Soil Biology & Biochemistry
Soil Aggregation and Associated Organic
Carbon Fractions as Affected by Tillage in a
Rice–Wheat Rotation in North India
Mamta Kumari
Debashis Chakraborty
Division of Agricultural Physics
Indian Agricultural Research Institute
(IARI)
Mahesh K. Gathala
International Rice Research Institute
(IRRI)
India Office
1st Floor, CG Block
NASC Complex
New Delhi 110012, India
H. Pathak
Division of Environmental Sciences
Indian Agric. Research Institute (IARI)
B.S. Dwivedi
Division of Soil Science and Agricultural
Chemistry
Indian Agric. Research Institute (IARI)
Rakesh K. Tomar
R.N. Garg
Ravender Singh
Division of Agricultural Physics
Indian Agric. Research Institute (IARI)
Jagdish K. Ladha*
International Rice Research Institute
(IRRI)
India Office
1st Floor, CG Block
NASC Complex
New Delhi 110012, India
Soil samples were obtained from a long-term trial conducted on a silty loam at Sardar Vallabhbhai Patel University
of Agriculture & Technology, Modipuram (Meerut), in 2007–2008 to study the effects of various combinations
of conventional and zero-tillage (ZT) and raised-bed systems on soil aggregation and associated organic C
fractions in the 0- to 5-cm and 5- to 10-cm depth in a rice–wheat (Orysa sativa L.–Triticum aestivum L.) rotation.
Macroaggregates increased under a ZT rice (direct-seeded or transplanted) and wheat rotation with the 2- to 4-mm
fraction greater than that of the 0.25- to 2-mm fraction. Bulk and aggregate associated C increased in ZT systems
with greater accumulation in macroaggregates. The fine (0.053–0.25 mm) intra-aggregate particulate organic
C (iPOM-C), in 0.25- to 2-mm aggregates, was also higher in ZT than conventional tillage. A higher amount
of macroaggregates along with greater accumulation of particulate organic C indicates the potential of ZT for
improving soil C over the long-term in rice-wheat rotation.
Abbreviations: AS, aggregate stability; DSR, direct drill seeding; GMD, geometric mean diameter;
iPOM-C, intra-aggregate particulate organic matter carbon; MWD, mean weight diameter; NaPT,
sodium polytungstate; POM-C, particulate organic matter carbon; RCTs, resource-conserving
technologies; SOC, soil organic carbon; TPR, transplanted; ZT, zero-tillage or no-tillage.
T
he rice–wheat cropping system, one of the largest agricultural production
systems in the world, accounts for nearly one-third of the cultivable area of
both rice and wheat grown in South Asia (12 Mha in India). The system produces
staple food grains for more than 20% of the world’s population (60% in India). It
is characterized by two contrasting edaphic environments, puddling in rice, which
destroys the soil structure (Sharma et al., 2003), and excessive tillage in wheat, resulting in its late planting (Hobbs and Gupta, 2003) and loss of soil organic matter
(Gupta et al., 2003). In the post-Green Revolution era, soil organic carbon (SOC)
content has declined from 5 to 2 g kg−1 in the major rice–wheat growing regions
in northwestern India (Sinha et al., 1998), with concomitant reports of a decline
in yield (Nambiar, 1994; Abrol et al., 2000; Yadav et al., 2000; Manna et al., 2005).
Conventional tillage practices are widely reported to adversely affect soil C stocks,
thereby degrading the soil resource base and threatening the potential productivity
of the rice–wheat cropping system (Ladha et al., 2003). In recent years, efforts have
been made to develop and evaluate various tillage and crop establishment technologies, like reduced or ZT, on flat-or raised-beds with direct-seeding, often termed
resource-conserving technologies (RCTs). These technologies provide an alternative to the intensive tillage practices commonly practiced in rice–wheat systems
(Ladha et al., 2009). Whereas information is available to demonstrate the impact
of alternate tillage practices on crop productivity and farmers’ incomes, little is
known about their effects on soil properties, especially soil C storage.
Soil Sci. Soc. Am. J. 75:560–567
Posted online 18 Feb. 2011
doi:10.2136/sssaj2010.0185
Received 1 May 2010.
*Corresponding author ([email protected]).
© Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by
any means, electronic or mechanical, including photocopying, recording, or any information storage
and retrieval system, without permission in writing from the publisher. Permission for printing and for
reprinting the material contained herein has been obtained by the publisher.
