Phototrophic Biofilm Decomposition Enhances Dissimilatory Nitrate Reduction to Ammonia in Paddy Soil

Dissimilatory nitrate reduction to ammonium (DNRA) plays a pivotal role in nitrogen (N) conservation by redirecting nitrate toward ammonium retention rather than losses through denitrification or hydrological export in paddy ecosystems. However, strategies to effectively enhance the DNRA activity remain underexplored. Here, we demonstrate that phototrophic biofilms (PB) colonizing under flooding conditions act as biogeochemical engineers during decomposition, creating localized, reducing microsites that amplify DNRA efficiency. Through controlled incubation experiments, PB amendment accelerated DNRA rates by maximally 4.4-fold compared to nonamended control, with dose-dependent effects. Mechanistically, PB decomposition induced (1) rapid soil Eh reduction, (2) sustained release of labile organic carbon with enhanced electron shuttle capacity, and (3) selective enrichment of DNRA-capable taxa while suppressing denitrifiers. Metagenomic profiling also revealed PB-driven restructuring of N cycling, characterized by higher DNRA genes and diminished assimilatory nitrate reduction and denitrification genes. These synergistic effects establish PB decomposition as a nature-based solution that reprograms N cycling, with ecological significance stemming from its ability to alter the balance between N retention and potential losses. Our findings provide a mechanistic framework for harnessing natural biofilms to enhance N bioavailability and guide sustainable agroecosystem management.