long-term manuring and fertilizer effects on depletion of soil organic carbon stocks under pearl...

land degradation & developmentLand Degrad. Develop. (2011)

Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ldr.1158

LONG-TERM MANURING AND FERTILIZER EFFECTS ON DEPLETION OF SOILORGANIC CARBON STOCKS UNDER PEARL MILLET-CLUSTER BEAN-CASTOR

ROTATION IN WESTERN INDIA

CH. SRINIVASARAO1*, B. VENKATESWARLU1, R. LAL2, A. K. SINGH3, S. KUNDU1, K. P. R. VITTAL4,J. J. PATEL5 AND M. M. PATEL5

1Central Research Institute for Dryland Agriculture, Santoshnagar, Saidabad (P.O.), Hyderabad 500 059, Andhra Pradesh, India2Carbon Management and Sequestration Center, The Ohio State University, Columbus, OH 43210, USA

3Indian Council of Agricultural Research, Krishi Anusandhan Bhawan (KAB-II), New Delhi, 110 012, India4National Institute for Abiotic Stress Management, Baramati, 413 115 Maharashtra, India

5All India Coordinated Research Project on Dryland Agriculture, Sardar Krushinagar, 385 506 Gujarat, India

Received: 18 July 2011; Revised: 7 October 2011; Accepted: 14 October 2011

ABSTRACT

Soil organic carbon (SOC) pools are important for maintaining soil productivity and reducing the net CO2 loading of the atmosphere. An 18-yearold long-term field experiment involving pearl millet-cluster bean-castor sequence was conducted on an Entisol in western India to examine theeffects of chemical fertilizers and manuring on carbon pools in relation to crop productivity and C sequestration. The data showed that even theaddition of 33.5Mgha�1C inputs through crop residues as well as farm yard manure could not compensate the SOC depletion by oxidation andresulted in the net loss of 4.4MgCha�1 in 18years. The loss of SOC stock in the control was 12MgCha�1. Conjunctive use of chemicalfertilizers along with farm yard manure produced higher agronomic yields and reduced the rate of SOC depletion. The higher average seed yieldsof pearl millet (809 kgha�1), cluster bean (576), and castor (827) over six cropping seasons were obtained through integrated use of fertilizers andmanure. For every Mg increase in profile SOC stock, there was an overall increase of 0.46Mg of crop yield, comprising increase in individualyield of pearl millet (0.17Mgha�1 y�1Mg�1 SOC), cluster bean (0.14) and castor (0.15). The magnitude of SOC build up was proportional tothe C inputs. Carbon pools were significantly correlated with SOC, which increased with application of organic amendments. Threshold C inputof 3.3MgCha�1 y�1 was needed to maintain the SOC stock even at the low antecedent level. Copyright © 2011 John Wiley & Sons, Ltd.

keywords: soil amendments; carbon sequestration; carbon pools; FYM; sustainable yield index; semi-arid tropics; India; soil organic carbon

INTRODUCTION

Soil organic matter (SOM) plays an important role in ecosys-tem services (Loveland and Webb, 2003; Lal, 2011) and inagronomic yields (Tiessen et al., 1994). Although the worldis facing the challenge of climate change because of therapidly increasing CO2 in the atmosphere, accumulation ofSOM and, hence, C sequestration (Schlesinger, 2000) havereceived attention since 1990s as a climate change mitigationoption at global (FAO, 2001) and regional scales (Smith,2004). Indeed, soil C sequestration is an important optionnot only to mitigate climate change but also to enhance soilfertility and the productivity of agro-ecosystems (Daweaet al., 2003; Janzen, 2006; Manlay et al., 2007). Therefore,maintenance of site-specific soil organic carbon (SOC)concentration above the threshold level is a prerequisite topreserving soil functions. Quantity and quality of SOC are

*Correspondence to: CH. Srinivasarao, Central Research Institute forDryland Agriculture, Santoshnagar, Saidabad (P.O.), Hyderabad 500 059,Andhra Pradesh, India.E-mail: [email protected]

Copyright © 2011 John Wiley & Sons, Ltd.

important indicators of soil health and productivity of agro-ecosystems (Liu et al., 2005).The threshold level of SOC concentration in cropland soils

in the temperate region is ~ 2 per cent (Loveland and Webb,2003), and soils may be at risks of structural collapse atSOC concentrations of <1 per cent. Some researchers haveargued that it would be difficult to attain the maximumagricultural production potential, regardless of the soil type,at SOC concentrations of <1 per cent (Kay and Angers,1999). Yet, a survey of SOC stocks conducted at 21 locationsacross the rain-fed tropics of India, covering eight productionsystems, showed that these soils contain low SOC concentrations(<0.5 per cent) and profile-based SOC stocks (Srinivasaraoet al., 2009). The SOC stock is comprised of labile or activelycycling pool and stable or passive/recalcitrant pools withvarying residence time. The labile pool is the fraction ofSOC with most rapid turnover rates. Its oxidation drives theflux of CO2 from soil to the atmosphere, fuels the soil foodweb and, therefore, greatly influences nutrient cycling formaintaining soil health and productivity (Chan et al., 2001;Mandal, 2005; Majumder et al., 2007; Mandal et al., 2007).

CH. SRINIVASARAO ET AL.

