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Soil & Tillage Research 92 (2007) 87–95

Carbon sequestration and relationship between carbon addition

and storage under rainfed soybean–wheat rotation in a

sandy loam soil of the Indian Himalayas

S. Kundu, Ranjan Bhattacharyya *, Ved Prakash, B.N. Ghosh 1, H.S. Gupta

Vivekananda Institute of Hill Agriculture (Indian Council of Agricultural Research), Almora 263601, Uttaranchal, India

Received 23 February 2005; received in revised form 19 January 2006; accepted 27 January 2006

Abstract

Soil organic matter (SOM) contributes to the productivity and physical properties of soils. Although crop productivity is

sustained mainly through the application of organic manure in the Indian Himalayas, no information is available on the effects of

long-term manure addition along with mineral fertilizers on C sequestration and the contribution of total C input towards soil

organic C (SOC) storage. We analyzed results of a long-term experiment, initiated in 1973 on a sandy loam soil under rainfed

conditions to determine the influence of different combinations of NPK fertilizer and fertilizer + farmyard manure (FYM) at

10 Mg ha�1 on SOC content and its changes in the 0–45 cm soil depth. Concentration of SOC increased 40 and 70% in the

NPK + FYM-treated plots as compared to NPK (43.1 Mg C ha�1) and unfertilized control plots (35.5 Mg C ha�1), respectively.

Average annual contribution of C input from soybean (Glycine max (L.) Merr.) was 29% and that from wheat (Triticum aestivum L.

Emend. Flori and Paol) was 24% of the harvestable above-ground biomass yield. Annual gross C input and annual rate of total SOC

enrichment were 4852 and 900 kg C ha�1, respectively, for the plots under NPK + FYM. It was estimated that 19% of the gross C

input contributed towards the increase in SOC content. C loss from native SOM during 30 years averaged 61 kg C ha�1 yr�1. The

estimated quantity of biomass C required to maintain equilibrium SOM content was 321 kg ha�1 yr�1. The total annual C input by

the soybean–wheat rotation in the plots under unfertilized control was 890 kg ha�1 yr�1. Thus, increase in SOC concentration under

long-term (30 years) rainfed soybean–wheat cropping was due to the fact that annual C input by the system was higher than the

required amount to maintaining equilibrium SOM content.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Carbon sequestration; Carbon addition and storage; Farmyard manure; Soybean–wheat cropping; Sandy loam soil; Sub-temperate

Indian Himalayas

1. Introduction

Sustaining soil organic matter (SOM) is of paramount

importance in terms of cycling plant nutrients and

improving the soil’s physical, chemical and biological

* Corresponding author. Tel.: +91 9412996534;

fax: +91 5962 241005.

E-mail address: ranjan_vpkas@yahoo.com (R. Bhattacharyya).1 Present address: Central Soil Water Conservation and Research

and Training Institute, Dehradoon, Uttaranchal, India.

0167-1987/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2006.01.009

properties. Maintenance of soil organic C (SOC) is

essential for long-term sustainable agriculture, since

declining levels generally lead to decreased crop

productivity (Allison, 1973). Soil organic C is an

important index of soil quality because of its relationship

to crop productivity (Campbell et al., 1996; Lal et al.,

1997). Optimum levels of SOM can be managed through

crop rotation, fertility maintenance including use of

inorganic fertilizers and organic manures, tillage

methods, and other cropping system components

(Huggins et al., 1995; Janzen et al., 1998). As SOC

S. Kundu et al. / Soil & Tillage Research 92 (2007) 87–9588

Table 1

Soil properties of initial conditions

Properties Soil depth (cm)

0–15 15–30 30–45

pH (soil:water, 1:2.5) 6.2 6.1 NA

EC (dS m�1) 0.08 0.07 NA

CEC [cmol (p+) kg�1 soil] 8.7 NA NA

Bulk density (Mg m�3) 1.32 1.33 1.36

Total organic C (g kg�1 soil) 5.8 5.6 5.3

C:N 12.8 12.1 12.3

Available N (mg kg�1 soil) 127 (125) 119 88

Available P (mg kg�1) 12 (7) 10 7

Available K (mg kg�1) 65 (55) 59 48

Texture Sandy loam Sandy loam NA

Data in parentheses indicate the critical values. NA: not available.

changes are generally directly related to the quantity of

crop residues returned to the land, agronomic practices

that influence yield and affect the residues returned to soil

are likely to influence SOC (Campbell et al., 1997, 2000).

