Tillage direction and mulching improve aggregate stability, carbon and nitrogen

Sloping croplands in the black soil region of Northeast China are experiencing decreases in aggregate stability and nutrient retention. To address this issue, a field experiment with four tillage-cover management treatments, i.e., downslope no-tillage with straw mulch (SM), cross-slope no-tillage with straw mulch (CM), downslope no-tillage with cover crop (SC), and conventional tillage (CK), was conducted to investigate the dynamics and patterns of water-stable aggregate size distribution and stability indices (water-stable aggregates > 0.25 mm (WR0.25), mean weight diameter (MWD), and geometric mean diameter (GMD)), as well as aggregate-associated soil organic carbon (SOC) and total nitrogen (TN) in the 0-60 cm soil profile across tillage directions and mulching measures. All the mulching-tillage treatments increased the WR0.25, MWD, GMD, aggregate-associated SOC and TN contents, and the contribution of > 0.25 mm aggregates to total SOC and TN; however, these improvements generally decreased with increasing soil depth. The aggregate stability indices were positively correlated with the SOC and TN stored in the aggregates, with the strongest relationships observed in the 0-20 cm layer. Overall, cross-slope no-tillage with straw mulch (CM) treatment was the most effective treatment for enhancing aggregate stability and promoting carbon and nitrogen accumulation in relatively larger aggregates.