Important components of labile pools used as indicators of soilhealth include: microbial biomass carbon (MBC), mineraliz-able C, and oxidizable organic C fractions. Particulate organicC (POC) is one of the important components of the labile or-ganic C pool. Whereas, dissolved organic carbon (DOC) isubiquitous in terrestrial and aquatic ecosystems, it representsonly a small proportion of the total organic carbon (TOC) insoils. It also plays a significant role in influencing the dynam-ics and interactions of nutrients and contaminants in soils andmicrobial functions, thereby serving as a sensitive indicator ofshifts in ecological processes (Bolan et al., 2011). This highlyrecalcitrant or passive pool is slowly altered by microbialactivities (Weil et al., 2003) and is not a good indicator ofagronomic productivity.Maintaining soil and crop productivity in the long-term

under continuous cropping is a major challenge in rain-fedarid and semi-arid regions of India, where summer tempera-ture of up to 50 �C are experienced for about a month(May). Low crop yields, and low or no biomass residues,coupled with long fallow periods that extend up to 7monthsin the year, result in adverse environments that do not sustainSOC levels. Therefore, crop and soil management practicesmust be designed to ensure long-term sustainability of crop/cropping systems. Addition of plant nutrients, organicamendments and inclusion of legumes in the rotation cycleenhance SOC and its sustainability. A rotation based oncereal-legume-non-edible oilseeds is profitable in the semi-arid part of Gujarat, India (Vittal et al., 2003). Pearl millet(Pennisetum glaucum), the fourth most important crop inIndia, is traditionally a component of the dry land systemand is grown in soils of low inherent fertility. The pearlmillet-based production system cover 9.8 million hectare(Mha) under tropical conditions of India (FAO, 2008). Re-search data on restoring SOC stock and biological activitythrough cropping, fertilization, and various nutrient manage-ment practices are widely available for soils of temperateregions (Angers et al., 1993). However, few if any, studieshave been conducted under tropical conditions where the turn-over rate of SOM is high (Ayanaba and Jenkinson, 1990;Goyal et al., 1993; Chander et al., 1997; Wani et al., 2003).Several studies have been conducted in soils of cooler

climates to assess changes in labile pools of SOC because ofdifferent management practices (Franzluebbers and Arshad,1996; Wu et al., 2003; Sherrod et al., 2005). In this context,data from long-term experiments are useful to monitorchanges in SOC stocks as influenced by different croppingsystems, soil management practices, fertilizer usage, C inputsfrom crop residues, and the like, because short-term experi-ments may not have discernible effects even on labile frac-tions. A few studies on soils of the tropics and subtropicshave indicated the importance of SOC stock on soil healthand long-term sustainability of crop production (Singh et al.,2004; Chaudhury et al., 2005; Mandal, 2005; Rudrappa


et al., 2005; Sharma et al., 2005; Majumder et al., 2007;Mandal et al., 2007). But, such studies are scanty for soils ofthe arid and semi-arid regions.Therefore, the present study was designed to assess the

effects of 18 years of chemical fertilizers and application offarm yard manure (FYM) on SOC stock, magnitude ofdifferent fractions, impact on agronomic productivity, andthe rate of SOC sequestration in an Entisols under semi-aridtropical conditions in western India.

MATERIALS AND METHODS

Details of the Experimental Location

A long-term field experiment with pearl millet-cluster bean orguar (Cyamopsis tetragonoloba)-castor (Ricinus communis)sequence (each crop was grown once in 3 years) wasconducted on Entisols. The site is located in the hot, semi-arid,dry tropics. The experiment was initiated in the rainy season of1988 at the Dryland Research Station, Sardar Krushi Nagar,Gujarat, India (24�30′ N latitude, 72�13′ E longitude at152.5m above mean sea-level) under the All India Coordi-nated Research Project on Dry land Agriculture. The experi-mental location falls under the agro-ecological region 2.Under this region, 36 Mha area is covered (Velayuthamet al., 1999). During the experimental period (1988–2005),the annual mean maximum and minimum air temperaturesat this location were 30.2 �C� 4.2 and 18.7 �C� 6.7, respec-tively. The mean annual precipitation for the 18 years was550mm� 333, of which 75 per cent was received during therainy season from June to August. The length of the growingperiod is 60–90 days. The soil is sandy-loam in texture(Entisol), slightly alkaline in reaction (pH 8.2) and it has alow profile SOC concentration (2.0 g kg�1 soil), low availableN (114.2 kg ha�1), medium available P (18.4 kg ha�1) and lowavailable K (94.1 kg ha�1). It has sand, silt and clay contentsof 85.4, 4.0 and 10.6 per cent, respectively, inorganic C con-centration of 13.4 g kg�1, and cation exchange capacity of7.28Cmol (+) kg�1 (Srinivasarao et al., 2006, 2009).

Treatments and Crop Management

Pearl millet-cluster bean-castor crop rotation was followedduring the rainy season (June–October) for the 18-year period(1988–2005). Crops were established with shallow tillageconsisting of plowing to an average depth of 0.15–0.20msoon after the first rainfall in the last week of June followedby blade harrowing. The experiment was laid out in a random-ized block design with the following treatments: T1-no fertil-izer (control); T2-100 per cent recommended dose of N(RDN) through chemical fertilizer (F); T3-50 per cent RDN(F); T4-50 per cent RDN through FYM; T5-50 per cent RDN(F) + 50 per cent RDN(FYM); T6-farmers method (5Mg ofFYMha�1 once in 3 years). Recommended varieties for theregion that were grown in this study were GHB 558, GCH 5

LAND DEGRADATION & DEVELOPMENT (2011)

LONG-TERM MANURING AND FERTILIZER EFFECTS ON SOC STOCKS

and RGC 1017 for pearl millet, cluster bean and castor, respec-tively. A spacing of 45� 12, 60� 30 and 90� 45 cm wasadopted for pearl millet, cluster bean and castor, respectively.Weeding was performed manually in all the plots as uniformlyas possible. The RDN at the rate of 80, 20, and 60kgha�1

was applied as urea to pearl millet, cluster bean and castor,respectively. Each treatment was replicated four times. Well-decomposed FYM was manually spread uniformly on thesurface of the specified plots (8� 6m) on a dry weight basisand mixed thoroughly in the soil using a power tiller. On thebasis of the analysis carried out once every third year, FYMincluded 380gkg�1 moisture and had a C :N ratio of 56.Fertilizer was broadcast and mixed in the soil before sowing.Grain and stover yields of the crops were recorded every yearand reported at air-dry moisture content.

