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Europ. J. Agronomy 43 (2012) 40– 48

Contents lists available at SciVerse ScienceDirect

European Journal of Agronomy

jo u rn al hom epage: www.elsev ier .com/ locate /e ja

oil carbon sequestration and agronomic productivity of an Alfisol for aroundnut-based system in a semiarid environment in southern India

h. Srinivasaraoa,∗, B. Venkateswarlua, Rattan Lalb, Anil Kumar Singhc, Sumanta Kundua, K.P.R. Vittald,. Balaguravaiahe, M. Vijaya Shankar Babue, G. Ravindra Charya, M.B.B. Prasadbabuf,. Yellamanda Reddye

Central Research Institute for Dryland Agriculture, Santoshnagar, Saidabad (P.O.), Hyderabad 500059, Andhra Pradesh, IndiaCarbon Management and Sequestration Center, The Ohio State University, Columbus, OH 43210, USAIndian Council of Agricultural Research, Krishi Anusandhan Bhawan (KAB-II), New Delhi 110012, IndiaNational Institute for Abiotic Stress Management, Baramati 413115, Maharashtra, IndiaAgricultural Research Station, Anantapur 515001, Andhra Pradesh, IndiaDirectorate of Rice Research, Rajendranagar, Hyderabad 500030, Andhra Pradesh, India

r t i c l e i n f o

rticle history:eceived 15 March 2012eceived in revised form 1 May 2012ccepted 4 May 2012

eywords:arbon inputsarbon sequestration

a b s t r a c t

Soil organic carbon (SOC) is a strong determinant of soil quality and agronomic productivity especiallyunder harsh arid and semiarid environments of the tropics. Thus, a 20-year experiment was used toassess the impact of rainfed groundnut (Arachis hypogeae) monocropping, fertilization and manuringon soil quality, SOC sequestration, and crop yield sustainability on an Alfisol in southern India. Fivetreatments with 4 replications were: (1) control (no fertilizer or manure), (2) 100% recommended doseof fertilizer (RDF, 20:40:40 kg ha−1 of N:P2O5:K2O), (3) 50% RDF + 4 Mg ha−1 of groundnut shells (GNS), (4)50% RDF + 4 Mg ha−1 of farmyard manure (FYM) and (5) 5 Mg ha−1 of FYM. The SOC concentration to 1-m

−1 −1

roundnut shellsustainable yield indexainfed groundnutemi-arid tropics

depth increased from 2.3 to 3.5 g kg (52.2%) in 50% RDF + 4 Mg ha GNS over control and mean SOCsequestration rate was 0.57 Mg C ha−1 yr−1. Higher mean pod yield of groundnut (Mg ha−1) was obtainedwith 50% RDF + 4 Mg ha−1 FYM (1.03). The rate of increase in groundnut pod yield was 13 kg ha−1 yr−1

for every one Mg increase in profile SOC stock. A minimum of 1.12 Mg C ha−1 yr−1 input was needed tomaintain the zero change in SOC. Hence, combined use of chemical fertilizers and organic manure isessential to enhancing SOC sequestration in monocrop regions in semi arid tropical conditions.

. Introduction

India, similar to China and other countries, is facing a dual chal-enge of reducing CO2 emissions and enhancing the gross domesticroduct (GDP) by 20–25% by 2020 compared with the 2005 base-

ine. In this context, the importance of sustainable management ofoils of agroecosystems to enhance soil organic carbon (SOC) stocksy sequestering atmospheric CO2 cannot be over-emphasized. Bothhe magnitude and quality of SOC stock are critical to improv-ng soil quality, increasing crop productivity, and off-setting CO2missions (Lal, 2004; Smith, 2007). Optimum levels of SOC cane managed through the adoption of appropriate crop rotations

Wright and Hons, 2005), fertility management, using inorganicertilizers and organic amendments (Schuman et al., 2002; Mandalt al., 2007; Majumder et al., 2008) and conservation tillage

∗ Corresponding author. Tel.: +91 40 24530161x218;ax: +91 40 24535336/24531802.

E-mail address: cherukumalli2011@gmail.com (Ch. Srinivasarao).

161-0301/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.eja.2012.05.001

© 2012 Elsevier B.V. All rights reserved.

methods (Lal, 2009). In rainfall-scarce environments of tropical andsub-tropical regions characterized by arid and semi-arid climates,soils are inherently low in SOC stock, and food security is stronglyrelated to soil quality (Srinivasarao et al., 2012a,b). Therefore, cur-tailing depletion and enriching SOC stock are essential to adaptingand mitigating climate change, buffering agroecosystems in harshclimates against extreme events (drought), and stabilizing agro-nomic productivity by ensuring some returns even during the badseasons.

Crop cultivation adversely affects the distribution and stabil-ity of soil aggregates and reduces SOC stock (Kong et al., 2005).The magnitude of reduction in SOC due to cropping, however,varies among climates and cropping systems (Lal, 2004). Becauseof the prevailing high temperatures, soils of tropics generallyemit more oxidative products (i.e., CO2) per unit SOC stock thanthose of temperate and cooler regions. However, crop species

also play an important role in maintaining quantity and qualityof SOC stock despite diverse nature of crop residues with highlyvariable turnover or residence time in the soil (Mandal et al.,2007).

Ch. Srinivasarao et al. / Europ. J. Agronomy 43 (2012) 40– 48 41

Table 1Monthly rainfall (mm) received during the experimental period in a long-term rainfed groundnut monocropping system.