Many studies have indicated a strong positive

relationship between the amount of C incorporated

into soil, either from crop residues or from external

sources such as manure, and total SOC content

(Paustian et al., 1992; Havlin et al., 1990). Changes

in SOC were linearly related to gross C input to the soil

(Rasmussen and Collins, 1991; Buyanovsky and

Wagner, 1998). Annual C input was 22% of the

harvestable above-ground biomass of soybean in a

Vertisol in Central India (Kundu et al., 2001). However,

there are no data on quantifying rhizodeposition in

soybean using tagged C (14C), although many reports

are available on other crops including wheat. In a green

house study with 11 plant species, Shamoot et al. (1968)

reported that rhizodepositon of C from root turnover

and exudates represented 5–20% of the above-ground

biomass. However, in soybean around 13% of the total

biomass (roots and tops) was found in roots alone by

Kemper et al. (1998).

The net change in SOC depends not only on the

current management practices but also on the manage-

ment history of the soil. Long-term experiments are the

primary source of information to determine the effect of

cropping systems, continuous cropping, and retention of

residues in soils and fertilizer/manure addition on

changes in SOC (Leigh and Johnston, 1994). These

experiments are usually the only source of information

for determining agricultural sustainability (Barnett

et al., 1995) and to define land use effect on SOC

(Paul et al., 1997). For getting meaningful estimates of

the rate of added biomass incorporation into SOM and

decay rates of native SOC, it is desirable to have

experiments with full records of gross C inputs into the

soil through various sources. Therefore, quantification

of SOC in relation to various soil management practices

is of value in identifying the pathways of C sequestra-

tion in soils.

Several experiments began in the 1970s in Asian

countries under tropical conditions with rice (Oryza

sativa L.) and wheat system under irrigated conditions.

The data of many such experiments have been analyzed,

but mostly restricted to yield trends and soil nutrient

status (N, P and K) and SOC content. Several authors

suggested declining rice and wheat yields due to gradual

decline in the supply of soil nutrients and SOC (Cassman

et al., 1995; Yadav et al., 1998, 2000; Duxbury et al.,

2000). Others reported an increase in SOC due to addition

of organic sources of nutrients along with high inputs of

NPK fertilizers (Gami et al., 2001; Bhandari et al., 2002).

We found no report of C sequestration, the relationship

between C addition and storage and estimation of decay

rates of native SOC under sub-temperate climate of the

Indian Himalayas with low input agriculture. Hence our

objectives were: (i) to quantify the effects of different

combinations of mineral fertilizer application and FYM

addition along with mineral fertilizers on C sequestra-

tion, and (ii) to establish a relationship between annual C

addition and storage.

2. Materials and methods

2.1. Experimental site

A long-term field experiment was initiated in June

1973 on a Typic Haplaquept at the experimental farm of

Vivekananda Institute of Hill Agriculture, located in the

Indian Himalayan region at Hawalbagh (298360N and

798400E with 1250 m above mean sea level), in the state

of Uttaranchal, India. Initially the purpose of the

experiment was to study the effect of manure

application along with fertilizers on yield of rainfed

soybean–wheat cropping and soil nutrient status. Soil

characteristics based on analysis of preserved soil

samples taken in 1973 are given in Table 1. The climate

is sub-temperate, characterized by moderate summer

(May–June), extreme winter (December–January) and

general dryness, except during the southwest monsoon

season (June–September). Based on the records for the

period 1973–2003, average kharif (monsoon) season

rainfall was 704 mm (e.g., 68% of the average annual

rainfall). During soybean growth, average monthly

rainfall (mm) was: June (146); July (257); August (187)

and September (119), whereas, during wheat growth

average monthly rainfall (mm) was: October (27);

S. Kundu et al. / Soil & Tillage Research 92 (2007) 87–95 89

November (6); December (28); January (53); February

(56); March (48) and April (42). The mean monthly

temperature ranged from a minimum of 0.4 8C in

January to a maximum of 31.5 8C in May. The field was

newly reclaimed for cultivation in 1973. Prior to 1973,

the field was native grassland that was yearly cut and

grazed.