Soil Sampling and Analysis

Three representative field-moist soil samples were collectedwitha tube-auger duringMay, 2006 at 0.2m increments to 1-m depthfrom each plot and replication. Samples were composited foreach depth and replication, then hand-crushed, passed througha 2-mm sieve, stored at 4 �C, and used within 24h for estimatingthe MBC. A portion of the field-moist sample was air-dried,finely ground, passed through 0.2mm sieve, and used for analy-sis of C and total N concentrations. Additional triplicate sampleswere taken from all five depths using a core sampler (0.05m indiameter, 0.08m in length) for measuring soil bulk density(Blake and Hartge, 1986). All determinations were performedin triplicate, and the results were expressed on oven-dry-basis.

Estimation of Total and Soil Organic Carbon Stock

Soil samples were air-dried, ground, and passed through a2.0mm sieve, and the visible organics (FYM, leaf, stubbleand roots) were dried, finely ground in a mechanical grinder,and analyzed for total C using a LECO CHN analyzerW (FossHeraeus Elementar Analyzer CHN-O-RAPID, Hanau,Germany) (Nelson and Sommers, 1996). Soil samples werealso analyzed for inorganic C by back titrating the excessacid with standard alkali after treating them with dilute HCl(Bundy and Bremner, 1972).Total SOC stock of the profile (Mgha�1) for each of the

five depths (0–0.2, 0.2–0.4, 0.4–0.6, 0.6–0.8, and 0.8–1.0m)was computed by multiplying the SOC concentration (g kg�1)(obtained by SOC=LECO C-HCl C) by the bulk density(Mgm�3), depth (m), and factor by 10 (Equation 1).

Profile SOC stock = SOC concentration (g kg�1)� bulkdensity (Mgm�3)� depth mð Þ � 10 (1)

Analysis of Total Nitrogen, Microbial Biomass Carbon andParticulate Organic Carbon

Soil samples were analyzed for total N by Kjeldahl digestion(Bremner, 1960), microbial biomass carbon by fumigation


extractionmethod (Vance et al., 1987), and POC (Cambardellaand Elliott, 1992). Themicrobial quotient (MQ) was calculatedas the ratio of MBC :TOC. POC/TOC was computed andexpressed as per cent.

Carbon Inputs through Plant and Manure

On the basis of biomass yield of pearl millet, cluster bean andcastor, annual C inputs to the soil through leaf-fall, stubble,roots, rhizodeposition, and nodules were computed. Leaf-fallfrom all treatments was manually collected for cluster beanand castor during 1999–2002 from 45days after sowing untilharvest, dried, and the dry weight recorded. On average, theleaf-fall contributed 14.1, 18.2, 16.4, 17.5, 20.4, and 16.4 percent of total harvestable above ground biomass in cluster beanfor control, 100 per cent RDN(F), 50 per cent RDN (F),50 per cent RDN (FYM), 50 per cent RDN (F) +50 per centRDN (FYM), and farmers method (5Mg of FYMha�1 oncein 3 years), respectively. Relative contribution of leaf-fallbiomass to the residue C inputs was 14.8, 21.3, 17.4, 18.3,22.5, and 17.5 per cent, respectively in the treatments listedabove. Leaf samples contain 41.1 and 35.4 per cent C incluster bean and castor, respectively. Pearl millet stubbleconstituted 2.8, 2.4, 2.1, 2.3, 2.6, and 2.2 per cent of the aboveground stover in control, 100 per cent RDN (F), 50 per centRDN (F), 50 per cent RDN (FYM), 50 per cent RDN (F) +50per cent RDN (FYM) and farmers method (5Mg of FYMha�1

once in 3 years), respectively. Stubble of pearl millet contains41.9 per cent C.Root biomass in three crops and nodule biomass in cluster

bean was calculated using the root: shoot and nodule: rootbiomass ratios recorded from the experiments. Root biomasswas measured immediately after harvest of the crop followingthe core-sampling procedure (Franzluebbers et al., 1999). Theroot biomass represented 18.9, 18.0, 17.2, 16.2, 15.7, and 17.4per cent of the harvestable above ground biomass in pearlmillet, 40.5, 42.0, 41.3, 36.5, 34.3, and 38.2 per cent of theharvestable above ground biomass in cluster bean, and 39.1,42.0, 43.3, 37.5, 35.3, and 39.4 per cent in castor, respectivelyin the treatments listed above. The nodule biomass of clusterbean was 5.3, 7.1, 6.4, 6.7, 8.1, and 6.1 per cent of its rootbiomass. Root samples contain 40.7, 42.3, and 34.8 per centC in pearl millet, cluster bean and castor, respectively.During 1999–2004, nodules were excavated at 65 days, and

roots at 85 days after sowing from all replications in eachtreatment and analyzed for total C content. Rhizodepositionof C from root turnover and exudates was assumed to be 10per cent of the harvestable above ground biomass in clusterbean and castor, and 12 per cent of root biomass of pearl millet(Shamoot et al., 1968). Rhizodeposition of pearl millet, clusterbean and castor contains 36.5, 35.5, and 35.6 per cent C,respectively. During crop growth, weeds were either removedor killed by herbicides. Hence, C inputs from roots andrhizodeposition by the weeds were not considered. Using all


Figure 1. Sustainable yield index (SYI) of different crops as affected by dif-ferent fertilization and manurial treatments during 1988–2005. This figure is

available in colour online at wileyonlinelibrary.com/journal/ldr


the above measurements, plant-derived C inputs as well as Cinputs for each treatment through organic material accumu-lated into the soil were estimated.

Calculations for C Budgeting

Calculations of C build-up (Equation 2), C build-up rate(Equation 3), C stabilization (Equation 4) and C sequestra-tion (Equation 5) are given below:

C build-up %ð Þ ¼ Cfertþorg or Cfert � Ccont

Ccont� 100 (2)

Where: Cfert+org represents SOC stock (Mgha�1) in RDN(F)+FYM treatment and Cfert and Ccont are the SOC stock (Mgha�1)in fertilizer and control treatments, respectively.