Month Year

1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

June 9 122 106 10 12 62 123 58 41 3 0 290 116 13 45 121 15 18 8 15July 78 21 2 130 334 18 14 58 18 30 195 69 15 152 17 71 10 7 28 26Aug 31 16 35 253 8 24 6 67 115 39 355 149 70 244 126 104 76 49 77 98Sept 25 132 122 182 231 71 87 61 64 35 150 209 308 105 204 55 242 54 44 52Oct 0 51 134 36 0 104 201 44 152 123 51 254 67 97 99 183 234 175 107 84

230

A

cbaahiocasiSecmtt

3B(icwddtmic2cildtitmtiaptwb

tstnb

(i) Control (no N–P–K fertilizers or organics),(ii) 100% recommended dose of fertilizer (RDF) (20:40:40 N, P2O5,

K2O),

Table 2Mean mineral composition of groundnut shells and farm yard manure used in theexperiment (Mean ± SD).

Nutrients Groundnut shells Farm yard manure

N (%) 1.0 (±0.05) 0.75 (±0.03)P (%) 0.25 (±0.01) 0.2 (±0.01)K (%) 1.1 (±0.06) 0.5 (±0.02)S (%) 0.2 (±0.01) 0.16(±0.02)

Total 143 342 399 611 585 279 431 288 390

verage of 20 years = 450 mm yr−1 (range of 143–971 mm).

The duration and timing of “fallowing” within a cropping systeman also affect the magnitude of SOC stock (Halvorson et al., 2002),ecause of the differences in cropping intensity and specific man-gement practices. Once the pathways of C sequestration in soilsre identified, suitable agricultural strategies may be identified thatave the potential to enhance SOC stocks, attenuate CO2 loading

nto the atmosphere, and mitigate global warming (Lal, 2009). Mostf the research done thus far on SOC sequestration in soils of agroe-osystems is confined to cold and temperate regions. There is little ifny research information available on this theme in the tropical andub tropical regions (Srinivasarao et al., 2012a,b,c,d, 2011a), includ-ng the South Asian countries (Velayutham et al., 2000; Lal, 2004).ome research conducted in the Indian subcontinent (Majumdert al., 2007; Mandal et al., 2007) is mostly limited to irrigated agri-ulture. Yet, there are no experimental data on SOC stock and itsanagement in arid/semiarid climate and rainfed farming prone

o drought stress, high temperatures and low net primary produc-ivity (NPP).

Alfisols occur mainly in southern India, and constitute about0% of the soils under rainfed farming (Virmani et al., 1991).eing light-textured and shallow, their available water capacityAWC) is low. Furthermore, hard-setting and structural instabil-ty exacerbate surface sealing and crusting. These soils in semiaridlimate support a single rainy season crop (kharif or summer)ith productivity levels of 0.7–0.8 Mg ha−1 under semiarid con-itions. These soils are characterized by low SOC and N stocksespite large variations in the cropping system, soil type, rainfall,emperature and supplementary management practices such as

anuring and fertilization. Maintaining soil and crop productiv-ty in the long-term under continuous monocropping is the majorhallenge in rainfed regions of southern India (Srinivasarao et al.,012b,d). Low crop yields, low or no biomass residue retention,oupled with long fallow periods which extend up to 9 monthsn a year, result in adverse environments that do not sustain SOCevels. However, the magnitude of decline or enhancement of SOCue to continuous cultivation depends on the balance betweenhe loss by oxidative forces during tillage; the quantity and qual-ty of crop residues that are returned, and the organics added tohe soils (Srinivasarao et al., 2011b,c). Therefore, crop and soil

anagement practices have to be tailored to ensure long-term sus-ainability. The use of plant nutrients, organic amendments and thenclusion and cultivation of legumes support SOC and its sustain-bility. However, crop residues are used for numerous competingurposes in India, and therefore, not always available for agricul-ure. But some crop by-products such as groundnut shells (GNS)hich do not have any major alternative uses are available as

iofertilizers.Therefore, the objective of the present study was to assess

he impact of soil fertility management on SOC stock under

emiarid conditions. The long-term experiment was designed toest the hypothesis that the magnitude of SOC stock and agro-omic yield in semiarid regions are proportional to the inputs ofiomass-C.

751 971 576 611 491 534 577 303 264 275

2. Materials and methods

2.1. Site description

A long-term field experiment with groundnut monocropping onan Alfisol was initiated in 1985 at the Agricultural Research Station,Anantapur, Andhra Pradesh, India (77◦40′ longitude and 14◦42′ lat-itude, at 350 m mean sea level (MSL). The site is characterized bya hot, semi arid, dry tropical region. This field experiment wasconducted under the aegis of the All India Coordinated ResearchProject on Dryland Agriculture (AICRPDA). During the 20-periodof the experiment (1985–2004), the mean maximum and mini-mum annual air temperatures were 34 ◦C and 21.5 ◦C, respectively.The mean annual precipitation during the 20 years was 566 mm(SD = 218; CV = 38.5%), of which, 70% of the rainfall (399 mm withSD = 178; CV = 44.5%) was received during the rainy season (June-October). Monthly rainfall received during the cropping season over20 years of the experiment is shown in Table 1.

Soil of the experimental site is classified as Rhodustalfs (Voy-alpadu soil series). It is sandy loam in texture, slightly acidicin reaction (pH 6.1), low profile SOC (2.6 g kg−1 soil), low avail-able N (139 kg ha−1), and medium available P (20 kg ha−1) andK (155 kg ha−1) contents. It contains sand, silt and clay of 72.2,5.8 and 22.0%, respectively, soil inorganic carbon (SIC) concentra-tion of 0.3 g kg−1, and cation exchange capacity (CEC) of 14.7 C mol(P+) kg−1 (Srinivasarao et al., 2006, 2009). The site has a gentle slopeof <1%.

