Drastic suppression of benthic nitrogen fixation by restructuring diazotrophic communities following tidal wetland conversion to aquaculture ponds

Abstract Tidal wetland reclamation is a widespread anthropogenic disturbance that alters sediment biogeochemistry; however, its impacts on biological nitrogen fixation (BNF) and diazotrophic communities remain poorly understood. Here, we integrated 15 N isotope labeling, metagenomic sequencing, and network analysis across a bare tidal flat ( B ), moderate‐density ( M ), and high‐density ( H ) aquaculture ponds to examine these effects. Biological nitrogen fixation rates declined by 53.8% ± 29.1% ( M ) and 70.0% ± 59.2% ( H ) relative to B (0.99 ± 0.06 μ mol kg −1 h −1 ), driven more by reclamation than by seasonal variation. Microbial communities shifted from diazotroph dominance (e.g., Pseudomonadota) to an increased contribution of lineages without detected nif genes (e.g., Thermodesulfobacteriota ), accompanied by reduced diversity and functional gene expression. nifH phylogeny showed a shift toward aerobic/facultative bacterial nitrogenases, highlighting archaeal sensitivity to aquaculture. This study reveals key mechanisms by which aquaculture suppresses sediment BNF and suggests that anfD may serve as a microbial indicator for wetland restoration monitoring.