Warming Tends to Promote Nitrogen Conservation but Stimulate N2O Emissions in Mangrove Sediments

While climate change significantly influences nitrogen cycling and its related microbial diversity, the effects of warming on nitrate reduction processes and their related microbial communities and functional gene abundances in mangrove sediments are not fully understood. In this study, mangrove sediment slurry was incubated under six controlled temperatures for 28 days to simulate warming trends. Following the incubation, rates of denitrification (DNF), anaerobic ammonium oxidation (ANA), and nitrate decomposition reduction to ammonium (DNRA), and net nitrous oxide (N₂O) production, functional gene abundances, and the structure of functional microbial taxa were investigated using a ¹⁵N tracer method, high-throughput sequencing, and qPCR methods. DNF’s optimal temperature was 25 °C, but ANA’s ranged from 25 to 35 °C. The DNRA rates; nosZ, nirS, and nrfA gene abundances; nosZ/(nirK + nirS) ratios; and, in particular, net N₂O production in the mangrove sediment significantly increased with increasing temperature. Furthermore, DNRA’s contributions to nitrate reduction increased from 26.70% at 10 °C to 44.42% at 40 °C, suggesting that the DNRA process transforms more nitrate to ammonia and retains more nitrogen within mangrove sediments than the other processes do. Meanwhile, microbial taxa changed significantly in relation to DNRA, indicating that DNRA is enhanced as temperature increases. Also, temperature explained most of the variance in the dominant bacterial communities (68.3%), nitrate reduction functional genes (91.8%), and process rates (79.9%). Thus, warming promotes nitrogen conservation in mangrove sediments but stimulates N₂O emissions, which in turn exacerbates global warming.