Soil organic carbon sequestration in relation to organic and inorganic fertilizationin rice–wheat and maize–wheat systems

S.S. Kukal *, Rehana-Rasool, D.K. Benbi

Department of Soils, Punjab Agricultural University, Ludhiana 141 004, India

Soil & Tillage Research 102 (2009) 87–92

A R T I C L E I N F O

Article history:

Received 13 December 2007

Received in revised form 30 May 2008

Accepted 27 July 2008

Keywords:

C sequestration

Farmyard manure

Inorganic fertilizers

Maize–wheat

Rice–wheat

A B S T R A C T

Soil organic carbon (SOC) pool is the largest among the terrestrial pools. The restoration of SOC pool in

arable lands represents a potential sink for atmospheric CO2. The management and enhancement of SOC

is important for sustainable agriculture. The cropping system and soil type influence crop biomass under

different fertilization. Data from two long-term field experiments on rice–wheat and maize–wheat

systems in progress since 1971, were analyzed to assess the impact of fertilization practices on SOC stocks

in sandy loam soils (typic ustipsament). The treatments in rice–wheat included (i) farmyard manure

(FYM alone @ 20 t ha�1, applied at the time of pre-puddling tillage), (ii) N120P30K30 (120 kg N, 30 kg P2O5

and 30 kg K2O ha�1), (iii) N120P30 (same as in (ii) except that K application was omitted), (iv) N120 (same

as in (ii) except that P and K application was omitted) and (v) control (without any FYM or inorganic

fertilizer). Similar treatments were studied in maize–wheat except that the amounts of N, P2O5 and K2O

were 100, 50 and 50 kg ha�1, respectively. In rice–wheat system, the SOC concentration at different

depths in 0–60 cm soil profile was higher (1.8–6.2 g kg�1) in FYM-treated plots followed by 1.7–

5.3 g kg�1 in NPK plots, compared to 0.9–3.0 g kg�1 in unfertilized plots. Balanced fertilization improved

the SOC concentration. Similar trend was found in maize–wheat system. In the 60-cm soil profile the total

SOC stocks in both the cropping systems were highest in FYM (31.3 and 23.3 Mg ha�1 in rice–wheat and

maize–wheat system) followed by balanced fertilization (29.6 and 21.3 Mg ha�1) and lowest in

unfertilized control (21.4 and 18.7 Mg ha�1). The SOC concentration in rice–wheat soils was 54 and 30%

higher in FYM and NPK plots than in maize–wheat system. Improved SOC content enhances soil quality,

reduces soil erosion and degradation, and increases soil. The soils under rice–wheat sequestered 55%

higher SOC in FYM plots and 70% higher in NPK plots than in maize–wheat. These results document the

capacity of optimally fertilized rice–wheat system to sequester higher C as compared to maize–wheat

system.

� 2008 Elsevier B.V. All rights reserved.

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1. Introduction

Currently, biotic carbon sequestration is being considered aviable option for mitigating CO2 emission to the atmosphere.Though carbon emissions from agricultural activities contribute tothe enrichment of atmospheric CO2 (Kimble et al., 2002), yetcarbon sequestration in agricultural soils, through the adoption ofimproved management practices, can mitigate this trend (Lal et al.,1998). Agricultural activities have profound influence on changesin soil organic carbon (SOC) both in the short and the long terms.The estimated potential of agricultural intensification on SOCsequestration in soils of India ranges between 12.7 and

* Corresponding author. Tel.: +91 161 2401960; fax: +91 161 2400945.

E-mail address: sskukal@rediffmail.com (S.S. Kukal).

0167-1987/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2008.07.017

16.5 Tg C year�1 (Lal, 2003). Improved SOC density productivity(Nieder et al., 2003). Recently, Benbi and Chand (2007) have shownthat each Mg of SOC in the 0–15 cm soil layer increased wheatproductivity by 15–33 kg ha�1 in semi-arid India. There is thus astrong need to increase SOC density to improve the quality ofnatural resources (soil, water and atmosphere) and for sustainablecrop productivity.