2.2. Experimental design

The experiment included two crops per year, soybean

(June–September) and wheat (October–April), with six

treatment combinations (in kg ha�1): no fertilizer and no

manure (unfertilized control); 20 N + 35 P (NP); 20

N + 33 K (NK); 20 N + 35 P + 33 K (NPK), 20 N +

FYM at 10 Mg ha�1 (N + FYM, commonly used by

the local farmers) and NPK + FYM at 10 Mg ha�1

(NPK + FYM). Treatments were distributed in a ran-

domized block design with six replications over three

uniformly level terraces. The net plot size was

5.4 m � 2.0 m. Fertilizers used were urea for N, single

super-phosphate for P and murate of potash for K. Based

on the chemical analysis of every fifth year, FYM had

370 g moisture kg�1 and contained 7.1–7.5 g N kg�1,

2.1–2.4 g P kg�1 and 5.3–5.8 g K kg�1 on oven-dry

weight basis.

2.3. Crop management

Farmyard manure (10 Mg ha�1 on a fresh weight

basis) and fertilizers were applied before sowing of

soybean. Soybean (80 kg seed ha�1) was sown in early

June each year after the land was tilled twice by hand

with a spade to a depth of 15 cm. Soybean cultivar

‘‘Bragg’’ was used from 1973 to 1998 and thereafter it

was replaced with VLS 2 (1999–2002). The crop was

sown in rows 40 cm apart to a depth of 4–5 cm by hand.

Propanil (3,3,4-dichloro-propionanilide) was sprayed

two days after sowing at the rate of 1.5 kg a.i. ha�1 in all

plots to control weeds. Other plant protection measures

were taken as needed to control diseases and pests for

both crops. Soybean was harvested manually just above

the ground in the first week of October using sickles and

above-ground biomass was removed from the field.

Grain yield was adjusted to 90 g moisture kg�1 for

soybean.

Wheat was grown on residual fertility. Wheat (cultivar

Sonalika from 1973–1974 to 1988–1989; VL 421 from

1989–1990 to 1998–1999; VL 616 from 1999–2000 to

2002–2003) was sown (100 kg seed ha�1) in rows

22.5 cm apart to a depth of about 5–6 cm by hand in

early October each year. Other field operations were as

with soybean. Wheat was harvested at 5 cm above the

soil surface in late April and straw was removed from the

plots and dry weights were recorded. Wheat straw below

5 cm (stubble) was incorporated into the soil during land

preparation for soybean. Grain yield was adjusted to

120 g moisture kg�1 for wheat.

2.4. Soil and plant sampling and analysis

Initial soil samples were collected in 1973 prior to

the start of the experiment. Soil was analyzed for pH in

1:2.5 soil:water suspension (Jackson, 1973), oxidizable

SOC by the method of Walkley and Black (1934), total

SOC (from the stored soil samples) by dry combustion

(Nelson and Sommers, 1982) with a CHN analyzer,

total N by standard procedure using a FOSS Tecator

(Model 2200), available P following the method of

Olsen et al. (1954), and available K by 1 N NH4OAc

using a flame photometer (Jackson, 1973). Core sampler

was used for soil bulk density determination. Soil

texture was determined by Bouyoucos hydrometer

(Bouyoucos, 1927).

After the harvest of wheat in 2003, depth-wise (0–15,

15–30 and 30–45 cm) triplicate soil samples were

collected from all plots, combined, ground, dried

(65 8C) passed through 0.2-mm sieve, and total C

content measured. Soil organic C concentrations were

converted from g kg�1 to Mg C ha�1 using measured

soil bulk density. Annual rate of change in SOC

concentration over initial soil condition was calculated.

Soil organic C sequestration in different treatments was

calculated as the net increase of SOC content

(Mg C ha�1) over the unfertilized control (Aulakh

et al., 2001).

Using biomass yield of soybean and wheat, annual

inputs of C added to the soil was computed. In case of

soybean, unmeasured biomass was through leaf-fall,

roots and nodules. Leaf-fall from all treatments was

manually collected during the last 5 years of experi-

mentation from 45 days after sowing until harvest, dried,

and dry weight recorded. Root and nodule biomass of

soybean were calculated using the root/shoot and nodule/

root biomass ratios recorded from a pot experiment

(Kundu et al., 2001). The pot experiment was conducted

using 10 kg capacity earthen pots filled with soil

collected during 1999–2002 from the field experiment.

