Biogenic carbon input reshapes nitrogen fate and N₂O emission in estuarine mangrove wetlands
State Key Laboratory of Marine Environment Science, Fujian Provincial Key Laboratory for Coastal Ecology and Environmental Studies, College of the Environment and Ecology, Xiamen University, Xiamen, 361102, China

Within the river-estuary-bay continuum, estuarine mangrove wetlands act as a critical biogeochemical filter, where sedimentary nitrate reduction pathways determine whether nitrogen is permanently removed via denitrification or retained as ammonium through dissimilatory nitrate reduction (DNRA). The balance between removal and retention has profound implications for downstream water quality. However, ecological restoration may also perturb this filtering function by releasing large amounts of biogenic carbon during Spartina alterniflora eradication. Here, we combined two-year field observations, anaerobic incubations, ¹⁵N isotope tracing, and metagenomic analyses to examine how this restoration-induced carbon input regulates nitrate reduction pathways and nitrous oxide (N₂O) dynamics in estuarine mangrove sediments. We found that early-stage residue input increased labile carbon availability and stimulated denitrification, leading to rapid nitrate consumption but also a transient pulse of N₂O emissions. As nitrate became depleted and carbon-to-nitrate availability increased, nitrate reduction was redirected from denitrification toward dissimilatory nitrate reduction to ammonium (DNRA), promoting ammonium retention rather than permanent nitrogen removal. This pathway shift was accompanied by enrichment of DNRA-related genes and sulfate-reducing microbial potential, and the system gradually changed from a short-term N₂O source to a later-stage N₂O sink. These results show that restoration-induced carbon inputs can drive a biogeochemical succession in mangrove nitrogen cycling, shifting sediments from enhanced nitrate removal with short-term greenhouse gas risk toward increased internal nitrogen retention, with important implications for eutrophication control, N₂O mitigation, and coastal ecosystem resilience.