The overall aim should be to increase SOC density, distributionof SOC in the subsoil and improve aggregation. These objectivescould be achieved through a wide range of land use and soil/vegetation management options (Lal, 1997) such as integratednutrient management, mulch farming, conservation tillage anddiverse crop rotations based on legumes and cover crops in therotation cycle (Lal, 2004). Another option for increasing SOCdensity is by increasing carbon (C) input through intensification ofagriculture and recycling of crop residues and animal manures.

Table 1Basic characteristics of the experimental soils

Soil characteristics Soil depth (cm)

0–15 15–30 30–45 45–60

Rice–wheat

Soil texture sl sl sl sl

Sand (%) 60.1 � 3.6 65.1 � 2.9 58.2 � 4.5 61.1 � 5.1

Silt (%) 33.0 � 2.2 30.3 � 3.6 35.1 � 1.9 33.1 � 3.4

Clay (%) 6.9 � 0.9 4.6 � 1.2 6.7 � 1.1 5.8 � 0.8

Bulk density (Mg m�3) 1.69 � 0.06 1.75 � 0.09 1.76 � 0.04 1.75 � 0.10

Mass water content (%)

FC 15.3 � 2.9 14.7 � 2.7 12.5 � 1.9 11.4 � 1.7

PWP 7.64 � 1.1 7.32 � 2.0 6.24 � 1.8 5.68 � 0.9

pH 8.3 � 0.4 7.8 � 0.3 8.1 � 0.2 8.5 � 0.2

Organic carbon (%) 0.29 � 0.06 0.23 � 0.08 0.19 � 0.06 0.11 � 0.04

Maize–wheat

Soil texture sl sl sl sl

Sand (%) 60.5 � 4.2 59.5 � 3.3 60.5 � 3.9 55.4 � 3.0

Silt (%) 23.9 � 2.7 25.7 � 2.9 24.0 � 1.9 27.0 � 1.3

Clay (%) 15.6 � 1.6 14.8 � 1.3 15.0 � 0.9 17.6 � 0.8

Bulk density (Mg m�3) 1.69 � 0.07 1.75 � 0.05 1.72 � 0.06 1.73 � 0.05

Mass water content (%)

FC 20.4 � 1.6 23.4 � 0.9 22.2 � 1.3 21.1 � 0.9

PWP 9.15 � 1.0 10.7 � 1.1 9.07 � 0.9 11.7 � 0.7

pH 8.1 � 0.5 7.9 � 0.7 7.90 � 0.4 8.07 � 0.3

Organic carbon (%) 0.27 � 0.05 0.17 � 0.02 0.16 � 0.03 0.13 � 0.03

S.S. Kukal et al. / Soil & Tillage Research 102 (2009) 87–9288

Higher crop productivity under intensive agriculture increasesplant residue input into the soils and thus has the potential ofincreasing SOC level (Franzluebbers, 2005). The improvement indepth distribution of SOC can be achieved by planting deep-rootedcrop species with large belowground biomass production andconservation tillage. Conservation tillage increases SOC sequestra-tion by improving micro-aggregation and deep placement of SOCin the subsoil horizons (Lal and Kimble, 1997).

The use of crop residues and animal manures returns the much-needed C back to the field and thus results in increased SOC densityand soil quality. Manuring and application of biosolids, as cropresidue or compost, also enhances soil aggregation (Benbi et al.,1998). Swarup et al. (1998) reported that the rate of SOCsequestration calculated for NPK + manure over that of the controlwas at rates of 15 to 120 kg C ha�1 year�1. The low rates areattributed to low soil water, high soil temperature and high rate ofoxidation. The attainable C sequestration is essentially limited bythe quantity of C input into the soil system. The fertilizer input intothe soil, which determines the C from crop productivity input, willtend to increase the attainable level to near the potential level.Cropping system, particularly involving upland versus lowlandcrops could influence SOC accumulation by influencing the amountof C input and rate of decomposition (Nieder et al., 2003).Decomposition rate increases with temperature, while it decreaseswith increasingly anaerobic conditions as is the case in rice-basedcropping systems. There is thus a real possibility of increasing Csequestration through improved soil management.