560
SSSAJ: Volume 75: Number 2 • March–April 2011
Experiments suggest that zero or no-tillage usually favors
the formation of soil aggregates and minimizes the potential
for rapid oxidation of SOC due to reduced soil disturbance
(Chaney et al., 1985; Elliott, 1986; Beare et al., 1994; Six et
al., 2000). Macroaggregates (>0.25 mm) that provide better
physical protection to SOC are regarded as the best predictor
of potential C responses to tillage and residue management
practices (Angers and Giroux, 1996; Jastrow et al., 1996). Several
studies (Angers and Giroux, 1996; Beare et al., 1994; Gale, 1997;
Jastrow et al., 1996; Six et al., 1998) suggest that soil C gets
incorporated first into macroaggregates and then forms the core
of new microaggregates (Oades, 1984). This physical protection
of C within macroaggregates limits its oxidation (Balesdent et
al., 2000) by creating a less favorable environment for microbial
activity (Franzluebbers et al., 1994) and thus can reduce its
decomposition rate by half or more (Hassink, 1997; Balesdent et
al., 2000; Grandy and Robertson, 2006).
The particulate organic matter C (POM-C) fraction is
regarded as an intermediate pool of SOC (Beare et al., 1994;
Franzluebbers et al., 1999), serving as an early indicator of
changes in SOC levels that influence aggregation and nutrient
dynamics in soil (Franzluebbers et al., 1995, 1995a; SalinasGarcias et al., 1997; Six et al., 1999). Though the effects of
conservation tillage on POM-C vary, fine intra-aggregate
POM-C (iPOM-C) from both micro- and macroaggregates in
surface (0–5 cm soil) is generally greater in ZT (Six et al., 1999,
2000). This fraction is also identified as a potential indicator of
increased C-sequestration under ZT compared with POM-C
per se (Six et al., 2000; Denef et al., 2007). Hence, iPOM could
further play a key role in the global C cycle by providing a sink
for atmospheric CO2 when appropriate management practices
are adopted (Paustian et al., 1998, 2000).
Therefore, this study aimed at evaluating medium- to
long-term consequences of various alternate tillage and crop
establishment options, such as ZT and reduced-tillage, on
flat or raised bed with direct-seeding of rice and wheat visà-vis conventional-tillage with puddle transplanted rice and
broadcasted wheat on soil aggregation and organic C pools. A
carefully designed replicated 7-yr experiment conducted in the
Indo-Gangetic Plains of North India (Bhushan et al., 2007) was
utilized for this study.
MATERIALS AND METHODS
Site Description
An ongoing, long-term, field experiment on the rice-wheat system
was established in 2002 (Bhushan et al., 2007) at Sardar Vallabhbhai
Patel University of Agriculture & Technology, Modipuram, Meerut
(29°01′N, 77°45′E, 237 m asl), India. It was used in the current study
during 2007–2008. The experiment has been continuing since 2002
with the same set of treatments and rice–wheat varieties, and almost
similar management practices over the years. The climate is semiarid,
with normal annual precipitation of 800 mm. During the study period,
June was the hottest and January the coolest month, with average air
temperatures of 35.0 and 13.4°C, respectively. The months of June, July,
and August receive the majority of the rainfall (72% of annual normal),
with an average relative humidity of 70%. According to initial samples
collected during 2002, the soil (0–15-cm) is classified as a silty loam
in texture (Typic Ustocrept) with 55, 26, and 19% sand, silt, and clay
content, respectively. It had pH 8.1; EC 0.4 dS m−1; exchangeable
sodium percentage (ESP) 13.5 g kg−1; total C 8.3 g kg−1; total N 0.88 g kg−1;
Olsen P 25 mg kg−1; and 1 M NH4OAC-extractable K 121 mg kg−1.
The soil retained 18 and 7% water (mass basis) at −30 and −1500 kPa
water potential, and was uniform in topography.
Experimental Details and Management
Six treatments (T1 to T6) involving three tillage methods
(puddling and ZT planting on flat-beds and on raised-beds) and two rice
establishment methods (direct drill seeding, DSR, and transplanting,
TPR) were evaluated in a rice–wheat rotation, replicated thrice in a
randomized complete block design. Raised beds were permanent, that
is, beds were not disturbed every season; they were reshaped at the time
of wheat sowing in a single pass of a tractor. The plot size was 15.0 m ×
6.7 m. The full experimental details were previously published (Bhushan
et al., 2007). Treatment details are given in Table 1. In T1 (CT-TPR
rice), plots were irrigated (5-cm depth of standing water) daily for the
first 2 wk and thereafter irrigation (5-cm depth of standing water)
was applied on the appearance of hairline cracks on the soil surface.
The same irrigation schedule was followed in T2 (CT-TPR AWD), T4
Table 1. Description of treatments.