2.2. Treatments and crop management

The groundnut (Arachis hypogeae) crop (variety: TMV-2) wasgrown every year during the rainy season (June-October) overthe 20 year period (1985–2004). Tillage consisted of ploughing to0.15–0.20 m depth soon after rainfall in June, followed by blade har-rowing. The experiment was laid out in a randomized block designwith the following treatments:

Ca (%) 1.2 (±0.07) 0.32 (±0.01)Mg (%) 0.35 (±0.02) 0.3 (±0.01)Zn (mg kg−1) 40 (±0.31) 20 (±0.8)B (mg kg−1) 40 (±0.28) 10 (±0.5)

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iii) 50% RDF + 4 Mg ha−1 of GNS,iv) 50% RDF + 4 Mg ha−1 of FYM, and(v) 5 Mg ha−1 FYM.

Each treatment was replicated four times. Crushed GNS andell decomposed FYM were spread uniformly on the surface of

ach plot (10 m × 4.8 m). These amendments were applied on dry-eight basis, and were mixed thoroughly with the soil using aower tiller. Based on the analysis conducted every third year,YM contained 380 g moisture kg−1, and were enriched in plantutrients (Table 2). The C:N ratio of FYM and GNS was 44 and2, respectively. The RDF comprising of N, P and K were applieds urea, diammonium phosphate and muriate of potash, respec-ively. The fertilizer was broadcast and mixed with soil beforeowing. Weeds were controlled by application of a pre-emergenceerbicide (pendimethalin), and one hand weeding as an intercul-ural operation. The average plant population was 21 plants/m2 or10,000 ha−1. After harvesting of groundnut in the first week ofctober, the above ground biomass was removed from the field,nd pod and haulm yields of groundnut were recorded every year.

.3. Soil sampling and analysis

Three representative field-moist soil samples were obtainedith a tube auger from each plot at 0.2 m increments to 1-mepth during February 2005. Samples were composited for eachepth and replication, hand crushed; passed through a 2.0 mmieve; and air dried. Additionally, three samples were obtainedrom all five depths by a core sampler (0.05 m in diameter, 0.08 mn length) to measure bulk density (BD) (Grossman and Reinsch,002). Antecedent (1985) and the final (2005) BD values are shown

n Table 3.Other soil properties (viz., pH, CaCO3 and CEC) were measured

s per the standard procedures (Jackson, 1973). Soil texture wasetermined by Bouyoucos hydrometer method (Bouyoucos, 1927).oil samples were also analyzed for available N (Subbiah and Asija,956), P (Bray and Kurtz, 1945) and K (Hanway and Heidel, 1952).ll determinations were performed in triplicate and the resultsxpressed on the oven-dry basis.

.4. Estimation of carbon in soil and organic materials andomputation of SOC stock

A portion of the soil sample (<2 mm) was air dried, gentlyround, and passed through a 2.0 mm sieve, while the organicsFYM, GNS, leaf, stubbles and roots) were dried in oven in 40 ◦Cnd finely ground in a mechanical grinder (Nelson and Sommers,996). They were analyzed for C by a LECO CHN analyzer (Elementarario EL Cube, DonaustraBe 7, Hanau, 63452, Germany). Soil sam-les were also analyzed for the SIC concentration by back titratinghe excess acid with standard alkali after treating them with diluteCl (Loeppert and Suarez, 1996).

Total SOC stock of the profile (Mg ha−1) for each of theve depths (0–0.2-m, 0.2–0.4-m, 0.4–0.6-m, 0.6–0.8-m, and

C stabilization (%) through plant residue with

C stabilization (%) through organic inputs with refe

.8–1.0-m) was computed by multiplying the SOC con-entration (g kg−1) (obtained by SOC = LECO C-HCl C)y the bulk density (Mg m−3), depth (m), and factor

Agronomy 43 (2012) 40– 48

by 10 (Eq. (1)).

Profile SOC stock = SOC concentration (g kg−1)

×Bulk density (Mg m−3)×depth (m)×10

(1)

2.5. Carbon inputs through plant residues and manure

The annual C inputs to the soil were computed by using theyield of groundnut biomass; leaf-fall, roots, rhizodeposition, andnodules. Leaf-fall from all the treatments was collected manuallyduring the years 1999–2002, from 45 days after sowing till the har-vest. The litter fall was dried, and the dry weight recorded. On anaverage, the leaf-fall in groundnut comprised of 9.1, 16.8, 22.8, 19.3and 13.8% of the above ground biomass for control, 100% RDF, 50%RDF + 4 Mg ha−1 GNS, 50% RDF + 4 Mg ha−1 FYM and 5 Mg ha−1 FYM,respectively.

The root and nodule biomass of groundnut was calculatedusing the root/shoot and nodule/root biomass ratios recordedfrom the experiments. Root biomass was measured immediatelyafter harvesting the crop, following the core-sampling proce-dure (Franzluebbers et al., 1999). The root biomass represented40.7, 37.4, 32.2, 34.5 and 38.1% of the harvestable above groundbiomass in control, 100% RDF, 50% RDF + 4 Mg ha−1 GNS, 50%RDF + 4 Mg ha−1 FYM and 5 Mg ha−1 FYM, respectively. The nodulebiomass was 15.2% of its root biomass with a C concentration of35.3%. Nodule biomass comprised of 11.2, 15.4, 12.9, 15.1 and 13.4%of the groundnut root biomass for control, which were treated with100% RDF, 50% RDF + 4 Mg ha−1 GNS, 50% RDF + 4 Mg ha−1 FYM and5 Mg ha−1 FYM respectively.

During the years 1999–2004, samples of groundnut leaves werecollected after senescence; nodules were excavated 65 days aftersowing, and roots were excavated 85 days after sowing from allreplications in each treatment and analyzed for total C content.Rhizodeposition of C from root turnover and exudates was assumedto be 10% of the harvestable above ground biomass of groundnut(Shamoot et al., 1968). During the growth of the crop, weeds wereeither removed or killed with herbicides and so C inputs from rootsand rhizodeposition by the weeds were not considered. Using allthe measurements described above, a treatment-wise estimate ofplant-derived C inputs, as well as C inputs through organics applied,are shown in Table 4.