To realize the vast SOC sequestration potential, which is in theinterest of India, it requires adoption of recommended manage-ment practices including integrated nutrient management andmanuring. The main focus should be on SOC fluxes in relation tosoil texture, land use and fertilizer management practices. While aconsiderable amount of research related to C sequestration andglobal climate change has been conducted in developed nations,comparatively fewer studies have been conducted in lesser-developed countries, especially in south Asia on impact of fertilizermanagement practices on soil C sequestration in different croppingsystems. The present investigation was conducted to estimate soilC sequestration in rice–wheat and maize–wheat systems undersimilar systems of nutrient management. Data from two long-termexperiments have been analyzed to assess the impact ofagricultural management on soil C pool.

2. Materials and methods

Two long-term field experiments involving the application offarmyard manure (FYM) and inorganic fertilizers in rice–wheatand maize–wheat system were selected for the SOC sequestrationstudies during 2004–2005. These experiments are in progress since1971 at the Punjab Agricultural University Research farm,Ludhiana, India (308540N latitude, 758480E longitude and 247 mabove mean sea level).

2.1. Climate and soil

The climate of the region is tropical semi-arid with an averageannual rainfall of 700 mm of which 80% is received during the 3-month period (July to September), which coincides with the rice/maize season. The mean maximum (39 8C) and minimum (4 8C)temperatures in the region occur during the months of June andJanuary, respectively.

The soils of the experimental site were sandy loam (typicustipsament), low in organic carbon, slightly alkaline in reaction,non-saline with low in KMnO4–oxidizable N (Subbiah and Asija,1956), NaHCO3 extractable Olsen and Sommers (1982) and high in

ammonium acetate extractable K (Merwin and Peech, 1950)(Table 1). While the two soils had similar sand content, thesediffered in silt and clay contents. The soil under rice–wheat hadrelatively higher silt content than that under maize–wheat, whichhad higher clay content. The field capacity (FC) and permanentwilting point (PWP) moisture contents were accordingly higher insoil under maize–wheat than under rice–wheat.

2.2. Crop establishment and treatments

2.2.1. Rice–wheat

The field was pre-irrigated in the second or third week of Juneeach year and tilled to 12–13 cm depth twice thereafter at fieldcapacity moisture to kill the germinated weeds and to level thefield for better puddling. It was inundated with 5–6 cm standingwater and ploughed twice under wet conditions by a tine cultivatorfollowed by a planking with a wooden plank. One-month-old riceseedlings were transplanted immediately after puddling with aspacing of 15 cm � 20 cm. The crop was raised as per therecommended practices of Punjab Agricultural University exceptfor the fertilization, which was done as per the treatments. Thetreatments in rice included:

T1 – farmyard manure (FYM alone @ 20 t ha�1, applied at thetime of pre-puddling tillage).T2 – N120P30K30 (application of 120 kg N, 30 kg P2O5 and30 kg K2O ha�1).T3 – N120P30 (same as in T2 except that K application wasomitted).T4 – N120 (same as in T2 except that P and K application wasomitted).T5 – control (without any FYM or inorganic fertilizer).

The treatments were allocated in bunded plots of size8 m � 3 m replicated four times in a randomized block design(RBD). Whole of P (as single super phosphate) and K (as muriate of

S.S. Kukal et al. / Soil & Tillage Research 102 (2009) 87–92 89

potash) and one-third amount of N (as urea) were applied at thetime of field preparation. The remaining amount of N was appliedat 21 and 42 days after transplanting. Well-decomposed and driedFYM was spread on the soil surface before pre-puddling tillage. Thecrop was kept submerged continuously for the first 15 days andthereafter irrigated 2 days after complete disappearance of pondedwater in the field. Weeds were well controlled using butachlor 6–7 days after transplanting. The crop was harvested in the fourthweek of October every year.

