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destroys the soil structure (Sharma et al., 2003), and excessive tillage in wheat, re- sulting in its late planting (Hobbs ... Mamta Kumari. Debashis Chakraborty.
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.

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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

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Same as T3 Same as T2 Same as T2

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(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 (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),

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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

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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 (0.25 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