2.6. Sample analysis for C and calculations for C budgeting

Sequestration of SOC was computed as per Eqs. (2)–(6) givenbelow:

C build-up (%) = Cfert+org or Cfert − Ccont

Ccont× 100 (2)

where, Cfert+org represents SOC stock of the profile in fertilizerNPK + FYM/GNS treatments and Cfert and Ccont are the SOC stockin fertilizer NPK and control treatments, respectively.

C build-up rate (Mg C ha−1 yr−1) = Cfert+org or Cfert − Ccont

Years of experimentation(3)

ect to control = Cfert+org or Cfert − Ccont

Cres× 100 (4)

to control = Cfert+org or Cfert − Ccont

CFYM or CGNS× 100 (5)

where, Cres represents C input through crop, CFYM or CGNS representC applies through FYM or GNS

C sequestered (Mg C ha−1 soil) = SOCcurrent − SOCinit (6)

Ch. Srinivasarao et al. / Europ. J. Agronomy 43 (2012) 40– 48 43

Table 3Change in soil bulk density (Mg m−3) in different treatments after 20 years of cropping (Mean ± Standard Deviation).

Depth (m) Initial (1985) At the end of experiment (Feb. 2005)

Control 100% RDF 50%RDF + 4 Mg ha−1 GNS 50% RDF + 4 Mg ha−1 FYM 5 Mg ha−1 FYM

0–0.2 1.33 ± 0.01 1.33 ± 0.01Ad 1.32 ± 0.01Ae 1.30 ± 0.01Be 1.31 ± 0.01Bd 1.29 ± 0.01Cd

0.2–0.4 1.35 ± 0.01 1.34 ± 0.01Ad 1.34 ± 0.01Ad 1.32 ± 0.01Bd 1.32 ± 0.01Bd 1.30 ± 0.01Cd

0.4–0.6 1.38 ± 0.01 1.38 ± 0.01Ac 1.38 ± 0.01Ac 1.38 ± 0.01Ac 1.38 ± 0.02Ac 1.38 ± 0.01Ac

0.6–0.8 1.42 ± 0.02 1.42 ± 0.02Ab 1.42 ± 0.02Ab 1.42 ± 0.02Ab 1.42 ± 0.02Ab 1.42 ± 0.02Ab

0.8–1.00 1.45 ± 0.02 1.45 ± 0.02Aa 1.45 ± 0.02Aa 1.45 ± 0.02Aa 1.45 ± 0.02Aa 1.45 ± 0.02Aa

Different capital letters within rows and different small letters within columns are significantly different at P = 0.05 according to Duncan Multiple Range Test (DMRT) forseparation of means.

Table 4Mean (1985–2004) annual C input into the soil for rainfed groundnut under different fertilizer and manurial treatments.

Treatment SYI C input (kg ha−1)

Leaf-fall Root Nodule RD Input through crop residue GNS/FYM Total

Control 0.25 ± 0.018D 95 195 25 190 505E – 505E

100% RDF 0.32 ± 0.022C 265 390 55 341 1051C – 1051D

50% RDF + 4Mg GNS ha−1 0.48 ± 0.034A 419 668 85 609 1781A 1680 3461A

50% RDF + 4Mg FYM ha−1 0.46 ± 0.032A 387 617 79 561 1644B 1320 2964B

5 Mg FYM ha−1 0.38 ± 0.027B 210 349 58 311 928D 1650 2578C

S yard

D ing to

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YI, Sustainable Yield Index; RD, rhizodeposition; GNS, Groundnut shell; FYM, Farmifferent capital letters within columns are significantly different at P = 0.05 accord

here, SOCcurrent and SOCinit indicate the SOC stocks in 2004current) and that onset of the experiment in 1985, respectively.ositive and negative values indicate SOC gains and losses respec-ively.

.7. Sustainable yield index

The total groundnut crop productivity was calculated through austainable-yield index (SYI) using yield-data of 20 years. The SYIas computed to offset annual variations in the yield, and to high-

ight the treatment impact over the 20 year period. The sustainableield index (SYI) is defined as per Eq. (7):

YI = Y − �

Ymax(7)

here, Y is the estimated average yield of a practice across the years. is its estimated standard deviation, and Ymax is the observed max-

mum yield in the experiment during the years of cultivation (Singht al., 1990).

.8. Statistical analysis

Statistical analysis was performed using the Windows basedPSS program (SPSS, 2001, Version 11.0, SPSS, Chicago, IL). ThePSS procedure was used to analyze variance and to deter-ine the statistical significance of treatment effects. The Duncanultiple-range-test was used to compare treatment means. Simple

orrelation coefficients and regression equations were also devel-ped to evaluate the relationships among the response variablesSYI, C inputs, profile SOC, C build up and C sequestration) usinghe same statistical package. The 95% probability level is regardeds significant, statistically.

. Results

.1. Growing season rainfall, pod yield and yield sustainability ofroundnut

Soil fertility management treatments produced significantlyore pod yield than control (P < 0.05). Moving averages of the

roundnut pod yields in relation to mean annual and seasonal

manure. Duncan Multiple Range Test (DMRT) for separation of means.

rainfall during 20 years of cropping, indicate that pod yieldduring the initial 2–3 years was similar in control and the5 Mg ha−1 FYM (Fig. 1). In subsequent years, however, the yieldgap between control and other treatments gradually widened.Higher mean pod yield of groundnut (Mg ha−1) was obtained in 50%RDF + 4 Mg ha−1 FYM (1.03) followed by 50% RDF + 4 Mg ha−1 GNS(1.02), 100% RDF (0.98) and the least in the control (0.78). The SYIwas also higher in 50% RDF + 4 Mg ha−1 GNS (0.48) followed by 50%RDF + 4 Mg ha−1 FYM (0.46), 5 Mg ha−1 FYM (0.38), 100% RDF (0.32)and the least in the control (0.25) (Table 4).