C build-up rate Mg C ha�1 y�1� �¼ Cfertþorg or Cfert � Ccont

Years of experimentation(3)

Total C stabilization (%) through plant residues andexternal C inputs with respect to

control ¼ Cfertþorg or Cfert � Ccont

CresþCFYM� 100 (4)

Where: Cres represents C (Mg ha�1) input through crop,CFYM represents C applies through FYM

C sequestered Mg C ha�1� � ¼ SOCf � SOCi (5)

Where: SOCf and SOCi indicate the SOC stocks in 2006 andthat at the initiation of the long-term experiment in 1988.Positive and negative values indicate SOC gains and losses,respectively.

Sustainable Yield Index

Total crop productivity of pearl millet, cluster bean and castorwas calculated through a sustainable yield index (SYI) using18 years of yield data (Figure 1). The SYI was computed tooffset any annual variations in yield and also highlight specificperformance of the treatments during the entire experimentalperiod. The SYI is defined as per Equation 6:

SYI ¼ Y � sYmax

(6)

Where: Y is the estimated average yield of a practice acrossthe years. s is its estimated standard deviation, and Ymax isthe observed maximum yield in the experiment during theyears of cultivation (Singh et al., 1990).


Statistical Analysis

Statistical analysis was performed by the Windows basedStatistical Package for the Social SciencesW (SPSS, 2001)program (Version 11.0, Chicago, IL, USA). The SPSSprocedure was used for computing the analysis of varianceto determine the statistical significance of treatment effects.Duncan multiple range test was used to compare treatmentmeans. Simple correlation coefficients and regressionequations were also computed to evaluate the relationshipsamong the response variables (SYI, MBC, POC, TOC)using the same statistical package. The 95 per cent probabil-ity level is regarded as statistically significant.

RESULTS

Depth Distribution of Soil Organic Carbon Concentration

The SOC concentration of the soil profile differed significantly(p< 0.05) among treatments and depths (Table I). In thesurface 0.2m layer, 50 per cent RDN (F) + 50 per cent RDN(FYM) contained the highest SOC concentration (2.7 g kg�1)followed by that in 50 per cent RDN (FYM) (2.2 g kg�1) and100 per cent RDN (F) (1.7 g kg�1). There was a significantreduction in SOC concentration with the sole application ofinorganic fertilizers (100 per cent RDN) compared with thosein the mixed organics and inorganics or sole FYM treatments.The lowest SOC concentration (1.3 g kg�1) in 0.2m depth

was observed in treatment of a continuous cropping of pearlmillet, cluster bean and castor over 18 years without anyamendments. The mean SOC concentration in the profileincreased from 1.2 g kg�1 in control to 1.8 g kg�1 in 50 per centRDN (F) + 50 per cent RDN (FYM).Despite these increases,the SOC concentration remains below the threshold levelrequired for a good soil health. Significant variations in SOCconcentration were observed even in the subsoil layers. Themean SOC concentration decreased from 1.8 g kg�1 in thesurface 0.2m layer to 1.1 gkg�1 in the 0.8–1.0m depth (Table I).


Table I. Changes in concentration of soil organic carbon after 18 years of cropping and use of amendments (Mean� SD)

Soil depth(m)

SOC (g kg�1)

MeanInitial(1988) Control

100 per centRDN (F)

50 per centRDN (F)

50 per centRDN (FYM)

50 per cent RDN(F) + 50 per centRDN (FYM)

Farmer’s method(5MgFYMha�1

once in 3 years)

0–0.2 3.1� 0.3 1.3� 0.1Ad 1.7� 0.2Ac 1.5� 0.2Ac 2.2� 0.2Ab 2.7� 0.2Aa 1.6� 0.2Ac 1.8� 0.2A

0.2–0.4 2.6� 0.2 1.2� 0.1Bd 1.5� 0.2Bc 1.4� 0.1Ac 2.0� 0.2Ab 2.3� 0.2Ba 1.4� 0.1Bc 1.6� 0.2B

0.4–0.6 1.9� 0.2 1.4� 0.1Ab 1.3� 0.1Cb 1.2� 0.1Bc 1.5� 0.1Ba 1.6� 0.1Ca 1.3� 0.1Bb 1.4� 0.1C

0.6–0.8 1.4� 0.1 1.2� 0.1Bb 1.1� 0.1Dc 1.3� 0.1Ba 1.2� 0.1Cb 1.2� 0.1Db 1.3� 0.1Ba 1.2� 0.1C

0.8–1.0 1.2� 0.1 1.0� 0.1Cb 1.0� 0.1Db 1.2� 0.1Ba 1.2� 0.1Ca 1.0� 0.1Db 1.2� 0.1Ca 1.1� 0.1D

Mean 2.0� 0.2 1.2� 0.1d 1.3� 0.2d 1.3� 0.1d 1.6� 0.1b 1.8� 0.2a 1.4� 0.1c

Different capital letters within columns and different small letters within rows are significantly different at p= 0.05 according to Duncan Multiple Range Testfor separation of means.SOC, soil organic carbon; RDN, recommended dose of N; FYM, farm yard manure.


Total Nitrogen Concentration

The data in Table II show that TN concentration in soilincreased significantly by application of fertilizer and organicamendments over control. Significantly, higher profile meanTN concentration was observed with 50 per cent RDN(F) + 50 per cent RDN (FYM) treatment (0.32 g kg�1)followed by sole application of chemical fertilizers (0.26gkg�1).

Microbial Biomass Carbon

The data on profile mean MBC concentration show largevariations among treatments (Table II). Among fertilitytreatments, the highest profile mean MBC was measured in50 per cent RDN (F) + 50 per cent RDN (FYM) (82.6mg g�1

soil) followed by that in 50 per cent RDN (FYM) (62.8mg g�1

soil), and the lowest in control (45.2mg g�1 soil).