Treatment
Rice
Wheat
Conventional puddled-transplanted rice; 21-d-old rice seedlings
transplanted at 20 × 20-cm plant-hill spacing; rice plots kept flooded
(5-cm water) for first 2 wk, followed by irrigation at hair-line cracking
Conventional tillage after rice harvest followed by
drill-seeded wheat at 20-cm row spacing (using press
drill with dry-fertilizer attachment)
T2
Same as T1, except that irrigation withheld for about 1 mo after
maximum tillering stage (at about 3 wk after transplanting)
Wheat seeded in zero-till plots at 20-cm row spacing
with zero-till press drill with dry-fertilizer attachment
T3
Direct drill-seeded rice on permanent raised-beds; 2 rows per bed at
20-cm row spacing; first irrigation applied 1 d after seeding, followed
by 3- to 4-d interval for 4 wk after germination, and thereafter irrigation
at the appearance of hair-line cracks at the bottom of furrow
Bed planter-seeded wheat on raised- beds at 20-cm
spacing
T1
T4
T5
T6
Transplanted rice on raised-beds in 2 rows at 12×20-cm hill spacing;
irrigations daily for 2 wk after transplanting, followed by irrigation at
hair-line cracking at the bottom of furrow
Same as T3, except that seeding was done on flat-beds
Same as T1, except that transplanting was done in zero-till plots
SSSAJ: Volume 75: Number 2 • March–April 2011
Same as T3
Same as T2
Same as T2
561
(bed-TPR), and T6 (ZT-TPR). In T2, irrigation during tillering to the
onset of flowering (20–70 d after transplanting) was applied at 10- to
15-d intervals. The irrigation schedule in T3 (bed-DSR rice) and T5
(ZT-DSR rice) was the same, that is, the first irrigation was applied 1
d after seeding, followed by irrigation at 3- to 4-d intervals for 4 wk
after germination, and thereafter irrigation at the appearance of hairline
cracks. The difference in irrigation between bed-planting and other
plots was in the amount and observance of hairline cracks. In rice on
raised-bed, hairline cracks were observed at the bottom of the furrow
and the furrows were completely filled with water during each irrigation,
avoiding over-topping of beds. Whereas, in other plots, flooding to a
depth of 5-cm was done. The irrigation schedule in wheat was the same
in all treatments, except that, while 6-cm irrigation was applied in
regular plots, furrows were completely filled with water in bed-planting,
thus avoiding the over-topping of beds.
In all plots, rice and wheat were harvested manually, leaving 15-cm
of stubble above the ground level. The leftover stubble, which accounted
for not more than 2 Mg ha−1 of residue, was completely incorporated in
the conventional systems, but was left undisturbed on the soil surface in
the ZT and bed-planting treatments.
Soil Sampling and Processing
Soil samples were collected from the 0- to 5- and 5- to 10-cm
depths during kharif ( July-October), 2007 and rabi (November-April),
2007–2008, at the harvest of rice and wheat, respectively. Soil moisture
(volumetric) at sampling ranged between 19.1 to 22.2% (0–5 cm) and
19.4 to 22.4% (5–10 cm) for rice; and 13.4 to 17.2% (0–5 cm) and 13.5
to 16.3% (5–10 cm) for wheat, at harvest, indicating nearly 3% variation
among the treatments. Samples from individual plots were thoroughly
mixed, air-dried, and passed through an 8-mm sieve, by gently breaking
apart the clods. Clods, aggregates, and residues >8-mm diam. were
discarded. Air-dried samples were placed in plastic bags, stored at
ambient temperature, and transferred to the laboratory for analysis.
Soil Aggregate Analysis
The air-dried soil sample (100 g) was placed on a 2-mm sieve and
submerged in water for 5 min to allow slaking (Kemper and Rosenau,
1986). The soils were then passed through a series of three sieves
of 2-, 0.25-, and 0.053-mm size (Six et al., 1998, 1999). Aggregate
separation was achieved by manually moving the sieves up and down
3-cm with 50 repetitions over a period of 2 min (Six et al., 1998). Large
(2–4 mm) and small (0.25–2 mm) macroaggregates, microaggregates
(0.053–0.25 mm), and ‘silt+clay’-sized fractions (<0.053 mm) were
separated. Floating organic material (>2 mm) was collected and
discarded and not considered as soil organic matter (Six et al., 1998).