3.2. Change in soil bulk density

Soil BD was lower in treatments receiving organics than min-eral fertilizers and unfertilized control (Table 3). The lowest BDwas observed in 0–0.2 m depth under 5 Mg ha−1 FYM (1.29 Mg m−3)and the highest in 100% RDF (1.33 Mg m−3). However, compared tothe initial values in 1985, BD was maintained in the control butdecreased in all other treatments. Irrespective of the treatment, BDincreased with increase in soil depth.

3.3. Carbon inputs level affected by different managementpractices

The highest C inputs through crop residues (internal) wereadded in 50% RDF + 4 Mg ha−1 of GNS (1781 kg ha−1), followed by50% RDF + 4 Mg ha−1 FYM (1644 kg ha−1) and the lowest was inthe control (505 kg ha−1) (Table 4). Combining external C inputsthrough GNS or FYM, the total C inputs ranged between 505 kg ha−1

in the control to 3461 kg ha−1 (50% RDF + 4 Mg ha−1 of GNS).

3.4. SOC concentration, stock and buildup

The SOC concentration of the soil profile differed signifi-cantly (P < 0.05) among treatments and depths (Table 5). Thehighest SOC concentration (6.2 g kg−1) was measured in 50%RDF + 4 Mg ha−1 GNS followed by that 50% RDF + 4 Mg ha−1 FYM

(5.9 g kg−1), 5 Mg ha−1 FYM (5.8 g kg−1), 100% RDF (4.0 g kg−1) andthe least in control (3.1 g kg−1) in the surface 0–0.2 m depth.Yet, the magnitude of SOC concentration in the manure-amendedtreatments was statistically on par with one another. Increase

44 Ch. Srinivasarao et al. / Europ. J. Agronomy 43 (2012) 40– 48

0

0.5

1

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2

2.5

0

200

400

600

800

1000

1200

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dn

ut

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ield

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Annual rainfall Crop Season rainfall 100% RDF

50%RDF+ 4 Mg ha-1 GNS 50%RDF+4 Mg h a-1 FYM 5 Mg h a-1 FYM

Control

lds of

itwtRamt

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Fig. 1. Mean annual and seasonal rainfall in relation to pod yie

n SOC concentration with reference to control was less withhe sole application of inorganic fertilizers (100% RDF) comparedith those receiving organics. The mean SOC concentration in

he profile ranged from 2.3 g kg−1 in control to 3.5 g kg−1 in 50%DF + 4 Mg ha−1 GNS. Significant differences in SOC concentrationmong treatments were observed even in the sub-soil layers. Theean SOC concentration decreased from 4.9 g kg−1 in the 0–0.2 m

o 1.6 g kg−1 in the 0.8–1.0 m depth.Total SOC stock in the profile was also significantly

P < 0.05) improved, and the highest stock was measured in0% RDF + 4 Mg ha−1 GNS (47.2 Mg C ha−1) followed by that

n 50% RDF + 4 Mg ha−1 FYM (45.9 Mg C ha−1), 5 Mg ha−1 FYM42.4 Mg C ha−1), 100% RDF (36.2 Mg C ha−1) and the lowestn the control (32.2 Mg C ha−1) (Table 6). The application of0% RDF + 4 Mg ha−1 GNS registered the highest SOC build up46.6%) followed by that in 50% RDF + 4 Mg ha−1 FYM (42.5%),

Mg ha−1 FYM (31.7%) and the lowest in 100% RDF (12.4%) com-ared with the control (Table 6). The SOC build up rate over 20ears of cropping also followed a similar trend, with the highestate in 50% RDF + 4 Mg ha−1 GNS (0.750 Mg C ha−1 yr−1) and theowest was in 100% RDF (0.200 Mg C ha−1 yr−1) (Table 6). Theuildup of SOC in different treatments is proportional to theotal C inputs viz., 100% RDF (1051 kg vs. 505 kg ha−1 in con-rol), 50% RDF + 4 Mg ha−1 GNS (3461 kg C vs. 505 kg), 50% RDF + 4

g ha−1 FYM (2964 kg vs. 505 kg), and 5 Mg ha−1 FYM (2578 kg vs.05 kg) (Table 4).

.5. SOC sequestration and stabilization

Monocropping of groundnut over 20 years in Alfisols underemiarid conditions without any organic and/or inorganic fertil-zer (control) depleted the SOC stock by 3.57 Mg C ha−1 (Table 6).

able 5hanges in SOC (g kg−1) concentration under different treatments after 20 years of mono

Treatment Depth (m)

0–0.2 0.2–0.4 0.

Initial SOC 3.4 ± 0.15 2.9 ± 0.11 2

Control 3.1 ± 0.14Ca 2.6 ± 0.12Cb 2100% RDF 4.0 ± 0.18Ba 3.2 ± 0.14Bb 250% RDF + 4 Mg ha−1 GNS 6.2 ± 0.29Aa 4.6 ± 0.22Ab 350% RDF + 4 Mg ha−1 FYM 5.9 ± 0.27Aa 4.3 ± 0.20Ab 25 Mg ha−1 FYM 5.8 ± 0.26Aa 3.9 ± 0.20Ab 2Mean 4.9 ± 0.22a 3.7 ± 0.16b 2

ifferent capital letters within columns and different small letters within rows are signeparation of means.

groundnut across the treatments over 20 years (1985–2004).

However, the SOC stock was maintained at the antecedentlevel (an increase of 0.43 Mg C ha−1) by 100% RDF application(Table 6). Yet, the addition of organic manures, either alone,or in combination with inorganic fertilizers, sequestered C atmuch higher levels. The highest amount of C sequestered was inthe 50% RDF + 4 Mg ha−1 GNS (11.43 Mg C ha−1) followed by thatin 50% RDF + 4 Mg ha−1 FYM (10.13 Mg C ha−1) and 5 Mg ha−1 FYM(6.63 Mg C ha−1).

