Soil organic matter pools and carbon fractions in soil under different land uses
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Soil & Tillage Research 126 (2013) 177182
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Soil & Tillage
Soil organic matter (SOM) plays an important role in maintainingthe productivity of tropical soils because it provides energy andsubstrates, and promotes the biological diversity that helps tomaintain soil quality and ecosystem functionality. SOM directlyinuences soil quality, due to its effect on soil properties (Wendlinget al., 2010). Once soil is cultivated for agricultural production,especially in the tropics and the semi-arid regions, SOM is rapidlydecomposed due to modications in conditions such as aeration,temperature, and water content (Ashagrie et al., 2007). This canaffect many soil functions that are either directly or indirectlyrelated to SOM, due to its capacity to retain water and nutrients.Although the breakdown rate of SOM can be faster in the tropics,regular inputs of organic amendments can promote a buildup ofSOM (Follett, 2001).
Soils with low natural fertility and reduced water availabilityhave been brought into production by means of high technologystrategies including irrigation and the intensive use of fertilizers and
pesticides. These management practices are very common in thetropics and can contribute to negative changes in soil quality, whichshould be investigated and monitored in order to reduce environ-mental impacts of agricultural activities as well as to promote a moresustainable use of soils. Due to the complexity of SOM compounds,the relationship between SOM characteristics and land use is poorlyunderstood. Hence, knowledge of different pools of SOM shouldrender the impacts of management practice more measurable.
Soil organic matter labile fractions have been used instead oftotal SOM as sensitive indicators of changes in soil quality (Bayeret al., 2002; Haynes, 2005), due to the many important interactionsof these components in the soil system. Non-humic substances arelabile compounds with relatively rapid turnover in soil, since theyare readily utilized as substrates by soil microorganisms (Schmidtet al., 2011). Humic substances are more stable organic mattercompounds, which make up a signicant portion of the total soilorganic C and N (Lal, 1994; Milori et al., 2002). Humic substancescan improve soil buffering capacity, increase moisture retention,and supply plants with available micronutrients. Moreover, thesecompounds can also bind metals, alleviating both heavy metaltoxicity and metal deciency in soils (McCarthy, 2001).
Humic substances represent approximately 4060% of the soilorganic matter, and include three different fractions, dened
changes in the content of humic substances in an Ultisol under different land uses, in the northeast
region of Brazil. Soil samples were collected from the 010 and 1030 cm layers, in three agricultural
areas (conventional coconut orchard, integrated coconut orchard, and citrus orchard). A native forest soil
was used as reference. Organic C and total N were determined to characterize the SOM. Humic
substances were chemically fractionated into fulvic acid, humic acid, and humin, based on solubility in
acid and alkali. Signicant loss (47.5%) of soil organic matter was observed in the surface layers of the
conventional coconut and citrus orchards, compared to the native forest. There was increased SOM
content in the integrated coconut orchard soil, due to the presence of cover crops as well as management
of crop residues. However, in the subsurface soil of the integrated coconut orchard, cultivation modied
the distribution of the more labile fractions of the soil organic matter, as measured by the ratio between
humic and fulvic acids (>1.0), indicating a substantial loss of fulvic acids. The degree of humication was
in the range 4097%. The distributions of the soil organic matter fractions varied in the ranges 1232.5%
(fulvic acids), 1234.5% (humic acids), and 4069.5% (humin).
2012 Elsevier B.V. All rights reserved.
* Corresponding author. Tel.: +55 79 2105 6984; fax: +55 79 2105 6929.
E-mail address: firstname.lastname@example.org (M.I.S. Gonzaga).