The treatment-wise soil samples collected from the field

experiment were processed to pass through 2.0-mm sieve

and equivalent amounts of fertilizer and manure were

applied as per the treatments. After harvest of above-

ground biomass (shoot), soil was removed gently by

placing the pot on a sieve and washed slowly using a

S. Kundu et al. / Soil & Tillage Research 92 (2007) 87–9590

Table 2

Soil bulk density and organic C at 0–45 cm as affected by 30 years of

fertilizer application in a rainfed soybean–wheat rotation

Treatments Soil bulk density

(Mg m�3)a

Soil organic C

Concentration

(g kg�1 soil)

Amount

(Mg ha�1)

Initial soil 1.34 5.54 33.3

Control 1.31 6.01 35.5

NP 1.28 6.70 38.7

NK 1.30 6.45 37.6

NPK 1.28 7.51 43.1

N + FYM 1.26 9.63 54.7

NPK + FYM 1.25 10.76 60.3

L.S.D. (P = 0.05) 0.02 0.09 0.8

a Soil bulk density at 0–45 cm layer = average soil bulk density of

0–15, 15–30 and 30–45 cm soil layers.

water jet. Root biomass with entangled nodules was

recovered, dried, and dry weights of root and nodules

recorded. From dry weights, root/shoot and nodule/root

ratios were estimated.

During 1999–2002, soybean leaves were collected

after senescence, nodules were excavated on 65 days

after sowing, and roots were excavated at 85 days after

sowing. Samples were analyzed for total C content using

a CHN analyzer for calculation of C inputs. Rhizode-

position of C from root turnover and exudates was

assumed to be 10% of the harvestable above-ground

biomass of soybean (Shamoot et al., 1968). Organic C of

FYM was 22% C on oven-dry weight basis.

Wheat biomass to soil was through stubble (3–4% of

the straw yield) and roots [30% of the harvestable

above-ground biomass as observed by Chander et al.

(1997)]. Samples of stubble and roots of wheat were

collected after harvest (1999–2000 to 2002–2003) from

all the plots and C inputs through stubble and roots were

calculated based on C determination with a CHN

analyser. The contribution of C through rhizodeposition

from wheat was estimated by multiplying the values of

total root C inputs with a factor of 1.4 as observed by

Regmi (1994). During growth of soybean and wheat,

weeds were removed so that C input from roots and

rhizodeposition by the weeds were not considered.

The relationship between C addition to the soil by

different sources and storage within the soil was

determined. The rate constant of annual C inputs

incorporated into SOM (h) and decay rate constant (k)

of native SOC were calculated from the following

equation (Jenkinson, 1988) assuming a single-pool, first-

order kinetic relationship between C addition and storage

dCs

dt¼ hA� kSOC (1)

where SOC is the soil organic C, t is time, and A is

annual C input to soil. The same first order kinetic

equation was also used by Parton et al. (1996) to explain

changes in SOC in terms of C added to the soil, wherein

they termed ‘h’ as ‘carbon storage fraction constant’.

From the single-pool first-order equation we estimated

the quantity of biomass C required to maintain equili-

brium SOC content. For computing the ‘A’ value, we

assumed that whole amount of biomass C from leaf fall

and FYM, and 90% of the biomass C from roots,

nodules and rhizodeposition remained within the 0–

45 cm soil layer for soybean. For wheat, we assumed

80% of the biomass contributed by roots and exudates

remained within 0–45 cm soil depth. Not all the C from

below-ground biomass and root exudates would remain

in the 0–45 cm depth of the soil profile as some C could

leach. Kundu et al. (2001) assumed 70% of the biomass

C from roots, nodules and rhizodeposition of soybean

and 70% of biomass C from roots and rhizodeposition

of wheat remained in the 0–30 cm layer in Central

India. The basis of choosing 10% higher value in case

of soybean over wheat was that the root system of

soybean is shallower than that of wheat. Kemper

et al. (1998) reported that roots of soybean and wheat

extended as deep as 60 and 225 cm, respectively.

Statistical analyses were done using standard

analysis of variance (Gomez and Gomez, 1984).

Treatment means were compared at 5% level of

significance using least significant difference.