Particulate Organic Carbon

The profile mean POC concentration ranged from 0.43 g kg�1

in control to 0.60 g kg�1 in 50 per cent RDN (F) + 50 per cent

Table II. Profile mean total nitrogen (g kg�1), microbial biomass carbon (quotient and POC : SOC ratio of the soil profile as affected by 18 years of

TreatmentTotal N(g kg�1)

MBC(mg kg�1)

P(g

Control 0.16� 0.01D 45.2� 2.2D 0.43100 per cent RDN (F) 0.26� 0.02B 61.0� 3.0B 0.4650 per cent RDN (F) 0.21� 0.01C 51.5� 2.5C 0.4350 per cent RDN (FYM) 0.21� 0.01C 62.8� 3.1B 0.5150 per cent RDN (F) + 50 per centRDN (FYM)

0.32� 0.02A 82.6� 4.0A 0.60

Farmer’s method(5MgFYMha�1 once in 3 years) 0.20� 0.01C 58.0� 2.8B 0.53

Different letters within columns are significantly different at p= 0.05 according toMBC, microbial biomass carbon; POC, particulate organic C; SOC, soil organic


RDN (FYM). However, POC concentration was higher in alltreatments receiving organic amendments, followed by thosereceiving 100 per cent RDN (F) and 50 per cent RDN(F) (Table II).

Carbon Inputs, Grain Yield and Sustainability

The highest C inputs through crop residues (internal) wasmeasured in 50 per cent RDN (F) + 50 per cent RDN (FYM)(6.6Mgha�1), followed by that in 100 per cent RDN (F)(6.3Mgha�1), and the lowest in control (3.7Mgha�1)(Table III). Combined with the external C inputs throughFYM, total C inputs ranged between 3.7Mgha�1 in controlto 33.5Mgha�1 in 50 per cent RDN (F) + 50 per cent RDN(FYM). The magnitude of C inputs through crop residueswas in proportion to the nutrients applied.Seed yields of pearl millet, cluster bean and castor were

significantly (p< 0.05) more in treatments receiving fertilizersand manure than those in control. The higher average seedyields of pearl millet (809 kg ha�1), cluster bean (576 kg ha�1),

mgkg�1), particulate organic carbon (g kg�1), C : N ratio, microbialcropping and fertilization under semi-arid conditions (Mean� SD)

OCkg�1) C :N ratio

Microbial quotient(kgCmic kg

�1Ctot)POC/SOC(per cent)

� 0.02D 7.9� 0.7A 0.037� 0.003B 31.4� 2.1D

� 0.02C 6.6� 0.6C 0.044� 0.004A 34.8� 1.9C

� 0.02D 6.8� 0.6B 0.038� 0.004B 32.4� 2.9C

� 0.02B 7.1� 0.7B 0.037� 0.004B 35.5� 2.9B

� 0.03A 5.3� 0.5D 0.045� 0.005A 38.5� 3.1A

� 0.03B 6.5� 0.6C 0.042� 0.004A 36.0� 3.5B

Duncan Multiple Range Test for separation of means.carbon; RDN, recommended dose of N; FYM, farm yard manure.


Table III. Cumulative C input, profile SOC, C build up, C build up rate and C sequestered in soil profile as affected by 18 years of croppingand fertilization under semi-arid conditions (Mean� SD)

Treatment

Cumulative C input(in Mg ha�1)

Profile SOC(Mg ha�1)

C build up(per cent)

C build up rate(MgCha�1 y�1)

C Sequestered(MgCha�1)

Throughcrops

ThroughFYM Total

Control 3.7 — 3.7 17.9� 1.6D — — �12.0� 0.8D

100 per cent RDN (F) 6.3 — 6.3 19.3� 1.8C 7.8� 0.06D 0.078� 0.007D �10.6� 0.7C

50 per cent RDN (F) 5.5 — 5.5 19.2� 1.7C 7.3� 0.06D 0.072� 0.006D �10.7� 0.8C

50 per cent RDN (FYM) 5.2 26.9 32.1 23.4� 1.9B 30.7� 2.4B 0.306� 0.021B �6.5� 0.5B

50 per cent RDN (F) + 50 percent RDN (FYM)

6.6 26.9 33.5 25.5� 2.1A 42.5� 3.1A 0.422� 0.030A �4.4� 0.3A

Farmer’s method(5Mg FYMha–1 once in3 years)

5.0 9.7 14.7 19.8� 1.7C 10.6� 0.09C 0.106� 0.001C �10.1� 0.8C

Different letters within columns are significantly different at p= 0.05 according to Duncan Multiple Range Test for separation of means.FYM, farm yard manure; SOC, soil organic carbon; RDN, recommended dose of N.

C build up (per cent) =Cfertþorg or Cfert�Ccont

Ccont� 100; C build up rate (MgCha�1 y�1) =

Cfertþorg or Cfert�Ccont

Years of experimentation; C sequestered (MgCha�1) = SOCf�SOCi..


castor (827 kg ha�1) over six cropping seasons were obtainedthrough integrated use of chemical fertilizers and organicmanure followed by those of 779, 453 and 796 kg ha�1, respec-tively, with 100 per cent RDN (F). The SYI of all three cropswas higher in 50 per cent RDN (F) + 50 per cent RDN(FYM) (0.30 for pearl millet, 0.69 for cluster bean and 0.46for castor) followed by 100 per cent RDN (F) (0.24, 0.46,0.40), 50 per cent RDN (FYM) (0.25, 0.46, 0.39), 50 per centRDN (F) (0.22, 0.36, 0.38), farmer’s practice (0.16, 0.34, 0.42)and lowest in control (0.14, 0.23, 0.32), respectively (Figure 1).