The soil material and water passing through the sieves were poured
onto a smaller mesh sieve, the sieving procedure was repeated, and the
material was retained. A subsample was taken from the collected soil
suspension that passed through the 0.053-mm sieve (‘silt + clay’-sized
fraction). This subsample, along with soil material retained on different
sieves, was collected, oven-dried (60 ± 5°C, 48 h), weighed, and stored
at room temperature for organic C analysis.
The protected microaggregates within the small and large
macroaggregates were isolated following the method of Six et al. (2000),
562
in which macroaggregates were completely broken, but disintegration
of microaggregates within these macroaggregates was minimized. The
macroaggregates (5 g) were placed on the 0.25-mm mesh sieve, immersed
in deionized water, and shaken with 125 glass beads (2 mm size) so as
to completely disrupt the macroaggregates. A continuous, steady flow
of water was maintained to flush the disrupted macroaggregates onto a
0.053-mm mesh sieve to avoid further breakup of the microaggregates
by the glass beads. The material present on the 0.053-mm mesh sieve
was wet-sieved manually following the procedure used for aggregate
separation. The material retained on the 0.25-mm (sand and coarse
POM) and 0.053-mm mesh sieves (microaggregates and fine POM)
was dried at 60 ± 5nC and stored at room temperature for organic C
analysis. Sand content (between 0.053 and 2 mm) of the aggregates
was determined on a subsample of aggregates that was dispersed with
sodium hexametaphosphate (5 g L−1).
Different particle-size fractions obtained from the aggregate
analysis were measured and converted into fractions. The fraction of
aggregates < 0.053 mm was calculated by summing up the total mass of
material retained on the sieves of >0.053 mm and this was subtracted
from the total weight of soil taken for wet-sieving analysis. Parameters
expressing the status of aggregation were determined as follows:
1. Macro- and microaggregates: The macroaggregates were
determined by adding the aggregates retained over 0.25- and 2-mm
sieves (Edwards and Bremner, 1967; Oades and Waters, 1991),
while the microaggregates referred to aggregates retained on 0.053to 0.25-mm sieves.
2. The mean weight diameter (MWD) and geometric mean
diameter (GMD) of aggregates (Kemper and Rosenau, 1986) were
calculated as:
(i) MWD = ∑ xi wi,
(ii) GMD = exp [(∑ wi log xi)/(∑wi)]
where wi is the proportion of each aggregate class in relation to the bulk
soil and xi the mean diameter of the aggregate class (mm).
3. The aggregate stability (AS) of soils (Castro-Filho et al., 2002)
was computed as
AS
ª weight of the aggregates–wp25–S º
«
» 100
¬« weight of the dry sample – S ¼»
where wp25 is the weight of aggregates < 0.25 mm (g) and S the weight
of particles between 2 and 0.053 mm (g), that is, sand content.
Soil Organic Carbon Fractionation
Aggregate-associated organic C of the small macroaggregates,
microaggregates, and microaggregates within macroaggregates was
separated by density floatation with 1.85 g cm−3 sodium polytungstate
(NaPT) solution (Six et al., 1998). A 5-g dry subsample was suspended
in water in a 35-mL graduated conical centrifuge tube, and slowly shaken
reciprocally by hand (10 times) without breaking the aggregates. The
extra material adhered to the cap and sides of the centrifuge tube was
SSSAJ: Volume 75: Number 2 • March–April 2011
washed into suspension with 10 mL of NaPT and kept under vacuum
for 15 min to evacuate air entrapped within the aggregates. The samples
were then centrifuged (~1250 g) at 20°C for 1 h. The floating material
in the tube was aspirated onto a filter paper, rinsed thoroughly with
deionized water to remove NaPT, and dried at 50°C. The heavy fraction
was rinsed twice with 50 mL deionized water and dispersed in 0.5%
hexametaphosphate by reciprocally shaking for 18 h on a mechanical
shaker. The dispersed heavy fraction was passed through 2-, 0.25-, and
0.053-mm sieves depending on the aggregate size being analyzed. The
material remaining on the sieve, intra-aggregate POM (iPOM) + sand
(0.053–0.25, 0.25–2, and 2–4 mm size), was dried and weighed. The
iPOM + sand in size classes 0.25 to 2 and 2 to 4 mm that was derived
from the 2- to 4-mm aggregates was pooled for the determination of
organic C concentration of iPOM (Six et al., 2000). The soil organic C
in each fraction of aggregates and bulk soil was determined following
the dry combustion method (Nelson and Sommers, 1982).
Statistical Analysis
All data were subjected to ANOVA (analysis of variance) using
standard procedures in the Statistical Analysis System (SAS Institute,
2001). Treatment means were compared by Tukey’s honest significant
difference test (P < 0.05) procedure (Gomez and Gomez, 1984).