3. Results and discussion

3.1. Soil bulk density

Soil bulk density (BD) was lower with FYM than

with inorganic fertilizers only or the unfertilized control

(Table 2). Highest BD was under the unfertilized

treatment at 30–45 cm depth (1.33 Mg m�3) and lowest

BD (1.24 Mg m�3) was under NPK + FYM treatment at

0–15 cm depth. Application of FYM might have

impacted soil aggregation, thereby reducing BD.

Reduction in soil BD with organic matter additions is

well documented (Sharma et al., 1995). Soil BD

decreased with FYM application due to higher SOC and

increased root biomass (Halvorson et al., 1999) that

resulted in better soil aeration and improvement of soil

structural properties.

3.2. Plant derived C inputs to soil

The highest mean harvestable above-ground biomass

of soybean (Table 3) and wheat (Table 4) was observed

S. Kundu et al. / Soil & Tillage Research 92 (2007) 87–95 91

Table 3

Annual input of soybean above-ground biomass (1973–2002), leaf-fall (1998–2002) and roots and nodules (1999–2002) under different fertilizer

treatments

Treatments Havestable above-ground biomass

(shoot) yield (kg ha�1)

Annual addition of biomass to soil (kg ha�1)

Leaf-falla Rootb Nodulec

Control 1307 124 549 62

NP 2139 344 898 103

NK 1601 115 672 78

NPK 3440 757 1280 196

N + FYM 5749 1150 1748 262

NPK + FYM 7024 1271 2374 318

L.S.D. (P = 0.05) 291 48 112 15

a From the pot experiment, we estimated that under the unfertilized control, NP, NK, NPK, N + FYM and NPK + FYM-treated plots, the leaf-fall

biomass constituted 9.5, 16.1, 7.2, 22.0, 20.0 and 18.1% of the harvestable above-ground biomass, respectively.b Under unfertilized control, NP, NK, NPK, N + FYM and NPK + FYM treatments, root biomass constituted 42, 42, 42, 37, 30 and 34% of the

harvestable above-ground biomass, respectively.c Under unfertilized control, NP, NK, NPK, N + FYM and NPK + FYM treatments, nodule biomass constituted 11.3, 11.5, 11.6, 15.3, 15.0 and

13.4% of the root biomass of soybean, respectively.

Table 4

Annual input of soybean above-ground biomass (1973–74 to 2002–03), stubble (1999–2000 to 2002–2003) and roots (1973–74 to 2002–03) under

different fertilizer treatments

Treatments Havestable above-ground biomass

(shoot) yield (kg ha�1)

Annual addition of biomass to soil (kg ha�1)

Stubblea Rootb

Control 2119 85 636

NP 2473 84 742

NK 2350 94 705

NPK 3025 91 908

N + FYM 4518 140 1355

NPK + FYM 5414 173 1624

L.S.D. (P = 0.05) 276 10 85

a We estimated that under the unfertilized control, NP, NK, NPK, N + FYM and NPK + FYM-treated plots, the stubble biomass constituted 4.0,

3.4, 4.0, 3.0, 3.1 and 3.2% of the straw yield of wheat, respectively.b We assumed that root biomass constituted 30% of the harvestable above-ground biomass of wheat.

Table 5

Estimated annual C inputs to soil (0–45 cm) from soybean under different fertilizer treatments

Treatments C inputs (kg ha�1)a Annual gross C input into

soil (kg C ha�1)Leaf-fall Root Nodule Rhizodeposition FYM

Control 47 174 18 131 – 371

NP 126 276 28 214 – 643

NK 45 200 22 160 – 427

NPK 291 404 58 344 – 1097

N + FYM 426 543 72 575 1587 3204

NPK + FYM 478 708 89 702 1587 3565

L.S.D. (P = 0.05) 18 35 4 31 – –

a We assumed that entire amount of biomass through leaf fall and FYM and 90% of the biomass contributed by roots, nodule, and rhizodeposition

remained within 0–45 cm soil depth.

S. Kundu et al. / Soil & Tillage Research 92 (2007) 87–9592

in the plots under NPK + FYM and the lowest under

unfertilized control. The higher above-ground biomass

in FYM amended soils was probably associated with

other benefits apart from N, P and K supply, such as

improvements in microbial activity (Ved Prakash et al.,

unpublished data) and soil physical conditions (Bhat-

tacharyya et al., unpublished data). FYM applied to rice

had residual positive effects on the next wheat crop

(Yadvinder-Singh et al., 1995).

