Impact of long-term residue burning versus retention on soil organic carbon sequestration under a rice-wheat cropping system

Despite rice residue burning is popularized against its retention under rice-wheat system in the Indo-Gangetic Plains (IGP), limited information is available on the impacts of long-term rice residue burning versus its retention on carbon stock, soil aggregation and aggregated-associated carbon in the region. Hence, a 18 (1998-2016) years old conservation agriculture (different combinations of tillage and residue management) experiment was selected to understand the impact of CA on soil aggregation, carbon pools and soil organic carbon (SOC) stock in a rice-wheat system. Experiment was laid down in a split-plot design with three replications having three tillage management viz. zero-tillage (ZT), conventional tillage (CT) and strip tillage (ST) as main plots and three residue management options namely residue retention (R), residue burning (RB) and no-residue (NR). The total SOC content was 20% higher (p < 0.05) under ZT and residues retention over CT and residue removal, in the 0-5 cm layer. The SOC stock increased with depth (from 0 to 5 cm to 5-15 cm). ZT with added residues had 25% higher (p < 0.05) SOC stock than CT without residues because of higher aggregation with lesser disturbance in ZT promoted SOC accumulation over time. Macro-aggregate fraction was higher in ZT by 18% (p < 0.05) than CT in the 0-5 cm soil depth. Whereas, CT without residues resulted in higher silt and clay size fraction than ZT. Labile carbon (very labile + labile fractions) was 7-25% increased (p < 0.05) under ZT+R than CT-R across depth. Recalcitrant carbon (less-labile + non-labile) followed the same trend. Notably, ZT with residues retention (conservation agriculture, CA) increased 32.6-41.7% potassium permanganate oxidizable carbon than CT without residues in the 0-5 cm and 5-15 cm, respectively. No mechanical tillage in ZT with added residues over the years promoted bridging between aggregates that brought about carbon accumulation. The study highlighted the importance of CA in enhancing SOC stock, soil aggregation and managing long-term SOC content.