0167-1987/$ see front matter 2012 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.still.2012.07.010Soil organic matter pools and carbon fr
Daniele Vieira Guimaraes a, Maria Isidoria Silva GoThiago Lima da Silva a, Nildo da Silva Dias b, MariaaAgronomy Department, Federal University of Sergipe, Sao Cristovao, SE 49100-000, Bb Environmental Science Department, Federal University of Semiarid, Mossoro, RN 596cGeology Department, Federal University of Bahia, Salvador, BA 40110-060, Brazil
A R T I C L E I N F O
Received 11 November 2011
Received in revised form 14 June 2012
Accepted 29 July 2012
A B S T R A C T
Changes in tropical land
suggested that alterations
soil use than total soil o
jou r nal h o mep age: w wtions in soil under different land uses
aga a,*, Tacio Oliveira da Silva a,aildes Silva Matias c
have profound effects on soil organic matter (SOM) status. It has been
the different fractions of SOM are more effective in indicating changes in
ic matter content. The main objective of this study was to investigate
l s evier . co m/lo c ate /s t i l l
according to different stability under acid hydrolysis andpermanganate oxidation (Paul et al., 2001). Humin (H) is theinsoluble fraction of humic substances; humic acid (HA) is thefraction that is soluble under alkaline conditions; and fulvic acid(FA) is the fraction that is soluble under both alkaline and acidicconditions (Sutton and Sposito, 2005). The chemically reactive andrefractory nature of humic substances contributes to theirpersistence in soils (Kiem and Kogel-Knabner, 2003; Rovira andVallejo, 2007), as well as to their important role in nutrient owsthrough ecological systems, and C emissions to the atmosphere(Lal, 2006). The relationship between concentrations of humic andfulvic acids (HA/FA ratio) is indicative of the potential mobility of Cin the soil system. In general, sandy soils present higher ratios dueto the solubility and selective loss of fulvic acids during thedecomposition of SOM. The (HA + FA)/humin ratio indicates the
D.V. Guimaraes et al. / Soil & Tillage Research 126 (2013) 177182178degree of illuviation of SOM (Benites et al., 2003).Changes in eld management practices can alter the chemical
properties of soil humic substances (Moraes et al., 2011). Spacciniet al. (2006) reported a progressive decrease in humic substanceconcentrations in soils that were converted from forest to arablefarming. Such decrease is commonly attributed to microbialoxidation of the organic materials previously protected in the soilaggregates destroyed by cultivation. Most studies report areduction in SOM and in its fractions when forest is convertedto other land uses (Navarrete and Tsutsuki, 2008).
In northeast Brazil, the production of tropical fruits occupies alarge area dominated by Ultisols. Management practices and yieldsvary among orchards, and soil quality has not been a focus of manyfarmers. In this region, there have been few studies that haveaddressed the impact of land use change on levels of humicsubstances in soils. The objective of this study was therefore toevaluate SOM characteristics as inuenced by land use. The degreeof humication of SOM, together with the contributions of thedifferent humic substance fractions, was determined using anoperationally dened chemical fractionation method, applied to anUltisol cultivated with tropical fruit trees. The same soil type undernative forest was used as a reference.
2. Materials and methods
2.1. Site characterization and experimental design
The work was conducted between August and October 2010 onirrigated farms in the state of Sergipe, in the northeast of Brazil. Theclimate is type BSh, according to the KoppenGeiger classication(Peel et al., 2007), with dry summers and rainfall concentrated inthe months of May to September. The mean annual precipitationand temperature are 1200 mm and 30 8C, respectively. The soil isan Ultisol (typic Hapludult). Physical and chemical properties ofthe soil under different uses are presented in Table 1. Soil texture
Table 1Physical and chemical characterization of the soils under different land uses.
Land use pH P*
Conventional coconut 6.4 21.0 6.40 76.4 15.8 7.78
Integrated coconut 6.5 54.1 10.4 80.1 12.4 7.55
Citrus 6.7 24.5 5.16 83.3 10.8 5.87
Native forest 5.0 0.21 5.91 85.6 10.2 4.22
Conventional coconut 5.5 4.20 4.27 67.6 23.3 9.04
Integrated coconut 6.3 18.4 7.25 74.8 16.5 8.66
Citrus 6.5 7.80 3.93 80.1 13.4 6.51
Native forest 4.7 0.08 3.30 84.4 10.1 5.53
* Mehlich 1 extractable P.a CEC: cation exchange capacity.was analyzed by the hydrometer method, pH was measured usinga 1:2.5 soil/solution ratio, P was extracted using Mehlich 1solution, and CEC was calculated by the sum of cations.
The study was carried out according to a fully randomizedfactorial design, with four types of land use (conventional coconut(Cocus nucifera L.) orchard, integrated coconut orchard, citrusorchard, and native forest) and 2 depths of sampling (010 cm and1030 cm), with 3 replicates. The conventional coconut orchardwas managed without any conservation practices, using mineralfertilization (NPK), mechanical and chemical weed control, andmicro-sprinkler irrigation (4.0 mm of water). The integratedcoconut orchard was managed with mineral fertilization andconservation practices that included organic fertilization (castorbean cake, cow manure), leguminous cover crops, and mulching.The main crop residues were dispersed on the soil surface, andirrigation employed a micro-sprinkler system (4.0 mm of water).The citrus orchard (Citrus sinensis L. Osbeck) was conventionallymanaged, with application of mineral fertilizer and pesticides.Weeds were controlled using tillage operations, and irrigation wasperformed via central pivot (4.0 mm of water). The nativevegetation was a remnant of the Atlantic Forest. The orchardswere implemented in 1998.