The highest fraction of C stabilization through input of cropresidues was measured in 5 Mg ha−1 FYM (55%) followed by thatin 50% RDF + 4 Mg ha−1 FYM (42.1%), 50% RDF + 4 Mg ha−1 FYM(41.7%), and the lowest in 100% RDF (19%) as compared to the con-trol. In comparison, the highest fraction of C stabilization throughexternal C inputs was measured in the 50% RDF + 4 Mg ha−1 FYM(51.9%) followed by that in the 50% RDF + 4 Mg ha−1 GNS (44.6%),and the lowest in 5 Mg ha−1 FYM (30.9%). The total magnitude of Cstabilization also followed a similar trend, with the highest valuemeasured in 50% RDF + 4 Mg ha−1 FYM followed by that in the 50%RDF + 4 Mg ha−1 GNS.

4. Discussion

4.1. Rainfall, pod yield of groundnut and sustainable yield index

Under arid and semi-arid conditions, crop yields are stronglyinfluenced by rainfall (Srinivasarao et al., 2012a,b). In the presentstudy, however, groundnut pod yields were not correlated withthe total or seasonal rainfall. Yet, there existed a strong corre-

lation between the pod yield and the rainfall received at thetwo critical stages of the crop growth-pegging (r = 0.47; P < 0.05)and pod formation (r = 0.50; P < 0.05) (Fig. 2). Further, applicationof organics in the form of FYM/GNS along with chemical fertil-

cropping of groundnut.

Mean

4–0.6 0.6–0.8 0.8–1.0

.9 ± 0.12 2.1 ± 0.09 1.7 ± 0.07 2.6 ± 0.12

.6 ± 0.12Bb 1.9 ± 0.09Bc 1.5 ± 0.07Bd 2.3 ± 0.12D

.6 ± 0.12Bc 1.9 ± 0.09Bd 1.5 ± 0.07Be 2.6 ± 0.13C

.0 ± 0.15Ac 1.9 ± 0.09Bd 1.6 ± 0.07Be 3.5 ± 0.18A

.7 ± 0.12Bc 2.1 ± 0.11Ad 1.8 ± 0.08Ad 3.4 ± 0.17A

.9 ± 0.13Ac 1.9 ± 0.09Bd 1.7 ± 0.08Bd 3.2 ± 0.14B

.8 ± 0.13c 1.9 ± 0.10d 1.6 ± 0.09d

ificantly different at P = 0.05 according to Duncan Multiple Range Test (DMRT) for

Ch. Srinivasarao et al. / Europ. J. Agronomy 43 (2012) 40– 48 45

Table 6Profile SOC, SOC buildup, SOC buildup rate, C stabilization and C sequestered of the soil profile under different fertilization treatments over 20 years.

Treatment Profile SOC(Mg ha−1)

C buildup (%) C buildup rate(Mg C ha−1 yr−1)

C stabilization (%) C Sequestered(Mg C ha−1)

Mean CSequestration rate(Mg C ha−1 yr−1)

Throughcropresidue

Throughexternalinput

Total

Control 32.2 ± 2.5D – – – – – −3.57 ± 0.45D −0.18 ± 0.01D

100% RDF 36.2 ± 2.7C 12.4 ± 1.6D 0.200 ± 0.015C 19.0 ± 1.6C – 19.0 ± 1.6C 0.43 ± 0.04C 0.02 ± 0.00C

50% RDF + 4 Mg ha−1 GNS 47.2 ± 3.8A 46.6 ± 4.1A 0.750 ± 0.055A 42.1 ± 3.2B 44.6 ± 3.7B 21.7 ± 1.8B 11.43 ± 0.91A 0.57 ± 0.03A

50% RDF + 4 Mg ha−1 FYM 45.9 ± 3.4A 42.5 ± 3.8B 0.685 ± 0.049A 41.7 ± 2.9B 51.9 ± 4.3A 23.1 ± 1.9A 10.13 ± 1.01A 0.51 ± 0.03A

5

D uncan

irtpm

4

ieonR

4a

tcaa

Sptp5amtt

Fa

5 Mg ha−1 FYM 42.4 ± 3.1B 31.7 ± 2.7C 0.510 ± 0.043B

ifferent letters within columns are significantly different at P = 0.05 according to D

zer significantly improved SYI of groundnut compared to use ofecommended rate of NPK (Table 4; Section 3.1), probably dueo a high moisture retention capacity in organic manure treatedlots as well as better nutrient supply by the integrated nutrientanagement treatment (Srinivasarao et al., 2011a,b,c).

.2. Change in soil bulk density

Soil BD decreased with the application of FYM because ofncrease in SOC concentration and the root biomass (Halvorsont al., 1999). The attendant increase in aggregation and macrop-rosity improved soil tilth and aeration (Du et al., 2009). Soil BD wasegatively correlated with SOC concentration (Y = −0.01X + 1.36;2 = 0.82; r = 0.91; P < 0.05).

.3. Carbon inputs level, SOC concentration, stock and buildupnd their relationships with C input

Amounts of C inputs through crop residues were proportionalo the nutrient supply. Higher biomass and C input in 50% RDFombined with 4 Mg GNS or FYM could be due to increased avail-bility of deficient nutrients (i.e., N, K, Ca, Mg, S, Zn and B) with thepplication of organic manure (Srinivasarao and Vittal, 2007).

