Dissimilatory nitrate reduction to ammonium (DNRA): A unique biogeochemical cycle to improve nitrogen (N) use efficiency and reduce N-loss in rice paddy

Microbially driven Dissimilatory Nitrate Reduction to Ammonium (DNRA) is a part of the nitrogen (N) cycle in rice paddies which could conserve N by transforming nitrate (NO3−) into ammonium (NH4+) and restrict the loss of NO3− through denitrification (DEN), leaching, and runoff processes. Additionally, DNRA coexists with DEN in various natural systems and competes for the reduction of NO3− and oxidizing carbon (C) sources in an integrative manner. DNRA is mostly reported in oceans, estuaries, marine sediments, wetlands, and floodplains of terrestrial habitats but is not well understood in rice soil compared to DEN. Our understanding of factors such as microsites, fertilization regimes, soil redox potential, conductivity, C to N ratio, C to NO3− ratio and DNRA-associated microbiomes influencing DNRA activity in rice soil is limited. Similarly, factors influencing the abundance of the nrfA gene (a universal marker of DNRA) in rice paddy and its relationship with other genes linked to DEN and Anaerobic Ammonia Oxidation (ANAMMOX) processes are not well established. This systematic review summarizes the recent advancement in the mechanistic way of understanding DNRA, highlighting the current knowledge on microbial community composition and diversity relationship with DNRA activity and key drivers associated with DNRA in rice soil. We also discussed how the DNRA pathway is co-related with N2-fixation, DEN, and ANAMMOX as part of the overall N cycle in rice paddy. Additionally, we outline some futuristic strategies that can be applied to increase DNRA activity and enhance N use efficiency while reducing N2O emissions from rice paddy ecosystems.