RESULTS AND DISCUSSION
the conventional tillage practices. In the 5- to 10-cm layer,
the fraction of macro- and microaggregates were similar.
However, in all cases, large macroaggregates contributed to 54
to 63% of total macroaggregates, indicating reduced turnover
of aggregates under ZT systems (Six et al., 2000), possibly due
to less disturbance of top layers. There were significantly fewer
microaggregates and silt + clay size (<0.053 mm) fractions in
the top (0–5 cm) layer. On the contrary, conventional tillage
resulted in fewer large (2–4 mm) but greater small (0.25–2
mm) macroaggregate and microaggregate (0.053–0.25 mm)
fractions. Results are explained by the possible breaking of
large macroaggregates into smaller fractions and constituent
primary particles (silt- and clay-sized fractions) with increasing
tillage intensity, as argued by others (Six et al., 2000; Grandy and
Robertson, 2007).
The aggregate-size distribution recorded at wheat harvest
(Table 3) showed a significantly higher fraction of large
macroaggregates in T5 and T6 in both 0- to 5- and 5- to 10-cm
depths (0.527 and 0.566; and 0.444 and 0.561 g g−1 of dry soil in
0 to 5 and 5 to 10 cm, respectively), with a proportional decrease
in the small macroaggregate fraction. No significant difference
was observed in either microaggregate or silt + clay size fractions
in these layers.
Size Distribution of Aggregates
Aggregation Indices
Irrespective of treatments and depths, the fraction of soil
macroaggregates was greater than microaggregates in both rice
and wheat crops. In ZT systems, the larger size of aggregates
increased with a concurrent and proportionate decrease in
smaller size fractions (Table 2), resulting in the fraction of
macroaggregates (>0.25 mm) being 2 to 4 times higher than
microaggregates (0.053–0.25 mm). In rice, under ZT (T5 and
T6), macroaggregates (0–5-cm) accounted for 0.80 and 0.78 g g−1
of dry soil, respectively; both were significantly higher than
Impact of tillage could be differentiated through aggregation
indices of which the MWD of aggregates was the most sensitive
(Tables 2 and 3). Significantly higher values of MWD, GMD,
and a 20 to 30% improvement in aggregate stability (significant
only at rice harvest) indicated better aggregation under ZT
systems (T5 and T6). Compared with puddle transplanted
rice (T1), marginally higher values were recorded in raised-bed
transplanted rice (T4), only in the 0- to 5-cm layer, although
results were inconsistent.
Table 2. Soil aggregation status at rice harvest as affected by tillage practices.
Soil
depths
Treatments†
Size distribution of aggregates, mm
2–4
cm
0–5
5–10
T1
T2
T3
T4
T5
T6
T1
T2
T3
T4
T5
T6
0.25–2
0.053–0.25
g aggregate g−1 dry soil
0.150b‡ 0.392a
0.358a
0.138b
0.417a
0.326ab
0.110b
0.427a
0.311ab
0.230ab 0.360a
0.266ab
0.498a
0.297a
0.166b
0.421ab 0.363a
0.157b
0.091c
0.419a
0.329a
0.062c
0.485a
0.291a
0.103bc 0.458a
0.223a
0.128abc 0.372a
0.260a
0.365a
0.252a
0.215a
0.344ab 0.297a
0.207a
<0.053
0.100abc
0.120ab
0.152a
0.144a
0.038c
0.059bc
0.161a
0.163a
0.216a
0.239a
0.168a
0.152a
Aggregation
indices
MWD
Macroaggregates Microaggregates
Aggregate
stability
GMD
mm
0.95b
0.699c
0.94b
0.693c
0.86b
0.659c
1.14ab 0.729bc
1.86a
1.071a
1.70a
0.998ab
0.80a
0.638a
0.78a
0.649a
0.86a
0.651a
0.85a
0.607a
1.42a
0.801a
1.40a
0.826a
–––––g aggregate g−1 dry soil––––
0.542b
0.358a
0.555b
0.326ab
0.537b
0.311ab
0.590ab
0.266ab
0.796a
0.166b
0.784a
0.157b
0.510a
0.329a
0.547a
0.291a
0.561a
0.223a
0.501a
0.260a
0.617a
0.215a
0.640a
0.207a
0.47b
0.54b
0.55b
0.62ab
0.78a
0.80a
0.53a
0.58a
0.69a
0.63a
0.70a
0.71a
†Refer Table 1 for treatment details.
‡Values followed by a similar letter within an aggregate size fraction are not significantly different at P < 0.05 level of significance according to
Tukey’s HSD mean separation test.