The same field was again pre-irrigated in the first week ofNovember and disked twice at field capacity moisture. It wasdisked and ploughed twice the next day and thereafter plankedonce. Wheat was sown with a drill at 5 cm depth in second or thirdweek of November and the crop was raised as per therecommended practices of Punjab Agricultural University, exceptfertilization, which was done as per the treatments. The fertilizertreatments in case of wheat were the same as outlined above in therice crop. Same plots received similar treatments during both thecrop seasons. The crop was irrigated at 21, 49 and 131 days afterseeding (DAS) as per the recommended practice of PunjabAgricultural University. Weeds were well controlled by sprayingsulfosulfuron within a week after the first post-sowing irrigation.The crop was harvested in the second week of April each year.

2.2.2. Maize–wheat

The field was pre-irrigated 3–4 days prior to its preparation insecond week of June. Upon attaining the field capacity moisture, itwas ploughed twice and planked once to attain a fine seedbed.Maize was sown in rows 60 cm apart with a plant to plant spacingof 22 cm. The crop was raised as per the recommended practicesexcept fertilization, which was done as per treatments. Thetreatments included:

T1 – FYM (FYM alone @ 20 t ha�1).T2 – N100P50K50 (100 kg N, 50 kg P2O5 and 50 kg K2O ha�1).T3 – N100P50 (same as in T2 except that K application wasomitted).T4 – N100 (same as in T2 except that P and K application wasomitted).T5 – control (without any FYM or inorganic fertilizer).

The treatments were allocated in plots of size 8 m � 2.7 mreplicated four times in a randomized block design (RBD). The cropwas irrigated at 12, 22 and 46 DAS as per the recommendedpractices. The weeds were well managed by the use of Atrataf 50 @1250 g ha�1 within 2 days of sowing. The crop was harvested in thelast week of September each year.

The same field was again pre-irrigated in the last week ofOctober and disked twice at the field capacity moisture content. Itwas disked and ploughed twice the next day and thereafterplanked once. Wheat was sown with a drill at 80 kg seed ha�1 infirst week of November. The crop was raised as per therecommended practices of Punjab Agricultural University, exceptfertilization, which was done as per the treatments. The fertilizertreatments in case of wheat were the same as outlined above in themaize crop. Same plots received similar treatments during both thecrop seasons. The crop was irrigated at 21, 52 and 131 DAS as perthe recommended practice of Punjab Agricultural University. Theweed management practices were similar as for the crop in rice–wheat system as described above. The crop was harvested in thesecond week of April.

Soil samples were collected from each plot with a post-hole augerat depths of 0–15, 15–30, 30–45 and 45–60 cm at the harvest of rice,maize and wheat crops during 2004–2005. Each sample was acomposite of six spots in a plot. Samples were air-dried and ground

to pass 2-mm sieve. The soil samples were analyzed for organiccarbon by wet oxidation method (Walkley and Black, 1934). Soilorganic carbon stocks were calculated for each treatment separatelyat each depth. From total SOC stocks in 60-cm profile, the SOCsequestration rate was calculated separately for each treatment overunfertilized control on the basis of 32 years of experimentation. Thein situ bulk density of 0–15, 15–30, 30–45 and 45–60 cm soil layerswas determined at 4 places in each plot. For this purpose theundisturbed soil cores in one edge-sharpened galvanized iron(metallic) cylinders of 7.5 cm internal diameter and 7.5 cm heightwere collected at the harvest of maize and wheat crops with the helpof a small hammer and a piece of wood from all the treatments in 0–15, 15–30, 30–45 and 45–60 cm soil layers. Precautions were takento reduce the disturbance of soil within the metallic cylinder duringsampling, which was done at 10–12% soil moisture content. The soilcores along with the metallic cylinder were excavated with the helpof a spade and the extra soil was removed on both sides of theexcavated cores and these were carefully preserved in polythenebags. The soil cores were dried in an oven at 105 8C for 24 h to get dryweight of the soil. The ratio of dry weight of soil core and internalvolume of the metallic cylinder was expressed as bulk density inMg m�3. The data so obtained for each observation were analyzed byanalysis of variance technique using factorials in randomized blockdesign (RBD) as described by Steel and Torrie (1960).