Application of inorganic fertilizers (100% RDF) also improved theOC concentration in the surface 0–0.2 m depth and maintained therofile mean SOC concentration at the antecedent level. Applica-ion of different amendments over 20 years of cropping improvedrofile mean SOC concentration over control by 13% in 100% RDF,2% in 50% RDF + 4 Mg ha−1 GNS, 48% in 50% RDF + 4 Mg ha−1 FYM

−1

nd 39% in 5 Mg ha FYM (Table 5). Twenty years of continuousonocropping without any inorganic or organic inputs depleted

he SOC concentration and stock in the surface soil comparedo the antecedent level (3.4 g kg−1). The plough-induced pertur-

y = 3.9x + 676.4r=0.47; P<0.05

y = 3.9x + 584.2r=0.50; P<0.05

0200400600800

100012001400160018002000

0 50 100 150 200

Pod

yiel

d (k

g ha

-1)

Rainfall (mm)

Rainfall (mm) received at pegging stage

Rainfall (mm) received at pod forma�on stage

ig. 2. Relationship between rainfall received at pegging and pod formation stagend mean pod yields of groundnut between 1985 and 2004.

5.0 ± 4.3A 30.9 ± 2.7C 19.8 ± 1.6C 6.63 ± 0.53B 0.33 ± 0.02B

Multiple Range Test (DMRT) for separation of means.

bation decreases amount and stability of aggregates (Sparlinget al., 1992), accentuates SOC oxidation (Mandal et al., 2007), andaggravates losses by erosion (Lal, 2011). The loss in SOC concen-tration by ploughing has been widely reported (Lal, 2009), but themagnitude of depletion depends upon the geographical location,crops/cropping systems, inherent soil properties, cropping dura-tion and land use, etc. (Davidson and Ackerman, 1993; Guo andGifford, 2002; Mandal et al., 2008; Srinivasarao et al., 2012a,b,c,d;Srinivasarao et al., 2011a).

Total C input was significantly correlated with the profile meanSOC concentration (Y = 0.0004X + 2.12, R2 = 0.99; r = 0.99; P < 0.05),SOC stocks in the profile (Y = 0.005X + 31.78; R2 = 0.48; r = 0.69;P < 0.05) explaining 99 and 48%, respectively, of the total variabil-ity (Table 7). Similarly, total SOC sequestered over 20 years wasstrongly correlated with total C inputs (Y = 0.005X − 5.57; R2 = 0.98;r = 0.99; P < 0.05), explaining 98% of the variability in C sequestered.A higher C retention in manure-amended than in other plots wasprobably because of a high degree of decomposition and thus a highproportion of chemically recalcitrant organic compounds (Paustianet al., 1992).

A significant relationship between the crop residue C, exter-nal C, and the total organic C in the profile indicated that theC input positively influences both the SOC stock and C buildup. However, the availability of adequate quantities of organicamendments under arid and semiarid conditions is a major con-straint due to a lower biomass production; the competing use ofdung as a fuel, and crop residues as animal feed and other uses.Residues estimated at 0.6–0.8 Tg (million Mg) of GNS are avail-able in Anantapur district annually The GNS are presently beingburnt, but contain plant available nutrients (Table 2), and can beused as an amendment to improve SOC, and increase crop yield.The GNS can also be used to generate energy by pyrolysis andbiochar thus produced can be fortified with fertilizer and used as anamendment.

4.4. Sustainable yield index and its relationships with C inputsand SOC

The trend in SYI was similar to that of the SOC stock.Thus, there existed a positive relationship between SOC stockto 1-m depth and cumulative yield of the groundnut pod(13 kg ha−1 yr−1 Mg−1 of SOC) (Fig. 3). A highly significant cor-relation was observed between SYI and internal C inputs (cropresidues) (Y = 0.0002X + 0.17; R2 = 0.91; r = 0.95; P < 0.05), total Cinputs (−0.0001X + 0.222, r = 0.97; R2 = 0.95, P < 0.05), profile meanSOC concentration (Y = 0.18X − 0.17; r = 0.99; R2 = 0.98, P < 0.05) and

SOC stock (Y = 0.008X + 0.051; r = 0.70; R2 = 0.48, P < 0.05) (Table 8).Thus, the maintenance of SOC through regular inorganic or organicinputs determines the sustainability of rainfed production systemsin arid and semiarid environments.

46 Ch. Srinivasarao et al. / Europ. J. Agronomy 43 (2012) 40– 48

Table 7Relationships of profile SOC concentration, SOC stock and carbon sequestration rate with total C input over 20 years.

Parameters Regression equation Correlation

(r) (R2)

Total C inputs (X) Profile mean C content (Y) Y = 0.0004X + 2.12 0.99 0.99***Total C inputs (X) Profile C stocks (Y) Y = 0.005X + 31.78 0.69 0.48*Total C input (X) CSR (Y) Y = 0.005X − 5.57 0.99 0.98***

*, **, *** denotes significance at p < 0.05, 0.01 and 0.001, respectively.

Table 8Relationships of C inputs, profile C content, stocks, sequestration rate and different forms of carbon with SYI in 20 years old long term experiment.

Parameters Regression equation Correlation

(r) (R2)

Internal C inputs (X)

SYI(Y)

SYI = 0.0002X + 0.17 0.95 0.91***Total carbon inputs (X) SYI = −0.00007X + 0.222 0.97 0.95***Profile mean C content (X) SYI = 0.18X − 0.17 0.99 0.98***Profile C stocks (X) SYI = 0.008X + 0.051 0.70 0.48*

I = 0.0

*

4i

s(tataaCda−(

icc(TsCt2p

Ft

CSR (X) SY

, **, *** denotes significance at p < 0.05, 0.01 and 0.001, respectively.