Profile SOC Stock, C Build-up, Stabilization andSequestration

The profile SOC stock was the highest in 50 per centRDN (F) + 50 per cent RDN (FYM) (25.5MgCha�1)followed by that in the 50 per cent RDN (FYM) (23.4MgCha�1), and the lowest in the control (17.9MgCha�1)(Table III). The highest rate of SOC build-up from the C inputswas observed in treatments receiving 50 per cent RDN(F) + 50 per cent RDN (FYM) (42.5 per cent) followed by thatin the 50 per cent RDN (FYM) (30.7 per cent), farmer’spractice (10.6 per cent), 100 per cent RDN (F) (7.8 per cent),and the lowest in 50 per cent RDN (F) (7.3 per cent). A similartrend was also observed in the profile SOC stock and the rateof SOC build up in the respective treatments (Table III). Incomparison with the control, the mean rate of SOC build-upduring the 18 years of cropping was the highest in 50 per centRDN (F) + 50 per cent RDN (FYM) (0.422MgCha�1) andthe lowest in 50 per cent RDN (F) (0.072MgCha�1). It wasestimated that 23 per cent of applied C through FYM wasstabilized, and the rest (77 per cent) was lost through oxida-tion. The SOC stock was depleted in all treatments, and the


highest magnitude of depletion of 12MgCha�1 was observedin the control.

DISCUSSION

Soil Carbon Build-Up and Carbon Inputs

Irrespective of the treatments, 18 years of continuous croppingcaused a net depletion of TOC in the surface soil comparedwith the antecedent concentration (3.1 g kg�1). The depletionwas exacerbated by plowing, which disturbs the soil andreduces the amount and stability of aggregates. Reduction inaggregation accentuates oxidation of SOC. Plowing—induced depletion of SOC is widely reported, but the magni-tude of depletion varies depending upon the climate,geographical location, crops/cropping systems, soil proper-ties, cropping history, duration of the fallow period, and inputs(Davidson and Ackerman, 1993; Guo and Gifford, 2002;Mandal et al., 2008). In the present study, the magnitudeSOC depletion was reduced by application of chemicalfertilizers alone or in combination with organic amendments.In comparison with the control, the magnitude of SOC depletionwas lower in treatments involving 50 per cent RDN (F)+50 per cent RDN (FYM) (13 per cent), 50 per cent RDN(FYM) (29 per cent), 100 per cent RDN (F) (45 per cent), and50 per cent RDN (F) (52 per cent).Similar to the concentration, SOC stock of the profile was

also significantly (p< 0.05) more in the amended treatmentscompared with the control. Higher biomass and C input inthe 50 per cent RDN treatment through fertilizer combinedwith FYM may be due to increased availability of deficientnutrients such as N, K, Ca, Mg, S, Zn, and B with additionof organic manure (Srinivasarao and Vittal, 2007).


3.30 Mg C ha-1 yr-1

igure 2. Critical C input value and its influence on soil C sequestrationnder pearl millet-based systems in semi-arid tropical conditions of India.his figure is available in colour online at wileyonlinelibrary.com/journal/ldr


The build up of SOC stock with C inputs in differentamendments is in proportional to the total C inputs. The totalC inputs was significantly correlated with the SOC stocks inthe profile (Y= 4.5X� 77.1; R2 = 0.93; p< 0.05). Similarly,the total SOC sequestered over 18 years was directly relatedto total C inputs (Y=0.21X� 12.4; R2 = 0.93; p< 0.05), and93 per cent of the variability in sequestered C was explainedby the magnitude of C inputs. Relatively higher SOC retentionin manure-amended plots may be due to highly decomposedmanure that contained a lower proportion of chemically labileorganic compounds (Paustian et al., 1992). A significant rela-tionship was observed between C inputs (through crop residueC, external C, and the total C) and the SOC stock in the profileand the magnitude of C build up (per cent). Another studyindicated a significant improvement in SOC followingincorporation of a cover-crop (horse gram) biomass grownduring the off-season rainfall for 10 years in a rain-fed Alfisolunder semi-arid tropical conditions in India (Venkateswarluet al., 2007). However, the lack of availability of organicamendments in adequate quantities under arid and semi-aridconditions is a major constraint. In addition to lower biomassproduction, there are competing uses of dung as a fuel and ofcrop residues as animal feed and fuel.

Carbon Sequestration and Requirement of Critical C Input

The cultivation of the pearl millet, cluster bean and castor cropover 18 years in Entisols under semi-arid conditions withoutusing any organic and/or inorganic fertilizer input (control)strongly depleted the SOC stock, with a mean depletion of12.0MgCha�1 (Table III). High temperatures 45–50 �Cduring peak summer months accentuate oxidation especiallyin combination with the disrupting effect of mechanicalseedbed preparation. However, the magnitude of depletionwas lower with addition of organic manures used either aloneor in combination with inorganic fertilizers. The annual rate ofSOC depletion in another study was 0.178MgCha�1 y�1 inunfertilized control, compared with a net gain of 0.572MgCha�1 y�1 in the 50 per cent RDF+4Mgha�1 of groundnutshells (Bhattacharyya et al., 2009).These data show that SOC depletion occurred even after

18 years of continuous addition of biomass C at the rate of0.20 to 1.86MgCha�1. Therefore, higher rates of C inputsare needed to compensate the losses by decomposition. Eachsoil with a different C loading might lead to the attainmentof a new steady state of SOC stock over time (Six et al.,2002). Assessment of SOC stock for these treatments atperiodic, perhaps at decadal intervals, might provide insightsinto a more relevant strategy of C management for soils. Theslope of the curve (Figure 2) represents the rate of conversionof inputs of biomass C into the SOC stock, which is about 21per cent of each additional input ha�1 in the pearl millet-cluster bean-castor cropping sequence. These rates comparewith 14.0–21.0 per cent reported by Rasmussen and Collins