The average annual total C input to soil from

soybean (Table 5) and wheat (Table 6) varied with

above-ground yield responses of both crops under

different fertilizer application. Roots of soybean crop

contributed the highest amount of annual addition of

biomass to soil. Kundu et al. (1997) also estimated that

root biomass of soybean added the highest amount

(31% of the above-ground biomass) to soil.

Mean C concentration of soybean leaves, roots and

nodules was 38, 34 and 32%, respectively. The roots and

rhizodeposition were major components of estimated C

inputs from soybean (Table 5). Without fertilizer total C

input from soybean was 371 kg ha�1 yr�1. Total C was

1097 kg ha�1 yr�1 with NPK and 3565 kg ha�1 yr�1

with NPK + FYM. Apart from FYM’s own contribution

(1587 kg C ha�1 yr�1); an additional gain of 872 kg

C ha�1 yr�1 occurred under soybean with NPK + FYM

compared to NPK only.

Like soybean, roots and rhizodeposition from wheat

also contributed the highest amount of C input to soil

(Table 6). Total C input from unfertilized wheat was

520 kg ha�1 yr�1 and 1286 kg ha�1 yr�1 from wheat

grown on residual fertility of NPK + FYM. The

residual effect of FYM resulted in an additional gain

of 563 kg C ha�1 yr�1 in wheat over that under NPK

only. The harvestable above-ground biomass yield

of wheat varied due to variation in residual

fertility from different fertilizer treatments applied

to soybean.

Table 6

Estimated annual C inputs to soil (0–45 cm) from wheat under different fe

Treatments C inputs (kg ha�1)a

Stubble Root

Control 36 201

NP 37 238

NK 40 232

NPK 37 286

N + FYM 58 439

NPK + FYM 74 505

L.S.D. (P = 0.05) 4 26

a We assumed that entire amount of biomass through stubbles and 80% of th

soil depth.

Average C input from wheat to soil (Table 6) was

24% of the harvestable above-ground biomass

(R2 = 0.998*, P < 0.01). Kundu et al. (2001) reported

a value of 32%. Kemper et al. (1998) reported a root/

shoot ratio of 0.25 in wheat. In wheat, considerable

amount of C input (on average 445 kg C ha�1 yr�1) to

soil originated from rhizodeposition. Study with 14C

indicated that rhizodeposition during the growing

season of wheat was 30% of the total C accumulated

above-ground (Lucas et al., 1977; Jenkinson and

Rayner, 1977; Keith et al., 1986).

3.3. Soil carbon

Soil organic C concentrations remained positively

changed from the initial values in plots under

unfertilized control (Table 2). This was due to annual

C addition through the roots and crop residues (Ved

Prakash et al., 2002). At the end of 30 years, soils under

NPK + FYM-treated plots contained higher SOC by

43.8 and 40.1% in the 0–15 and 0–45 cm soil layers,

respectively, over NPK-treated plots (Table 2). Total

SOC stocks in the 45 cm layer were 60.3 Mg ha�1 for

NPK + FYM-treated soils compared with 43.1 Mg ha�1

for NPK-treated plots. Thus, there was

573 kg C ha�1 yr�1 increase in SOC contents in the

plots under NPK + FYM treatment as compared to

those under NPK-treated plots. Soil organic C content at

0–45 cm soil layer in the plots under NPK + FYM

treatment was 15% higher than that under N + FYM-

treated plots (Fig. 1).

At the end of 30 years, application of balanced dose

of NPK showed significantly higher SOC (7.51 g kg�1)

over the unfertilized control (6.01 g C kg�1) in 0–45 cm

soil layer. Application of NPK also showed significantly

higher SOC concentrations over either NP or NK. There

was significant improvement in SOC in NP

(6.70 g C kg�1) over NK (6.45 g C kg�1) in the same

rtilizer treatments

Annual gross C input

into soil (kg C ha�1)Rhizodeposition

282 519

333 608

324 596

401 723

614 1111

707 1286

40 69

e biomass contributed by roots and exudates remained within 0–45 cm

S. Kundu et al. / Soil & Tillage Research 92 (2007) 87–95 93

Fig. 1. Annual change in soil organic C content during 30 years as affected by different fertilizer treatment.

