Nitrogen-Transforming Bacteria and N₂O Dynamics in Tropical Rice Amended with Plant Residues

Abstract

Purpose: In tropical rice systems, when reactive nitrogen (N) exceeds crop uptake, it results in inefficient N use and elevated nitrous oxide (N₂O) emissions, leading to experimental use of plant residues with nitrification-inhibitory (NI) properties to enhance fertilizer efficiency and mitigate nitrate leaching and denitrification by promoting N retention in the soil during the study.

Methods: Five treatments, comprising inorganic fertilizer (100% NPK, T1), fresh neem leaves (T2), green manure, Sesbania (T3), used tea leaves (T4) and calcium carbide (T5), were selected and applied in well prepared rice fields for three seasons (pre-monsoon, monsoon and summer), to evaluate their efficacy towards reducing N2O emission via microbial process.

Results: A marked reduction in N₂O emissions was observed in T4-treated fields during the pre-monsoon, while T2 recorded the lowest emissions during monsoon and summer rice. Across all ecosystems, microbial propagule densities were significantly lower in amended soils than in 100% NPK treatments, indicating reduced net N mineralization. Denitrifier populations were lowest in T4 during pre-monsoon and summer, and in T2 during monsoon. Nitrifier populations were consistently lowest at T2 across ecosystems. A significant correlation between soil NO₃⁻ oxidizers and N₂O emissions was observed, and the reduced populations of NH₄⁺ and NO₃⁻ oxidizers in T2 and T4 suggest strong inhibition of ammonium oxidation, likely due to allelochemical effects.

Conclusions: The findings underscore the potential of residue-mediated biological nitrification inhibition (BNI) as a sustainable strategy to improve nitrogen use efficiency and mitigate greenhouse gas emissions across varying seasonal rice cultivation systems.