surface layer (30 cm depth) and the N and Ca 2+

    content of the soil.

    Regression analysis showed that SOC significantly accounted for < 40 % of the

    variability in > 1.00 mm WSA. This shows that although SOC is important in the aggregation of

    these WSA, other cementing agents than C are equally involved in the stability of

    macroaggregates of cultivated soils. Mbagwu (1989) reported that the role of SOC as an

    aggregating agent diminishes in the presence of other dominating aggregating agents, such as

    polyvalent metals and silicate clay. Remarkably, the non significant effect of SOC on the

    variability in 0.05-0.25 mm and < 0.25 mm WSA under both land uses indicates that SOC does

    not contribute to the instability of these aggregates. However, SOC became more influential and

    was able to explain up to 54 % and 76 % of the variability in 1.00-0.50 mm WSA and

    microporosity, respectively (Table 20).

  • 66

    CHAPTER FIVE

    SUMMARY, CONCLUSION AND RECOMMENDATION

    The main aim of this study was to assess the potential of various aggregate size fractions

    of varying soil textures and depths to sequester C under different land uses. The hypothesis was

    that SOC sequestration is a function of soil texture and soil aggregation; and that SOC is similar

    between soil phases (cultivated and uncultivated) of the same soil series. The data presented

    support the first hypothesis, and rendered the second invalid. The results showed that great

    variations exist in SOC, TSN and C/N ratio associated with WSA classes of the cultivated and

    uncultivated soils. The N content of the soils was far much lower when compared with C content.

    The C/N ratios of the soils were also, generally lower although higher at 0-10 cm. At 0-30 cm

    soil depth, SOC content increased with increase in WSA of uncultivated soils, but in cultivated

    soils, > 2.00 mm WSA sequestered the highest C, followed by 0.50-1.00 mm, 0.25-0.50 mm and

    < 0.25 mm, while the least was 1.00-2.00 mm WSA. Hence, > 2.00 mm WSA contained the

    highest concentration of SOC in both soil phases. Land use affected SOC concentration in 1.00-

    2.00 mm and < 0.25 mm WSA at 0-30 cm depth; SOC in 1.00-2.00 mm WSA at 0-10 cm; and

    SOC in 1.00-2.00 mm and 0.25-0.50 mm WSA at 20-30 cm soil depth. Unlike in SOC, TSN

    associated with WSA was more in < 1.00 mm WSA than in > 1.00 mm WSA: it (generally)

    increased with decrease in WSA classes at 0-30 cm depth and varied significantly in the 1.00-

    2.00 mm WSA between the soil phases. The 1.00-2.00 mm and 0.50-1.00 mm WSA were

    significantly affected by land use at 0-10 cm soil depth. Therefore, whereas > 1.00 mm WSA at

    0-30 cm soil depth contained 8.20 to 9.30 g kg-1

    C / 1.58 to 1.59 g kg-1

    N and 10.10 to 10.80 g kg-

    1 C / 1.70 to 1.83 g kg

    -1 N in cultivated and uncultivated soils, respectively, < 1.00 mm WSA

    contained 8.50 to 8.60 g kg-1

    C / 1.92 to 2.60 g kg-1

    N and 7.20 to 8.70 g kg-1

    C / 2.14 to 2.69 g

    kg-1

    N in cultivated and uncultivated soils, respectively.

    Soil depth exerted a strong influence on the spatial distribution of SOC pool, unlike land

    use that did not show any significant effect on SOC pool. In both cultivated and uncultivated

    soils, SOC pool significantly decreased with soil depth; hence highest SOC pool was observed at

    0-10 cm soil depth. Soil organic carbon content to the depth of 30 cm differed distinctly among

    the study sites. Uncultivated soils recorded higher SOC than the cultivated soils except at 20-30

    cm depth which may be due to structural alteration of C distribution resulting from anthropogenic

    disturbance. Hence, loss of C caused by cultivation amounted to 10 % and 9 % at 0-10 cm and

    10-20 cm soil depth respectively, with an average of 5 % at 0-30 cm.

  • 67

    The rate of C sequestration differed significantly (P = 0.05) with soil texture across soil

    depths and land uses. The trend followed thus; clay loam > clay > sandy loam > loam > sandy

    clay loam. Among the three sampled depths, soil texture effects on SOC pool indicated that SOC

    was most sequestered at 10-20 cm depth, followed by 0-10 cm and the least at 20-30 cm depth.

    Finally, whereas WDC, micro porosity, MWD, soil pH, K+, C/N ratio, > 2.00 mm, 1.00-2.00

    mm, and 0.50-1.00 mm WSA significantly correlated with SOC concentration in cultivated soils;

    only Na+, N, and CEC significantly correlated with SOC concentration in uncultivated soils thus

    indicating the relative contribution of these properties to C accumulation and stabilization. Soil

    organic carbon accounted for 54 % and 76 % of the variability in micro aggregates and micro

    porosity (respectively) of the cultivated soil, but could not significantly account for that in

    uncultivated land use.

    In conclusion, SOC sequestration is a function of soil texture, depth and aggregation.

    Cultivation does not affect the stability of microaggregates; however, the effect of cultivation as

    a result of tillage to the depth of 30 cm promotes macroaggregate breakdown and consequent loss

    of SOC to the atmosphere which could accelerate global warming. The C and N associated with

    1.00-2.00 mm WSA are mostly lost as a result of increased decomposition and erosion rate due to

    change in aggregate structure associated with cultivation. Notably, high proportion of clay

    content in soils, and prolonged land fallow may increase the rate of carbon sequestration in soils.

    Therefore, for the potentially erodible soils of southeastern Nigeria, substantial C sink might be

    achieved if the rate of macro- and micro- aggregates disruption is minimized. Also, the adoption

    of recommended soil management practices such as no-till or reduced tillage, use of cover crops,

    residue mulching, addition of organic manure, etc will not only increase soil fertility but also will

    reduce agricultural related CO2 emissions and consequently, mitigate climatic change.

    Although this study has succeeded in quantifying SOC pool and understanding the

    distribution of SOC and TSN across aggregate size classes as stratified by land use, soil texture,

    and soil depth of some soils of southeastern Nigeria, there is still the need for more advanced

    research using physical fractionation techniques of particulate organic matter to identify how

    aggregation under forest vegetation or other soil management systems contribute to SOC pools

    stabilization.

  • 68

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  • 87

    Appendix 1: Final analysis of variance of a 4 x 2 x 3 factorial experiment in CRD showing

    sources of variation and degree of freedom only

    Source of Variation d.f.

    Treatment Combinations

    23

    Factor A (Location) 3

    Factor B (Land use) 1

    Factor C (Soil depth) 2

    First-Order Interaction

    A x B 3

    A x C 6

    B x C 2

    Second-Order Interaction

    A x B x C 6

    Error 48

    Total 71

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