.5. SOC sequestration, stabilization and requirement of critical Cnputs

Monocropping of groundnut over 20 years in Alfisols underemiarid conditions without any organic and/or inorganic fertilizercontrol) depleted the SOC stock by 3.57 Mg C ha−1 (Table 6). Highemperatures (45–50 ◦C during summer) and ploughing accentu-te SOC oxidation. However, the SOC stock was maintained athe antecedent level (an increase of 0.43 Mg C ha−1) by 100% RDFpplication (Table 6). Yet, the addition of organic manures, eitherlone, or in combination with inorganic fertilizers, sequestered

at much higher levels. The C sequestration potential (CSP),efined as the rate of increase in the SOC stock vis-à-vis thentecedent baseline stock in the 0–0.2 m depth, ranged from0.18 Mg C ha−1 yr−1 (unfertilized control) to 0.57 Mg C ha−1 yr−1

50% RDF + 4 Mg ha−1 GNS) (Bhattacharyya et al., 2009).Significant and positive linear relationships between changes

n SOC and total cumulative C inputs (external organics plusrop residue) (Y = 0.25X − 5.57; R2 = 0.99, P < 0.001) (Fig. 4) indi-ate that even after 20 years of continuously adding biomass0.505–3.461 Mg C ha−1 yr−1), the C sink capacity is not saturated.herefore, with the extremely low antecedent levels of SOC, theseoils have a large C sink capacity and high technical potential of

sequestration. Each soil with a different C loading might lead to

he attainment of a new steady state SOC level over time (Six et al.,002). Assessment of SOC stock for these treatments at periodic,erhaps at decadal intervals, might provide insights into strategies

y = 0.013x + 0.414R² = 0.69; P< 0.05

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

30 35 40 45 50

Gro

undn

ut P

od Y

ield

s (M

g ha

-1)

Profile SOC St ock (Mg ha-1 )

ig. 3. Influence of profile SOC stocks on yield of groundnut pods in 20 years longerm experiment under semiarid conditions.

15X + 0.30 0.99 0.98***

of C management in these soils. The slope of the curve (Fig. 4) repre-sents the rate of conversion of biomass-C inputs into the SOC stock,which is about 25% of each additional Mg of C input ha−1 in thegroundnut mono-cropping system. Although for a different ecore-gions, these results are comparable to those reported by Rasmussenand Collins (1991) (14.0–21.0%) from a temperate regions of USAand Canada, but higher than those obtained by Kong et al. (2005)(7.6%) under Mediterranean climate, and from the humid Indo-Gangetic plains of India (14%) under irrigated rice–wheat system(Majumder et al., 2008). The analyses of data in Fig. 4 also showthat the critical amount of C input to the soil required for maintain-ing the SOC levels (zero change) is 1.12 Mg C ha−1 yr−1 for Alfisolsunder a groundnut cropping system. However, the antecedent levelis extremely low, and further enhancement to attain the thresh-old level may necessitate substantial inputs of biomass-C. Thelower levels of input of optimal C needed to maintain levels sim-ilar to the antecedent SOC under the present study, may be dueto lower initial SOC levels (the profile mean SOC concentration of2.6 g kg−1) (Srinivasarao et al., 2006). The quantity of crop residueinput (1.05 Mg ha−1 yr−1) in the 100% RDF treatments, maintainedSOC (no significant change) over 20 years of cropping.

In view of the required C input of 1.12 Mg C ha−1 yr−1, a bal-anced fertilization (100% RDF) with 1.05 Mg C ha−1 yr−1 annualinput, could also sustain the SOC at the antecedent level, although

it is extremely low and must be increased. However, the additionof organic manure in the form of 4 Mg ha−1 yr−1 GNS or FYM alongwith 50% RDF improved the SOC and maintained a higher rate ofSOC sequestration. Thus, these data suggest that 4 Mg ha−1 yr−1

y = 0.25x - 5.57R² = 0.99; P<0.001

-15

-10

-5

0

5

10

15

0 20 40 60 80

C Se

ques

tere

d (M

g ha

-1)

Cumula�ve C input (Mg ha-1 )

1.12 Mg C ha-1

yr-1

Fig. 4. Critical C input value and its influence on SOC sequestration in groundnutmonocropping system under semiarid conditions.

rop. J.

iiI

5

omot3omammtirtanHc

mma∼epc

A

R

R

B

B

B

D

D

F

G

G

H

H

H

JK

Ch. Srinivasarao et al. / Eu

nput of biomass/organic manure along with inorganic fertilizerss essential to improve soil quality in the hot semiarid tropics ofndia and elsewhere under similar soils and climatic conditions.

. Conclusions

Higher sustainable yields were obtained with the integrated usef nutrients involving 4 Mg ha−1 of groundnut shells, or farm yardanure along with 50% RDF ha−1 yr−1 (10–20–20 kg N:P2O5:K2O)

f chemical fertilizer compared with the unfertilized control. Con-inuous cropping without inputs resulted in the net depletion of.57 Mg C ha−1 during the 20 year period, but a substantial amountf SOC was sequestered with regular additions of organic amend-ents. Application of balanced dose of NPK could maintain the SOC

t the antecedent level and arrested the further depletion. Thateans ∼1 Mg C ha−1 required to maintain soil C levels is approxi-ately equivalent to the residues from the recommended fertilizer

reatment. But to sequester higher SOC in the soil, external C inputn the form of manure/amendments are required. Groundnut shells,eadily available in the region (0.60–0.80 Tg annually in this dis-rict) can be used as a soil amendment to increase pod yieldsnd enhance SOC stock. A critical C input of 1.12 Mg C ha−1 yr−1 iseeded to maintain SOC concentration at equilibrium (no change).owever, higher rates of biomass-C are needed to raise the SOConcentration above the threshold level (1.1–1.5%).

The data support the conclusion that continuous groundnutono cropping in the Alfisol regions of hot, semiarid, tropical cli-ate is possible with balanced use of N–P–K fertilizer, along with

dequate amounts of organics such as groundnut shells or FYM at4 Mg ha−1 yr−1. Application of chemical and organic fertilizers cannhance SOC stock, improve soil quality, and sustain agronomicroductivity even in these soils of low inherent fertility and harshlimate.

cknowledgement

The authors are thankful to Indian Council of Agriculturalesearch (ICAR), New Delhi for funding the project.

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