Response of oxidative stability of aggregate-associated soil organic carbon and deep soil carbon sequestration to zero-tillage in subtropical India

Conservation agriculture (CA) practices have been recognized to enhance soil health by optimizing key soil attributes and are being implemented worldwide. But, the potential of CA to soil organic carbon (SOC) sequestration and enhancing its oxidative stability (OXS) in deep layers and aggregates are less highlighted. Thus, our aim was to study the long-term (9 years) effects of contrasting tillage treatments and residue addition on soil aggregation, OXS in bulk soils (0-5, 5-15, 15-30 and 30-60 cm) and aggregates (0-5 and 5-15 cm), and C pools and sequestration (0-5, 5-15, 15-30 and 30-60 cm). For this an experiment was laid out in a split plot design with two tillage practices in main plots (zero-tillage: ZT and conventional tillage: CT) and four residue management practices in sub-plots (No residue: NR, wheat residue: WR, soybean residue: SR and wheat + soybean residue: WR + SR) in 2008. Soil samples collected from 0-5, 5-15, 15-30 and 30-60 cm layers were analyzed. The ZT plots had similar to 26 and 15% higher macroaggregate associated C than CT plots in 0-5 and 5-15 cm soil layers, respectively. In macroaggregates of first two soil layers, recalcitrant C was (a) similar to 57, 76 and 63%; and (b) 85, 59 and 40% greater in WR + SR, SR and WR plots over NR, respectively. Similar trend was observed for microaggregate associated recalcitrant C. The OXS of C in ZT was similar to 21% more than CT, but residue addition had no impact on OXS. In 15-30 and 30-60 cm soil layers, OXS in WR + SR plots were similar to 28 and 31% greater than NR. Although deep soil layers (15-60 cm) accounted for similar to 68% of sequestered C, tillage had no impact on deep soil C sequestration. About similar to 85% of total SOC stock was accumulated in soil aggregates of topsoil. Contrarily, of total sequestered SOC in top soil, only similar to 30% was sequestered by aggregates of that layer indicating the role of silt + clay fraction in SOC sequestration in tropical Inceptisol. Thus, SOC sequestration in tropical soil is very sluggish and would require long time to meet saturation deficit. Interestingly, crop yield was significantly influenced by active and labile C of bulk soils and aggregates, but not by recalcitrant C in ZT plots. Thus, ZT in combination with crop residue retention has great potential in enhancing deep soil C sequestration and C stability of soil aggregates to sustain soil health and crop production.