Application of plant residues in combination with nitrogen fertilizer to improve soil N availability, enhance nitrogen utilization efficiency (NUE) and grain productivity in tropical rice paddies

In agricultural environments, nitrification inhibitors (NIs), both synthetic and biological, are widely used to reduce nitrogen (N) losses through various mechanisms. Two plant residues were selected to evaluate their potential as biological NIs in tropical rice agroecosystems when combined with reduced nitrogen fertilization. Three amendments were imposed: conventional fertilizer − 100% NPK as the control (T1); 50% urea-N + 100% P₂O₅ + 100% K₂O + fresh neem leaves (T2); and 50% urea-N + 100% P₂O₅ + 100% K₂O + used tea leaves (T3). Fields were amended before transplanting with rice varieties Arize 6129 (V1) and Sona mashuri (V2). A significant improvement in soil physico-chemical properties were observed following the incorporation of plant residues without any varietal differences. Reduced soil nitrate (NO₃⁻–N) content, with higher nitrogen recovery (32.9–34.9%), improved nitrogen use efficiency (22.0–28.5 kg grain kg⁻¹ N), and increased grain yield in both the varieties under T2, were recorded with a positive correlation with enhanced nitrification inhibition rates (7.6–48.9%). These effects may be attributed to the antimicrobial properties of the plant residues, likely suppressing the activities and presence of nitrifying microorganisms, limiting soil N transformation, prolonged NH₄⁺–N availability, and thereby enhancing nitrogen uptake by the plants from soils. The findings suggest that amendment of neem leaf residues in combination with reduced urea-N can serve as a viable biological nitrification inhibitor (BNI) and improve soil health by mitigating N loss from tropical rice agroecosystem without compromising grain productivity.