SSSAJ: Volume 75: Number 2 • March–April 2011
563
Table 3. Soil aggregation status at wheat harvest as affected by tillage practices.
Soil
depths
Treatments†
cm
0–5
5–10
T1
T2
T3
T4
T5
T6
T1
T2
T3
T4
T5
T6
Size distribution of aggregates, mm
0.053–
2–4
0.25–2
<0.053
0.25
−1
g aggregate g dry soil
0.114b‡
0.191b
0.183b
0.244b
0.527a
0.566a
0.128b
0.166b
0.222b
0.169b
0.444a
0.561a
0.590a
0.514a
0.512a
0.519a
0.218b
0.196b
0.614a
0.508ab
0.442bc
0.556ab
0.332cd
0.213d
0.208a
0.192a
0.214a
0.173a
0.123a
0.116a
0.182a
0.220a
0.212a
0.176a
0.126a
0.120a
0.088a
0.104a
0.091a
0.066a
0.132a
0.122a
0.076a
0.106a
0.124a
0.100a
0.098a
0.106a
Aggregation
indices
MWD
GMD
mm
1.10b
1.18b
1.16b
1.35b
1.97a
1.94a
1.11b
1.11b
1.20b
1.16b
1.73a
1.94a
0.848b
0.816b
0.826b
0.903b
1.054a
1.008a
0.841ab
0.785b
0.792b
0.826ab
0.971ab
1.016a
Macroaggregates
Microaggregates
Aggregate
stability
g aggregate g−1 dry soil
0.704a
0.705a
0.695a
0.763a
0.745a
0.762a
0.742a
0.674a
0.664a
0.725a
0.776a
0.774a
0.208a
0.192a
0.214a
0.173a
0.123a
0.116a
0.182a
0.220a
0.212a
0.176a
0.126a
0.120a
0.46a
0.44a
0.56a
0.53a
0.67a
0.63a
0.44a
0.48a
0.48a
0.42a
0.65a
0.65a
†Refer Table 1 for treatment details.
‡Values followed by a similar letter within an aggregate size fraction are not significantly different at P < 0.05 level of significance according to
Tukey’s HSD mean separation test.
The relative aggregate-size fractions and the aggregation
indices demonstrated that ZT promoted macroaggregation,
while conventional-tillage, due to more mechanical disturbances,
had less aggregation. Higher values of aggregation indices were
also reported elsewhere under reduced-tillage or ZT systems
(Oyedele et al., 1999; Franzluebbers et al., 1999; Madari et al.,
2005; Oorts et al., 2007). These improvements in soil structural
stability with conservation-tillage could be attributed to higher
biotic activity, especially earthworms (Blevins and Frye, 1993;
Birkas et al., 2004).
Bulk Soil Organic Carbon
Organic C in bulk soil was significantly affected by tillage
practices; the difference was significant in the 0- to 5-cm layer
(Fig. 1). In this layer, ZT had significantly higher SOC of
7.37 and 7.86 g kg−1 in T5 and T6, respectively than those
of 5.81 and 6.14 g kg−1 in conventional tillage treatments T1
and T2, respectively. This indicated a greater potential for C
accumulation with ZT likely to be associated with factors such
as (a) a reduction in soil disturbance, (b) undisturbed left-over
stubbles on the surface, and their slow decomposition leading
to cooler soil temperature, and (c) increased soil water retention
(Kern and Johnson, 1993). All of these favored the formation
and stabilization of soil aggregates and their associated organic
C, which has been protected from rapid breakdown and
decomposition (Elliott, 1986; Six et al., 1999, 2000). Like rice,
soil C at wheat harvest was higher in ZT than conventional tillage.
In addition, raised-bed planting also resulted in higher C (6.79
and 6.83 g kg−1 in T3 and T4, respectively) than conventional
T1 and T2 (5.68 and 5.93 g kg−1 in T1 and T2, respectively).
This observation together with the better macroaggregate status
revealed that the raised-bed system presumably also preserved
soil structure. It is interesting to note that though C content was
generally higher at 0- to 5-cm than that at 5- to 10-cm depth, the
564
difference between two layers was almost double in ZT than that
of conventional tillage.
The mean weight diameter (MWD) increased by 62
and 76% in raised-bed and ZT systems, respectively, while in
conventional tillage, the increase was only 3% compared with
values at the start of the experiment (Mahesh K. Gathala,
unpublished data, 2010). Though no benchmark SOC data are
available, the improvement in MWD indirectly suggests the
potential for increasing soil C under zero-tillage and raised-bed
than that of conventional tillage.