Fig. 2. Change in SOC content as affected by estimated gross C input

to 0–45 cm depth of soil layer.

depth. Compared with the plots under NPK, total SOC

content increased significantly in plots where N + FYM

or NPK + FYM were applied (Table 2). The plots with

NPK + FYM treatment applied to soybean accumulated

highest SOC concentration (10.76 g kg�1) in 0–45 cm

soil depth, which was 43% higher than that of NPK-

treated plots (7.51 g kg�1) and 12% higher than that of

N + FYM-treated plots (9.63 g kg�1).

The benefits of sequestering SOC to sustain crop

productivity by applying organic amendments have

been well documented in the temperate regions

(Aulakh et al., 2001). In our study, use of both

fertilizers and FYM increased SOC due to increased

yields of roots and plant residues, and the direct

application of organic matter through FYM. Fertilizer

application in the NPK + FYM-treated plots was the

same as in the NPK treated ones. The difference in

SOC, therefore, was a result of added C through entry

of FYM and increased biomass of both the crops. A

larger proportion of the higher C content or C

sequestration in FYM added plots also might have

resulted from slower breakdown rate (less and constant

mineralization rate).

3.4. Rate constant (h) of added biomass C

incorporation into soil organic matter

The annual rate of change in SOC was positively

correlated (P < 0.01) with gross C inputs (Fig. 2). The

intercept (�61.3 kg C ha�1 yr�1) of the equation would

represent the annual loss of C from native SOM.

Equating this intercept with ‘k � Cs’ of the Jenkinson

(1988) equation and setting initial SOC content at

33.3 Mg C ha�1, the decay rate of native SOC was

0.0018. This indicated that C loss from native SOC

during 30 years of cultivation was 0.18% of the initial

SOC content.

3.5. Minimum C input to maintain SOC

When addition of C input to the soil equals loss of C

(i.e, dCs/dt = 0), annual C input was 321 kg ha�1 yr�1.

This amount would be required from plant parts or

manures to be incorporated to maintain SOM content at

equilibrium. Annual loss of C from native SOM in the

Pacific North-west of the USA was 200 to

2000 kg ha�1 yr�1 (Rasmussen and Albrecht, 1998)

S. Kundu et al. / Soil & Tillage Research 92 (2007) 87–9594

and in sub-tropical Central India was 888 kg ha�1 yr�1

(Kundu et al., 2001). Our relatively low value at

61 kg ha�1 yr�1 suggests a relatively slow decomposi-

tion rate of native SOM. Under unfertilized conditions,

average productivity (grain + straw yield) of soybean

(1.31 Mg ha�1) and wheat (2.12 Mg ha�1) contributed

C input of 371 and 520 kg ha�1 yr�1, respectively. The

total annual C input under unfertilized control treatment

was nearly three times more than the amount of C input

required (321 kg ha�1 yr�1) to maintain SOM at

equilibrium. So, even removing the above-ground

biomass of both crops, these production levels on

sandy loam soils in the Indian Himalayas would be

sufficient to supply enough C input to soil (provided soil

erosion be checked), so that SOC could be maintained.

4. Conclusion

Long-term (30 years) average (all treatments) above-

ground biomass of soybean and wheat was 3543 and

3317 kg ha�1 yr�1, respectively. The proportion of

mean harvestable above-ground biomass as C input

from soybean and wheat was 29 and 24%, respectively.

Application of inorganic fertilizer resulted in higher

SOC over the unfertilized control, due to greater crop

residue and root biomass produced. The annual rate of

increase in SOC content over initial condition was 74,

328, 711 and 900 kg C ha�1 under unfertilized, NPK,

N + FYM and NPK + FYM treatments. Thus, addition

of FYM resulted in higher SOC and higher productivity

of both soybean and wheat crops.

From the relationship of C addition and storage, it was

observed that (i) 19% of the gross C input contributed to

SOC accumulation, (ii) annual C loss during 30 years

from native SOM averaged 61 kg C ha�1, (iii) C loss

from native SOC during 30 years of cultivation was

0.18% of initial SOC content, and (iv) 321 kg C ha�1 was

needed to maintain SOM at equilibrium. Total annual C

input by the soybean–wheat rotation under unfertilized

control was 890 kg ha�1 yr�1. Thus, rainfed soybean–

wheat cropping in the sub-temperate Indian Himalayas is

a system that can sequester soil organic C.

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