(1991) for cooler, temperate regions of the USA and Canada,7.6 per cent by Kong et al. (2005) for the Mediterraneanclimate, and 14 per cent by Majumder et al. (2008) for thehumid Indo-Gangetic plains of India under irrigated rice-wheat system. The present study shows that the criticalamount of C input to the soil to maintain the SOC at theantecedent level (zero change) is 3.3MgCha�1 y�1 forEntisols under a pearl millet-based cropping system. Yet, theantecedent rate is minimal and lower than the threshold level.This rate of C input compares with 3.1Mgha�1 y�1 reportedby Kong et al. (2005) for Davis, CA, and 3.56Mgha�1 y�1

by Majumder et al. (2008) for irrigated rice-wheat systemsof the Indo-Gangetic plains. The lower levels of input of Cneeded to maintain SOC levels may be due to lower anteced-ent levels (3.4 g kg�1) (Srinivasarao et al., 2006). The anteced-ent SOC levels in the studies reviewed above were three to sixtimes higher (~6–15 g kg�1 soil).The addition of FYM along with 50 per cent RDN (F)

decreased the rate of depletion of SOC compared with controland sole application of inorganic fertilizer. These resultssuggest that application 4Mg of organics along with fertilizersis essential to improve soil health in the hot semi-arid tropicsof India and minimize the rate of depletion of SOC stock.Thus, the present rates of application of soil amendment arenot adequate and must be doubled to 8–10Mgha�1 to main-tain the SOC stock.

Microbial Biomass Carbon and Microbial Quotient

The MBC, which normally constitutes about 1–5 per centof the TOC, can provide an early warning for a possibledegrading and/or aggrading effect of different managementpractices on soil health (Powlson, 1994; Mandal, 2005). Thelower value of MBC in the control treatment (Table II) isrelated to nutrient mining by continuous cropping withoutany fertilization, whereas a higher value in the 50 per centRDN (F) + 50 per cent RDN (FYM) treatment may be due to

FuT



C enrichment through plant residue incorporation and FYMapplication (Grego et al., 1998). These results are in accordwith those reported by Hopkins and Shiel (1996), whoobserved that MBC was high in soils receiving annuallyFYM for nearly 100 years along with inorganic NPK than thatin NPK only, and it was the lowest in soils receiving only N orno fertilizers (control). In contrast, Hart and Stark (1997)reported a positive effect of N fertilizer on MBC,whereas Biederbeck et al. (1994) and Lovell et al.(1995) observed a negative effect, and Fauci and Dick(1994) observed no effect in the short-term but a negativeeffect in the long-term. Differences in experimentalconditions and agro-ecosystems of those studies may beresponsible for the contradictory results.The active C pool, represented by MBC, varied signifi-

cantly among different amendments and was significantlycorrelated with SOC concentration (Y= 635.7X� 30.9;R2 = 0.87, r = 0.93; p< 0.05). Regression equations withTOC explained 87 per cent of the variation, and indicate thatsmall changes in the MBC could make large changes in theSOC with the use of soil amendments after 18 years. Despiterelatively small quantity, MBC is one of the most labile poolsand is an important reservoir of plant nutrients such as N andP (Jenkinson and Ladd, 1981; Marumato et al., 1982; Waniet al., 2003). In general, MBC responds more rapidly thanSOM to changes in management that alter the annual soil inputof biomass-C (Powlson and Jenkinson, 1981). The highest in-crease in MBC observed was 147 per cent in the 50 per centRDN (F) +50 per cent RDN (FYM) followed by that of 79per cent in 50 per cent RDN (FYM), 74 per cent in 100 per centRDN (F) and the lowest of 37 per cent in 50 per cent RDN (F).Having the most rapid turnover rates, oxidation of MBCdrives the soil efflux of CO2. Yet, MBC is very

Table IV. Relationships between different forms of carbon, carbon inputmillet-based systems in semi-arid tropical conditions

Independent variable Regression

Carbon sequestration rate SYI Y(pearl millet) =Y(cluster bean) =Y(castor) = 0.01X

Total carbon inputs Y(pearl millet) =Y(cluster bean) =Y(castor) = 0.002

Profile mean SOC content Y(pearl millet) =Y(cluster bean) =Y(castor) = 1.64X

Microbial biomass carbon Y(pearl millet) =Y(cluster bean) =Y(castor) = 0.003

Particulate organic carbon Y(pearl millet) =Y(cluster bean) =Y(castor) = 0.55X

SYI, sustainable yield index; SOC, soil organic carbon.*, **, *** denotes significance at p< 0.05, 0.01 and 0.001, respectively.


important, because it fuels the soil food web, stronglyinfluences nutrient cycling to maintain the soil healthand its productivity (Majumder et al., 2008). The dataof the present study show that MBC is directly relatedto total C inputs (Y = 0.82X + 46.4; R2 = 0.65; p< 0.05)(Table IV). Microbial quotient ranged between 0.037and 0.045 kgMBC kg�1 TOC among the treatments, andsignificant improvement was observed with the use ofsoil amendments from 19 per cent in 100 per cent RDN(F) to 22 per cent in 50 per cent RDN (F) + 50 per centRDN (FYM) (Table II). These trends, however, arewithin the range of 0.010 to 0.050 kgMBC kg�1 TOCas reported by Anderson and Domsch (1980). HigherMQ values in chemical fertilizer + organics comparedwith control may be attributed to a better nutritional sta-tus. These results also suggest that the organic C under theformer treatments is more stable than that in the control(Sparling et al., 1992). A low value of MQ indicates animpaired capacity of the soil for C cycling, and degradationof its biological quality (Dalal, 1998). The MQ in the presentstudy ranged between 1 and 5 per cent, which is a normalrange (Powlson, 1994; Carter, 2002). The lowest value ofMQ in the unfertilized control indicated a poor soil health withimpairment of its C cycling capacity (Dalal, 1998; Chaudhuryet al., 2005). The better nutritional environment for the micro-bial population in the soils under balanced fertilization com-pared with control, sole N or even NP treatments increasedthe MQ (Rudrappa et al., 2005). Haynes and Tregurtha(1991) also reported a decline in MQ with decrease in SOCconcentration. Further, MBC was also significantly correlatedwith SYI (r=0.86–0.97; p< 0.05) explaining a variability of74–95 per cent (Table IV). These ratios are comparable withthose reported for other soils and cropping systems of the

s, soil carbon sequestration rate and sustainable yield index of pearl

equation Coefficient of determination (R2)