In the present study, total aboveground biomass (grain +
straw) yield of rice was, on average 17 Mg ha−1 under conventional
puddled transplanted system, followed by 15 Mg ha−1 under ZT,
and 11 Mg ha−1 under raised-bed planting. The average biomass
yield of subsequent wheat crops ranged between 11 and 12 Mg ha−1
and did not differ among the treatments (Mahesh K. Gathala,
unpublished data, 2010). All the aboveground biomass, except
residue/stubbles of <2 Mg ha−1, was removed. The increase in
organic C content as observed in the present study is, therefore,
attributed to improved soil aggregation especially higher
proportion of macroaggregates under zero/reduce-tillage.
Aggregate Associated Carbon
Soil C content increased with aggregate size, with maximum
C contents associated with large macroaggregates (2–4 mm),
which was 2 to 3 times higher than that with silt + clay-size
(<0.053 mm) aggregate fractions (Table 4). By and large, zerotillage (T5 and T6) had higher C in aggregates, though it was not
consistent for all aggregate size classes. At 0- to 5-cm soil depth,
C in 2- to 4-mm size aggregates was higher in T5 and T6 (8.4
and 8.2; 9.3 and 9.1 g C kg−1 soil in aggregate fraction at rice and
wheat harvest, respectively) than T1 and T2. The significantly
higher amounts of macroaggregate-associated C suggest slower
turnover rates of macroaggregates resulting from reduced soil
disturbance (Madari et al., 2005). This signifies the potential of
SSSAJ: Volume 75: Number 2 • March–April 2011
ZT to retain considerably higher soil C, which is
physically protected from microbial oxidation within
the macroaggregates (Franzluebbers et al., 1994;
Balesdent et al., 2000; Grandy and Robertson, 2006).
Moreover, a significantly higher C content also in
microaggregates indicated, by and large a better soil
C balance under ZT in a rice–wheat rotation.
Irrespective of tillage practices, the bulk SOC
in the 0- to 5-cm layer positively correlated with
the aggregation indices (data not presented). The
MWD and GMD also correlated with C associated
with 2- to 4-mm aggregates. An increase in soil C
corresponded to an increase in macroaggregates,
thereby improving the overall aggregation status.
These results suggest a potential for building soil
C in the long-run. The effect of ZT in developing a
stratification of C between the 0- to 5- and 5- to 10cm layers implied this could be a good indicator of
soil quality, as also argued by others (Franzluebbers,
2002; Madari et al., 2005).
Particulate Organic Carbon
Fig. 1. Soil organic C (g kg−1 of bulk soil) as influenced by tillage practices at (a) rice
and (b) wheat harvest. A similar letter between treatments within a soil layer indicates a
nonsignificant (P < 0.05) difference.
Tillage induced changes in the intraaggregate POM-C content was distinguishable
at 0- to 5-cm depth only (Fig. 2). Differences
were nonsignificant at the 5- to 10-cm depth.
On average, the iPOM C content in soil
was higher at wheat than at rice harvest, and
accumulated in greater portion as fine (0.053–
0.25 mm) than the coarse (0.25–2 mm) fraction.
A significantly higher particulate-C fraction was
recorded in the zero-till systems (T5 and T6),
and was associated more with the fine fractions
(20–30% higher than under conventional-tillage
T1 and T2). The iPOM-C is physically better
Table 4. Organic carbon content in aggregate size classes under different tillage practices at rice and wheat harvest.
Size distribution of aggregates, mm
Soil depths
Treatment†
2–4
cm
0–5
5–10
T1
T2
T3
T4
T5
T6
T1
T2
T3
T4
T5
T6
7.1b‡
7.4b
7.4b
7.0b
8.4a
8.2a
6.9bc
7.2ab
6.3c
6.9bc
7.6a
7.3ab
At rice harvest
0.25–2
0.053–0.25
6.1b
6.0b
6.8ab
6.9ab
7.6a
7.7a
6.5b
6.4a
6.2a
5.8a
6.5a
6.3a
<0.053
2–4
g C kg−1 soil in aggregate fraction
5.3b
2.8a
6.8c
5.4b
2.7a
7.7bc
5.5ab
3.1ab
7.6bc
5.3b
2.6a
8.0abc
5.6a
3.7b
9.3a
6.0a
3.9b
9.1a
5.2c
2.5a
5.8bc
5.1c
2.4a
5.3c
5.4abc
2.8a
6.3b
5.3bc
2.4a
5.9bc
5.7ab
2.6a
7.3a
5.8a
2.7a
6.4ab
At wheat harvest
0.25–2
0.053–0.25
<0.053
5.2c
5.4bc
5.8bc
7.0a
6.3ab
6.9a
4.2a
4.1a
4.2a
4.3a
4.6a
4.8a
2.8a
2.7a
2.6a
2.6a
2.9a
2.9a
2.1a
2.3a
2.3a
2.1a
2.4a
2.4a
4.5ab
3.8b
4.6ab
5.1a
5.5a
5.2a
3.9a
4.0a
4.1a
4.2a
4.3a
4.1a
†Refer Table 1 for treatment details.
