Impact of conservation tillage in rice-based cropping systems on soil aggregation, carbon pools and nutrients

Tillage intensive cropping practices have deteriorated soil physical quality and decreased soil organic carbon (SOC) levels in rice-growing areas of South Asia. Consequently, crop productivity has declined over the years demonstrating the need for sustainable alternatives. Given that, a field experiment was conducted for six years to assess the impact of four tillage based crop establishment treatments [puddled transplant rice followed by conventional tillage in wheat/maize (CTTPR-CT), non-puddled transplant rice followed by zero-tillage in wheat/maize (NPTPR-ZT), zero-till transplant rice followed by zero-tillage in wheat/maize (ZTTPR-ZT), zero-tillage direct seeded rice followed by zero-tillage in wheat/maize (ZTDSR-ZT)], two residue management treatments [residue removal, residue retention (~33%)], and two cropping systems [rice-wheat, rice-maize] on soil aggregation, carbon pools, nutrient availability, and crop productivity. After six years of rotation, in top 0.2 m soil depth, zero-till crop establishment treatments (ZTTPR-ZT and ZTDSR-ZT) had higher (p < 0.05) total organic carbon (TOC) over conventional tillage treatment (CTTPR-CT). Zero-till crop establishment treatments increased very-labile C faction (Cfrac ₁) by 21% followed by labile fraction (Cfrac ₂) (16%), non-labile fraction (Cfrac ₄) (13%) and less-labile fraction (Cfrac ₃) (7%). Notably, higher passive C-pool in conservation tillage practices over CTTPR-CT suggests that conservation tillage could stabilize the recalcitrant form of carbon that persists longer in the soil. Meantime, zero-till crop establishment treatments had higher (p < 0.05) water stable macro-aggregates, macro-aggregates: micro-aggregates ratio and aggregate carbon content over CTTPR-CT. The treatment NPTPR-ZT significantly increased soil quality parameters over CTTPR-CT. However, the effect was not as prominent as that of ZTTPR-ZT and ZTDSR-ZT. Retention of crop residue increased (p < 0.05) TOC (12%) and soil available nutrients mainly available-P (16%), followed by available-K (12%), DTPA-extractable Zn (11%), and available-S (6%) over residue removal treatment. The constructive changes in soil properties following conservation tillage and crop residue retention led to increased crop productivity over conventional CTTPR-CT. Therefore, conservation tillage (particularly ZTTPR-ZT and ZTDSR-ZT) and crop residue retention could be recommended in tropical rice-based cropping systems for improving soil quality and production sustainability.