0.02X+ 0.34 0.67*0.05X+ 0.84 0.77**+ 0.5 0.56*0.003X+ 0.17 0.450.008X�0.29 0.56*X + 0.36 0.452.01X�0.07 0.61*5.99X�0.44 0.77**+ 0.16 0.64*0.004X+ 0.004 0.74*0.01X�0.22 0.95***X + 0.21 0.86**0.0002X�0.009 0.101.81X�0.05 0.79**+ 0.013 0.79**


igure 3. (a) Influence of profile SOC stocks on cumulative yields of crops18-year long-term pearl millet-cluster bean-castor sequence in semi-arid

ondition. (b) Influence of SOC stocks to 1-m depth on yields of individualrops in 18-year long-term pearl millet-cluster bean-castor sequence in semi-rid condition. This figure is available in colour online at wileyonlinelibrary.

com/journal/ldr


tropics (Jenkinson and Ladd, 1981; Bolton et al., 1985;Chander et al., 1997).

Particulate Organic Carbon

The POC increased significantly in the slow turnover “C pool”among different amendments (Table II). The POC is moresensitive to changes in management practices than TOC(Bowman et al., 1999; Needelman et al., 1999), and is asensitive indicator of changes in soil health (Chan, 1997;Wilson et al., 2001). The ratio of POC : SOC improved31.4 per cent in control to 38.5 per cent in integrated use ofchemical fertilizers and FYM, 36.0 per cent in farmer’s prac-tice, and 35.5 per cent in 50 per cent RDN (FYM) treatment(Table II), indicating that the regular addition of soil amend-ments improved C and stabilized it in the fine-sized soil parti-cles (Denef and Six, 2005). A high correlation between POCand SOC (Y=0.39X� 0.01; R2 = 0.54; r=0.74; p< 0.05)indicates a strong association of these two forms of C, andexplained 54 per cent of the variation in SOC concentration.Similarly, POC concentration explained up to 79 per cent ofthe variability in SYI (Table IV). These trends indicate theusefulness of POC as an indicator for changes in TOC(Sherrod et al., 2005). Wander and Bollero (1999) alsoidentified POC as a promising soil health measure.Franzluebbers and Arshad (1996), working on soils in Albertaand British Columbia, Canada, observed that POC-C wasmore sensitive than SOC to tillage-induced changes in SOC.

Soil Organic Carbon Pools, C Input and Crop Productivity

High and significant correlations among different fractionsof SOC suggest that these components are in a dynamicequilibrium (Table IV). Depletion or enrichment in onewould shift the equilibrium and affect the size of the others.The SYI of all three crops was in accord with the SOC stock.Thus, there existed a positive relationship between SOCstock to 1-m depth and cumulative yield of the crops(0.46Mg ha�1 y�1Mg�1 of SOC) (Figure 3a) and individ-ual grain yield of pearl millet (0.17Mg ha�1 y�1Mg�1 ofSOC), cluster bean (0.14Mg ha�1 y�1Mg�1 of SOC) andcastor (0.15Mg ha�1 y�1Mg�1 of SOC) (Figure 3b). Inaccord with the observations indicating strong relationshipbetween SOC concentration/stock and yield (Bronsonet al., 1998; Yadav et al., 2000; Regmi et al., 2002; Singhet al., 2004), there was a linear relationship between theSYI and SOC fractions, especially between SYI and POCand MBC. These trends indicate the importance of thesefractions of SOC on crop yield through improvement in soilhealth. A significant relationship was also observed betweenSYI and total C inputs (r = 0.67 to 0.75; R2 = 0.45 to 0.56,p< 0.05) and the profile SOC stock (r = 0.78 to 0.88;R2 = 0.61 to 0.77, p< 0.05) (Table IV). Thus, the mainte-nance of SOC stock through regular addition of organicsdetermines the sustainability of rain-fed production systems.


b

a

Fincca

Increase in SOC stock also enhances water holding capacityof the soil profile (Du et al., 2009) that mitigates intermittentdroughts, a common feature in dry land agriculture. Asignificant correlation of SYI with the total N concentrationin the soil (r=0.84 to 0.96) explained 71–92 per cent ofvariability. Total N concentration in soil improved with theregular addition of amendments. However, 2–5 per centchanges in the C :N ratio were observed with the applicationof amendments compared to the control (Table II).

CONCLUSIONS

Sustainable yield index was measured with the integrated useof chemical fertilizer and FYM.However, even the addition of33.5Mgha�1 C inputs through crop residues and FYMresulted in the net depletion of 4.4MgCha�1 by 18 years ofcultivation. Treatment involving 50 per cent recommendeddose of N supplied through chemical fertilizers and another50 per cent through FYM reduced the depletion of SOC stocksand produced higher yields. Increase in SOC stock by 1Mgha�1 in 1-m depth increased cumulative grain yield by



0.46Mgha�1. However, most (�77 per cent) of the Csupplemented through FYM in this climate was mineralizedand only a small fraction (�23 per cent) was stabilized intoSOC stock. The threshold level of C input to maintain SOCat the antecedent level (with no change), was 3.3MgCha�1.However, the antecedent level is low and below the thresholdrequired for a good soil health. Thus, the rate of addition oforganic amendments should be at least doubled to reduceSOC depletion and increased considerably to enhance theSOC stock. Strong relationships were observed among thedifferent SOC fractions, of which MBC explained a highervariability in the SYI.

ACKNOWLEDGEMENT

The authors are thankful to the Indian Council of Agricul-tural Research (ICAR), New Delhi for funding the project.

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