‡Values followed by a similar letter within a column are not significantly different at P < 0.05 level of significance according to Tukey’s HSD mean
separation test.
SSSAJ: Volume 75: Number 2 • March–April 2011
565
protected than other POM-C fractions
in soil (Balesdent et al., 2000; John et al.,
2005; Grandy and Robertson 2006). In
our study, a significantly higher amount of
fine iPOM-C mostly associated with small
macroaggregates indicated slower turnover
under ZT, resulting in the formation and
stabilization of fine intra-aggregate C
particles. The increased fine iPOM-C
could be regarded as a potential indicator of
increased C accumulation (Six et al., 1999).
When expressed as C content per unit of
soil volume for 0- to 10-cm, both the coarse
and fine iPOM-C fractions were significantly
higher under T5 and T6 than those under
T1 and T2 (Table 5). The raised-bed systems
(T3 and T4) indicated higher soil C content
associated with macroaggregates compared with
conventional treatments (T1 and T2; Table 3),
but the iPOM-C showed no difference. Results
suggested that relatively less physical disturbance
in ZT caused slower turnover of aggregates and
greater accumulation of both coarse and fine
iPOM-C fractions. Furthermore, the higher
amounts of macroaggregates together with
increased iPOM-C indicate the potential of ZT
for improving the soil C stocks.
CONCLUSIONS
The accumulation of C in soil was related
to soil aggregation and the distribution of
C in aggregates. By significantly improving
soil aggregation and associated C content, Fig. 2. Intra-aggregate particulate organic matter (iPOM) C (g kg−1 of sand-free aggregates) in
the potential of ZT systems in a rice–wheat aggregate-size fractions at the 0- to 5-cm soil depth at (i) rice and (ii) wheat harvest. ‘(a)’ and
rotation for enhancing C storage was noted. The ‘(b)’ in legend refer to coarse (0.25–2 mm) and fine (0.053–0.25 mm) iPOM in the respective
size of aggregates. Treatment means are shown ± standard error (n = 3). A similar letter between
differences were prominent mostly in the top treatments within an aggregate size class indicates a nonsignificant (P < 0.05) difference.
(0–5-cm) soil layer, which is the most disturbed
Indian Agricultural Research Institute, New Delhi, by the senior author
layer under a conventional-tillage system. In a rice–wheat rotation,
during the course of the study is gratefully acknowledged.
being highly tillage-intensive, the losses of C from the surface soil
can partially be reversed or organic C pools in the soil conserved
Table 5. Coarse and fine intra-aggregate; particulate organic
through the adoption of ZT or alternate resource-conserving
matter carbon (iPOM-C) in 0- to 10-cm soil layer as affected
technologies such as direct-seeded or transplanted rice on
by tillage practices in a rice-wheat rotation.
permanent raised beds followed by wheat on the same beds, which
iPOM-C
offer less physical disturbance to soils. The iPOM-C in various size
Treatment†
At rice harvest
At wheat harvest
classes of aggregates was perceptively influenced by tillage and was
Coarse
Fine
Coarse
Fine
significantly higher in the zero-tillage system. Better aggregation
g C m−3
was associated with a higher amount of iPOM-C. Slow turnover of
T1
36.9a‡
41.7a
64.9a
70.6a
T2
37.6a
44.1a
61.1a
71.5a
aggregates under the zero-tillage system resulted in fine iPOM-C
T3
37.3a
47.2a
60.3a
72.6a
sequestered within aggregates, thereby increasing the potential of
T4
41.1a
45.0a
66.1a
72.8a
C-sequestration in the long run.
ACKNOWLEDGMENTS
T5
T6
We appreciate the support and help rendered by the scientific and
technical staff of the International Rice Research Institute, India Office,
Modipuram, Meerut, UP, India. The fellowship received from the
†Refer Table 1 for treatment details.
‡Values followed by a similar letter within an aggregate size fraction
are not significantly different at P < 0.05 level of significance.
566
53.5b
52.7b
65.3b
69.4b
74.0b
76.2b
93.1b
91.2b
SSSAJ: Volume 75: Number 2 • March–April 2011
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