Conservation tillage and residue management are the options for enhancing soil organic carbon stabilization by improving soil aggregation in tropical soils. We studied the influence of different combinations of tillage and residue management on carbon stabilization in different sized soil aggregates and also on crop yield after 5 years of continuous rice–wheat cropping system on a sandy loam reclaimed sodic soil of north India. Compared to conventional tillage, water stable macroaggregates in conservation tillage (reduced and zero-tillage) in wheat coupled with direct seeded rice (DSR) was increased by 50.13% and water stable microaggregates of the later decreased by 10.1% in surface soil. Residue incorporation caused a significant increment of 15.65% in total water stable aggregates in surface soil (0–15 cm) and 7.53% in sub-surface soil (15–30 cm). In surface soil, the maximum (19.2%) and minimum (8.9%) proportion of total aggregated carbon was retained with >2 mm and 0.1–0.05 mm size fractions, respectively. DSR combined with zero tillage in wheat along with residue retention (T6) had the highest capability to hold the organic carbon in surface (11.57 g kg−1 soil aggregates) with the highest stratification ratio of SOC (1.5). Moreover, it could show the highest carbon preservation capacity (CPC) of coarse macro and mesoaggregates. A considerable proportion of the total SOC was found to be captured by the macroaggregates (>2–0.25 mm) under both surface (67.1%) and sub-surface layers (66.7%) leaving rest amount in microaggregates and ‘silt + clay’ sized particles. From our study, it has been proved that DSR with zero tillage in wheat (with residue) treatment (T6) has the highest potential to secure sustainable yield increment (8.3%) and good soil health by improving soil aggregation (53.8%) and SOC sequestration (33.6%) with respect to the conventional tillage with transplanted rice (T1) after five years of continuous rice–wheat cropping in sandy loam reclaimed sodic soil of hot semi-arid Indian sub-continent.
Soil aggregation is an imperative mechanism contributing to soil fertility by reducing soil erosion and mediating air permeability, water infiltration, and nutrient cycling (Spohn and Giani, 2011 and Zhang et al., 2012). Soil aggregates are important agents of soil organic carbon (SOC) retention (Haile et al., 2008) and protection against decomposition (Six et al., 2000a). Quantity and quality of SOC fractions have an impact on soil aggregation (Lal, 2000) that in turn physically protect the carbon (C) from degradation by increasing the mean residence time of C (Bajracharya et al., 1997). Soil management through the use of different tillage systems affects soil aggregation directly by physical disruption of the macroaggregates, and indirectly through alteration of biological and chemical factors (Barto et al., 2010). Conventional tillage (CT) generally abrades the network of mycelium by mechanical breakdown of macroaggregates (Borie et al., 2006), and decreases the content of soil organic C (SOC), microbial biomass and faunal activities (Mikha and Rice, 2004, Sainju et al., 2009 and Curaqueo et al., 2011). Conservation tillage practices with minimal soil disturbance and residue retention are becoming economically and ecologically more viable option as they save energy and provide more favourable soil conditions (Husnjak et al., 2002) for sustainable crop production and SOC sequestration for future posterity.
Rice–wheat cropping rotation has been spread over an area of about 10 Mha in Indo-Gangetic Plains (IGP) of India (Kumar et al., 1998) and together contributes 85% to India's cereal production (Timsina and Connor, 2001). Intensive tillage, residue removal and burning practised during the whole crop season accelerate soil erosion, environmental pollution, soil degradation (Montgomery, 2007) and affects ecosystem functions (Srinivasan et al., 2012). Therefore, adoption of the rational cropping practices, such as crop residue recycling (Aoyama et al., 1999 and Blair et al., 2006), manure application (Hao et al., 2003 and Rudrappa et al., 2006), conservation tillage (Gale and Cambardella, 2000 and Six et al., 2000a), and farmland fallow (Nair et al., 2009), would be a century need for improving the soil quality and ecosystem function. Available database on on-station farm trials across the Indo-Gangetic Plains in India, divulges the wheat yield increment under conservation tillage ranging from 1% to 12% with an average of 240 kg ha−1 across the area of study (Erestein and Laxmi, 2008). Thus, the cultivation of rice (transplanted/direct seeded) and wheat crops grown rotationally with different tillage and residue management practices has been advocated to evaluate its long-term effect on yield attributes, aggregation and C stabilization in different size aggregates in reclaimed sodic soil of north Indian sub-continent. We hypothesize that direct seeded rice under reduced/zero tillage along with crop residue retention could lead to improved soil aggregation and C sequestration and sustainable yield increment for future posterity of the rice–wheat cropping systems.
Our study corroborates that DSR and wheat in zero tillage coupled with residue retention is a suitable management practice for enhancing soil C sequestration and sustainable yield increment even in reclaimed sodic soil of hot semi-arid zone of Indian sub-continent. This has a potential to increase total SOC content by 33.6%, equivalent wheat yield by 8.3%, water stable macroaggregates by 53.8% and macroaggregate associated C by 20.8% over conventional tillage with transplanted rice after five years of continuous rice–wheat cropping. The higher stratification ratio of total SOC and different aggregated C under this treatment ensured the greater SOC sequestration in surface than in sub-surface soil layer. Addition of crop residues along with no-disturbance favoured a higher amount of C to be preferentially stabilized in coarse macro and mesoaggregates by physical protection. The high SOC stabilized within the ‘silt + clay’ sized particles was manoeuvred by its chemical recalcitrance nature and may act as a stable soil carbon pool encouraging long-term SOC sequestration.