3. Results and discussion

3.1. Soil organic carbon

At all the sampling depths, SOC concentration in rice–wheatwas highest in FYM-amended plots and the least in unfertilizedcontrol (Table 2). The SOC in FYM-treated plots was significantly(P < 0.05) higher than in NPK plots. The magnitude of differenceamong the treatments was greater in the top soil layer (0–15 cm)and generally decreased with soil depth. For example, theapplication of FYM increased the SOC by 3.2 g kg�1 over controlin the 0–15 cm soil layer and by 0.9 g kg�1 at 45–60 cm depth. Theanimal manure not only added the valuable nutrients but alsoimproved soil physical environment for better uptake of water andnutrients from the soil, which resulted in higher above-ground andbelow-ground plant biomass. Parker et al. (2002) reported 7–20%of greater organic C in the surface 5 cm of soil in a cotton-ryecropping system with poultry litter than with commercial fertilizerapplication. Among the inorganic fertilizers, the SOC concentrationat all the sampling depths was highest in N120P30K30 plots rangingfrom 1.7 to 5.3 g kg�1 and least in N120 ranging from 1.0 to3.4 g kg�1. It was significantly higher even in N-plots than incontrol plots. Similar was the trend in the organic C concentrationobserved at the harvest of wheat. Balanced fertilization is expectedto increase SOC because of greater C input associated withenhanced primary production and crop residues returned to thesoil. Rudrappa et al. (2006) reported that balanced fertilizationimproved total SOC concentration over 50% NPK or NP alone in 0–15 cm soil layer. Though the average SOC concentration decreasedwith soil depth, the FYM and N120P30K30 treatments resulted insignificant increase in organic C even in 45–60 cm soil layer,whereas in N120 and N120P30 treatments, the increase wassignificant up to 30 cm soil depth. The decrease in SOCconcentration with soil depth is well documented (Liu et al.,2003; Brady and Weil, 2000).

The SOC concentration at the harvest of maize in maize–wheatsystem (Table 3) ranged from 3.9 to 1.4 g kg�1 in FYM and 3.7 to1.3 g kg�1 in N100P50K50 plots, being statistically at par. Among theinorganic fertilizers, the application of balanced fertilizers did notimprove the SOC content. The trend was similar at the harvest of

Table 2Soil profile organic carbon (g kg�1 of soil) as affected by FYM and inorganic

fertilizers in rice–wheat cropping system

Treatments Soil depth (cm)

0–15 15–30 30–45 45–60

Rice

Control 3.0 2.0 1.5 0.9

FYM 6.2 3.4 2.2 1.8

N120 3.4 2.5 1.7 1.0

N120P30 3.8 2.7 1.9 1.1

N120P30K30 5.3 3.0 2.0 1.7

Mean 4.3 2.7 1.9 1.3

LSD (0.05)

Fertilization = 0.23

Soil depth = 0.22

Fertilization � soil depth = 0.50

Wheat

Control 2.9 2.3 1.9 1.1

FYM 5.1 3.4 2.6 1.8

N120 3.9 2.7 2.2 1.3

N120P30 4.4 2.9 2.2 1.5

N120P30K30 4.9 3.1 2.4 1.6

Mean 4.2 2.9 2.3 1.5

LSD (0.05)

Fertilization = 0.29

Soil depth = 0.26

Fertilization � soil depth = 0.59

Table 4Grain and straw yield and total above-ground biomass production (Mg ha�1) of

rice–wheat and maize–wheat cropping systems in relation to FYM and inorganic

fertilization

Treatments Rice–wheat Maize–wheat

Grain Straw Total Grain Straw Total

Control 7.36 a 19.8 a 27.2 a 4.21 a 7.41 a 13.6 a

FYM 12.7 b 35.7 b 48.4 b 10.1 b 20.4 b 30.5 b

N100 9.84 c 26.6 c 36.4 c 8.66 c 12.6 c 21.3 c

N100P50 11.5 b 32.7 b 44.2 b 9.55 b 13.5 c 23.0 c

N100P50K50 12.1 b 35.1 b 47.2 b 9.78 b 14.8 d 24.6 d

Similar letters in the columns indicate non-significant whereas dissimilar letters

indicate significant differences among the treatments.