Each experimental area was divided into three plots of 100 m2.In each plot, ten soil samples were randomly collected at eachdepth, and mixed to obtain a composite soil sample. The soilsamples were homogenized, and transported in plastic bags to thelaboratory, where they were air dried, sieved through a 2-mmscreen, and stored prior to analysis.
2.2. Chemical fractionation of soil organic matter
The chemical fractionation was carried out following themethod proposed by the International Humic Substances Society(IHSS) (Hayes et al., 1989) to obtain humin (H), humic acid (HA),and fulvic acid (FA) fractions, based on the solubility in acid andalkali. Total soil organic C (TOC) and the C contents of thesefractions were determined according to Yeomans and Bremner(1988). The relationships HA/FA and (HA + FA)/H were alsocalculated. The degree of humication was calculated using theformula (HA + FA + H)/TOC 100.
Soil total N was determined by the Kjeldahl method, asdescribed by Bremner and Mulvaney (1982).
2.3. Statistical analysis
The data were analyzed following complete randomized designand using subplots. The soil use types and sampling depths wereconsidered as primary and secondary effects, respectively. Theresults were submitted to variance analysis for identication ofrelevant effects, and the means were compared using the Tukeytest (P < 0.50). All analyses were performed using SISVARsoftware, version 5.0 (Fereira, 2003).
3. Results and discussion
3.1. Total soil organic matter and stratication ratio
Total soil organic matter (SOM) concentrations in the 010 cmsoil layer differed signicantly among land uses (Fig. 1), andfollowed the order: integrated coconut > native forest > citrus = -conventional coconut. Considering the soil of the native forest as areference, the integrated coconut orchard showed an increase inSOM of approximately 37%. On the other hand, in the conventionalcoconut and citrus orchards the SOM concentrations were reducedby almost 40%. This was mainly because the different soil uses andmanagements affected the inputs of organic residues. Even within
residues, undisturbed soils offer a suboptimal decompositionenvironment, thus causing accumulation of SOM at the soil surface(Franzluebbers, 2002).
However, care must be taken when evaluating the SOMstratication ratio as an indicator of soil quality, because a highstratication ratio does not always reect good soil quality. HighSOM stratication ratios are expected to indicate relativelyundisturbed soil with good water inltration capacity, aggregatestability, microbial activity, and nutrient supply (Franzluebbers,2002). In the present work, this was as expected for the nativeforest soil. However, the conventional coconut and citrus orchardsoils also presented high SOM stratication ratios, but this time asa result of soil compaction (data not shown), which reduced therate of residue decomposition.
3.2. Soil total nitrogen
Fig. 1. Soil organic matter under different land uses, at different soil depths. Meansfollowed by the same uppercase letter (for a given soil depth), and lowercase letter
(for a given land use) are not signicantly different using Tukeys test at p < 0.05.
D.V. Guimaraes et al. / Soil & Tillage Research 126 (2013) 177182 179the same land use (as was the case for the two coconut orchards),SOM concentrations can be very different as a result of differentmanagement practices. In the integrated coconut orchard, the soilwas cultivated using conservation practices that included legumi-nous cover crop (Pueraria phaseoloides Benth), mulching, andfrequent inputs of organic and inorganic amendments, which alsoimproved plant biomass production and consequently contributedto greater amounts of residues added to the soil. These mimickednatural systems by increasing residue returns and minimizing Cremoval, both of which are recognized as important components ofsustainable systems (Lal, 2008). These practices were not appliedin the conventional coconut and citrus orchards. The differencesobserved across land uses can also be attributed to differences inmicroclimate, vegetation canopy, and litter input (Yao et al.,2010).In the 1030 cm soil layer, SOM concentrations weresignicantly lower than in the 010 cm layer, with values in theorder: native forest = integrated coconut > citrus = conventionalcoconut. The results obtained here differed from those of Schrothet al. (2002), who reported no signicant difference in Cconcentrations below 10 cm depth. Their study showed that theeffect of forest conversion to crops, as a result of agriculturalactivities, was largely restricted to the topsoil.Most of the SOMcontent was concentrated near the soil surface (010 cm), asshown by the stratication ratio, which varied from 2.0 to 3.4(Fig. 2). Accumulation of SOM at the soil surface was a r...