Conservation agriculture optimizes soil nitrogen stabilization and availability to crops under rice-based cropping

Conservation agriculture (CA) is increasingly promoted in the Indo-Gangetic Plains to address sustainability challenges in soil-plant-environment system. Efficient nitrogen (N) management plan under CA is one of those challenges, which restricts its wider adoption. We hypothesized that appropriate CA management option optimizes the balance between soil N stabilization and N availability to crops. To evaluate this, a field experiment was conducted with 15 treatments involving three tillage practices [i.e., zero tillage, reduced tillage, and conventional tillage] and five residue+nutrient combinations [0% residue + 100% NPK, 100% residue + 50% NPK, 100% residue + 75% NPK, 50% residue + 100% NPK, and 50% residue + 75% NPK] in a rice-wheat-green gram rotation. After three years, the changes in soil organic [hydrolyzable (amino acid, amino sugar, hydrolyzable ammonium, serine+threonine and hydrolyzable unknown) and non-hydrolyzable] and inorganic N fractions, N stabilization and N availability to crops were assessed. Observations showed that zero tillage or minimum tillage combined with 50–100% rice residue retention and 75–100% NPK enhanced N sequestration in soils, preferentially by increasing organic N content (16 to 24%). Besides, these CA practices stabilized N in soils by increasing the ratio of organic to inorganic N (61 to 106%) and non-hydrolyzable to hydrolyzable N (3 to 42%) and maintained N availability to crops by increasing hydrolyzable N fractions [i.e., potentially mineralizable fractions including amino acid, amino sugar and hydrolyzable ammonium (6 to 28%)]. Optimizing N stabilization and availability to crops under intensive cropping, this study provides choices for the selection of appropriate CA practice(s) by farmers and policymakers.