Tillage intensity and straw retention impacts on soil organic carbon, phosphorus and biological pools in soil aggregates under rice-wheat cropping system in Punjab, north-western India

The climate smart agricultural residue management practices which entail reduced tillage and in-situ crop residue managements are considered pre-requisite for improved soil health and ecosystems' sustainability. The adoption of rice residue management practices help reduce the risk of soil nitrogen (N) depletion, while sequestering and conserving more carbon (C) in soils due to crop residues retention as compared to the conventional tillage characterized by more intense tillage and residue removal and/or burning. We therefore, quantified the impact of tillage regimes and residue return on soil C, phosphorus (P) and biological pools in the surface (0-15 cm) soil layer under rice-wheat cropping system in north-western India. We studied the medium term (5-years) impacts of fertilizer-N application (viz. N-1 =75% and N-2 =100% recommended fertilizer-N) as main plot treatments, and tillage regimes viz. conventional tillage, rotavator tillage and zero tillage, rice residue removal and retention under conventional and rotavator tillage and RT vis-& agrave;-vis zero tillage with partial and complete rice residue retention for wheat as sub-plot treatments in a split-plot design. Residue retention and/or incorporation significantly (p < 0.05) increased the proportion of macro-aggregates (>0.25 mm), aggregate diameter and aggregate ratio than the conventional tillage. Zero tillage significantly increased the macro-, meso- and micro-aggregate associated C by similar to 22.2%, 25.7% and 18.6%, respectively than under conventional tillage. Macro-aggregates has higher concentration of very labile (by similar to 37.8% and 153.1%) and labile C (by similar to 29.7% and 80.4%) than the meso-aggregates (0.25-2.0 mm) as well as micro-aggregates (<0.25 mm). The passive C pool comprised the largest proportion of total organic C (TOC) pool; the highest in macro-aggregates (similar to 75.7% of TOC), while the lowest in macro-aggregates (similar to 64.4% of TOC). Macro-aggregates has higher dehydrogenase and alkaline phosphatase activity than the meso-and micro-aggregates; which was closely linked to soil related glomalin protein (R-2 =0.798-0.828 *; p < 0.05). These results revealed that rice residue retention significantly increased the water soluble-P concentration by similar to 60.1% and 57.9%, respectively in macro-aggregates compared to residual removal. The water soluble-P showed linear relationship with increased concentration of Fract. 1 in different sized aggregates (R-2 =0.789-0.975). The inorganic P pool (NaHCO3-P-I and NaOH-P-I) increased with increasing organic P pool (NaHCO3-P-O and NaOH-P-O). These results suggest that reduced tillage and residue retention is highly beneficial for the overall improvement of C, P and biological pools in soils under rice-wheat cropping system.