S.S. Kukal et al. / Soil & Tillage Research 102 (2009) 87–9290

wheat (maize–wheat). Unlike rice–wheat system, the increase inSOC with soil depth in maize–wheat system was significant onlyin surface (0–15 cm) layer of FYM and N100P50K50-plots whenobserved at the end of maize but the SOC content at the harvest ofwheat shows the increase to be significant up to 30 cm depth inFYM plots but it was significant only in surface layer ofN120P30K30-plots.

Table 3Soil profile organic carbon (g kg�1 of soil) as affected by FYM and inorganic

fertilizers in maize–wheat cropping system

Treatments Soil depth (cm)

0–15 15–30 30–45 45–60

Maize

Control 3.0 2.1 1.4 1.1

FYM 3.9 2.7 1.9 1.4

N100 3.1 2.3 1.5 1.3

N100P50 3.4 2.3 1.7 1.3

N100P50K50 3.7 2.5 1.7 1.3

Mean 3.4 2.4 1.6 1.1

LSD (0.05)

Fertilization = 0. 34

Soil depth = 0.30

Fertilization � soil depth = 0.68

Wheat

Control 2.7 1.7 1.6 1.3

FYM 3.9 2.2 2.1 1.5

N100 2.9 2.0 1.6 1.4

N100P50 3.0 1.9 1.8 1.3

N100P50K50 3.4 2.1 1.9 1.4

Mean 3.2 1.9 1.8 1.4

LSD (0.05)

Fertilization = 0.26

Soil depth = NS

Fertilization � soil depth = 0.50

The SOC concentration in rice–wheat soils was 54 and 30% higherin FYM and N120P30K30 plots than in similar treatments in maize–wheat soils despite the fact that in control plots it was similar in thetwo soils. It, however, was not much different in N120 and N120P30

plots of the two cropping systems. Even the higher clay content insoil under maize–wheat did not help gain SOC concentration(McConkey et al., 2003; Aurelie Metay et al., 2007). High SOCconcentration under rice–wheat as compared to maize–wheat maybe attributed to greater aboveground biomass production andfavorable water regime during rice season. The grain and straw yielddata for the year 2004–2005 showed that total above-groundbiomass production in rice–wheat was higher by 13.6–22.6 t ha�1 ascompared to maize–wheat (Table 4). The total above-groundbiomass production in balanced fertilization plots (NPK) was 47.2and 24.6 Mg ha�1 in rice–wheat and maize–wheat, respectively.Higher above-ground biomass production in rice–wheat probablyresulted in greater biomass of roots and rhizodeposition leading togreater C input in the soil. Root biomass yield and rhizodepositon indifferent crops has been shown to be related to the abovegroundbiomass harvests (Bronson et al., 1998; Majumder et al., 2007).Secondly, the rate of soil organic matter decomposition is lessened inlowland rice fields, apparently due to excessively reduced conditions(Watanabe, 1984).

Total SOC stocks in 60-cm soil profile under rice–wheat systemwere significantly higher in FYM (31.4 Mg ha�1) and N120P30K30

plots (29.6 Mg ha�1) than in control (21.3 Mg ha�1) plots (Table 5).Higher amount of SOC in FYM plots could be due to greater C inputthrough FYM and enhanced crop productivity (Kundu et al., 2007).Terra (2004) reported that application of dairy manure increasedSOC by 2.7 Mg ha�1 in a cotton-cotton rotation with cover crops.The SOC stocks in N plots was statistically similar to that in control.It increased significantly with balanced fertilization from25.5 Mg ha�1 in N120 to 29.6 Mg ha�1 in N120P30K30-plots. Balancedfertilization is expected to increase SOC because of greater C inputassociated with enhanced primary production and crop residuesreturned to soil. Similar trend of SOC stocks in 60-cm soil profilewas observed in maize–wheat except its magnitude. It was 26%higher in FYM plots of rice–wheat than in maize–wheat, whereasin NPK-plots it was 27% higher in rice–wheat than in maize–wheat.

3.2. Soil bulk density and aggregation

The bulk density of different soil layers as observed at theharvest of wheat did not vary in two systems in response todifferent treatments (Table 6). However, the aggregation in termsof mean weight diameter (MWD) improved significantly by FYM aswell as NPK (Table 7). The MWD in rice–wheat increased by 75%with FYM in 0–15 cm soil layer, whereas in maize–wheat itincreased by 67%. In lower layers, the increase in MWD with FYMwas higher (75–77%) in maize–wheat than in rice–wheat (41–62%). The increase in MWD due to NPK was lower than with FYM in

Table 5Total soil organic carbon stock (Mg ha�1) as affected by inorganic fertilizers and

FYM as observed at the harvest of wheat in rice–wheat and maize–wheat systems

Treatments Soil depth (cm)

0–15 15–30 30–45 45–60 Total

Rice–wheat

Control 7.35 6.04 4.99 2.89 21.4

FYM1 2.1 8.31 6.44 4.59 31.4

N120 9.65 6.80 5.68 3.37 25.5

N120P30 10.7 7.26 5.61 3.87 27.4

N120P30K30 11.8 7.67 6.05 4.10 29.6

Mean 10.3 7.22 5.75 3.76

LSD (0.05)

Fertilization = 4.3

Soil depth = 2.1

Fertilization � soil depth = NS

Maize–wheat

Control 6.84 4.36 4.13 3.37 18.7

FYM 9.18 5.28 5.10 3.69 23.3

N100 7.31 4.82 4.08 3.59 19.8

N100P50 7.38 4.73 4.53 3.31 20.0

N100P50K50 8.11 5.07 4.64 3.49 21.3

Mean 7.76 4.85 4.50 3.49

LSD (0.05)

Fertilization = 2.1

Soil depth = 2.5

Fertilization � soil depth = NS

Table 7Mean weight diameter (MWD) (mm) as affected by inorganic fertilization and FYM

treatments under rice–wheat and maize–wheat

Treatments Soil depth (cm)

0–15 15–30 30–45 45–60

Rice–wheat

Control 0.129 0.085 0.064 0.054

FYM 0.524 0.224 0.157 0.092

N120 0.154 0.114 0.069 0.064

N120P30 0.244 0.116 0.096 0.071

N120P30K30 0.309 0.213 0.141 0.088

Mean 0.272 0.150 0.105 0.074

LSD (0.05)

Fertilization = 0.006

Soil depth = 0.007

Fertilization � soil depth = 0.013

Maize–wheat

Control 0.070 0.046 0.038 0.026

FYM 0.217 0.203 0.153 0.114

N100 0.080 0.068 0.060 0.032

N100P50 0.128 0.111 0.062 0.038

N100P50K50 0.163 0.135 0.102 0.069

Mean 0.132 0.113 0.083 0.056

LSD (0.05)

Fertilization = 0.04

Soil depth = 0.04

Fertilization � soil depth = 0.09

S.S. Kukal et al. / Soil & Tillage Research 102 (2009) 87–92 91

both rice–wheat and maize–wheat. However, it was similar in 0–15 cm soil layer in the two cropping systems, but in lower layers itwas again higher in maize–wheat than in rice–wheat system. Theincrease in MWD due to FYM and NPK decreased with soil depth inrice–wheat, but in maize–wheat it was similar in all the depths.

3.3. SOC sequestration

The SOC sequestration in rice–wheat and maize–wheat(Table 8) showed that in both the cropping systems sequestrationrate was highest in FYM plots followed by NPK plots. In both the

Table 6Soil bulk density (Mg m�3) observed at the harvest of wheat as affected by inorganic

fertilizers and FYM in rice–wheat and maize–wheat systems

0–15 cm 15–30 cm 30–45 cm 45–60 cm

Rice–wheat

Control 1.69 1.75 1.75 1.75

FYM 1.58 1.63 1.65 1.70

N120 1.65 1.68 1.72 1.73

N120P30 1.62 1.67 1.70 1.72

N120P30K30 1.60 1.65 1.68 1.71

LSD (0.05)

Fertilization = 0.06

Soil depth = 0.04

Fertilization � soil depth = 0.05

Maize–wheat

Control 1.69 1.71 1.72 1.73

FYM 1.57 1.60 1.62 1.64

N100 1.68 1.69 1.70 1.71

N100P50 1.64 1.66 1.68 1.70

N100P50K50 1.59 1.61 1.63 1.66

LSD (0.05)

Fertilization = 0.11

Soil depth = 0.03

Fertilization � soil depth = 0.05

cropping systems, balanced fertilization with NPK enhanced Csequestration rate, but the increase was more in rice–wheat thanmaize–wheat cropping system. When evaluated across treat-ments, the SOC sequestration rate was about three times higher inrice–wheat than in maize–wheat. In rice–wheat, the FYM plotssequestered 0.31 Mg ha�1 year�1 SOC compared to 0.14 Mg ha�1

year�1 in maize–wheat cropping system. Similarly, with balancedfertilization C was sequestered at 0.26 Mg ha�1 year�1 under rice–wheat as compared to 0.08 Mg ha�1 year�1 under maize-wheat.This shows that the soils under rice–wheat sequestered 55% higherSOC in FYM plots than in maize–wheat, whereas in NPK-plots, theSOC sequestration was 70% higher in rice–wheat than in maize–wheat. Higher rate of C sequestration in rice–wheat is due to thesoils being under a unique aquic (flooded) moisture regime for 3–4months under rice crop and secondly due to higher biomassproduction in rice as compared to maize. Because of lack of oxygenunder submerged conditions even a modest oxygen demand formicrobial activity cannot be met if large pores are filled with water(Jenkinson, 1988) resulting in decreased rate of decomposition.Sahrawat (2004) based on a literature review concluded that thereis preferential accumulation of organic matter in submerged rice

Table 8Soil organic pool (Mg ha�1) and calculated soil organic carbon sequestration rate

(kg ha�1 year�1) as affected by inorganic fertilizers and FYM in rice–wheat and

maize–wheat systems (0–60 cm profile)

Treatments Rice–wheat Maize–wheat

SOC pool C sequestration SOC pool C sequestration

Control 21.4 – 18.7 –

FYM 31.4 310 23.3 140

N120 25.5 130 19.8 30

N120P30 27.4 190 20.0 40

N120P30K30 29.6 260 21.3 80

Mean 27.1 220 20.6 70

LSD (0.05) – 40 – 30

S.S. Kukal et al. / Soil & Tillage Research 102 (2009) 87–9292

soils as compared to aerobic soils due to incomplete decomposi-tion of organic materials, and decreased humification of organicmatter under flooded conditions. Consequently, the overall organicmatter decomposition rates are slower in submerged soils thanthose in aerobic soils. This results in net accumulation of organicmatter in soils that remain flooded for several years. Witt et al.(2000) also reported 11–12% greater C sequestration in soilscontinuously cropped with rice for 2 years than in the maize-ricerotation with the higher amounts sequestered in N-fertilizedtreatments.

4. Conclusions

In both rice–wheat and maize–wheat cropping systems,application of FYM or balanced fertilization with NPK resultedin higher C sequestration. Rice–wheat system has greater capacityto sequester C as compared to maize–wheat system probablybecause of greater C input through enhanced productivity. The SOCconcentration was higher with FYM than with NPK application inboth rice–wheat and maize–wheat systems after a period of 32years. In both systems, the balanced fertilization improved the SOCconcentration, total SOC stocks and SOC sequestration rate.

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