Nitrogen addition and warming rapidly alter microbial community compositions in the mangrove sediment

The mangrove ecosystem is an important CO₂ sink with an extraordinarily high primary productivity. However, it is vulnerable to the impact of climate warming and eutrophication. While there has been extensive research on plant growth and greenhouse gas emission in mangrove ecosystems, microbial communities, the primary biogeochemical cycling drivers, are much less understood. Here, we examined whether short-term experimental treatments: (1) eutrophication with a supplement of 185 g N m^-2·year^-1 (N), (2) 3°C warming (W), and (3) the dual treatment of N and W (NW) were sufficient to alter microbial communities in the sediment. After 4 months of experiments, most environmental factors remained unchanged. However, N had significant, strong effects on bacterial, fungal, and functional community compositions, while the effects of W on microbial communities were weaker. N increased bacterial richness, phylogenetic diversity, and evenness, owing to stronger stochastic processes induced by eutrophication. There were no interactive effects of N and W on bacterial, fungal, and functional community compositions, suggesting that joint effects of N and W were additive. Concomitant with higher N₂O efflux induced by N, the relative abundances of most bacterial nitrogen cycling genes were increased or remained changed by N. In contrast, N decreased or did not change those of most bacterial carbon degradation genes, while W increased or did not change the relative abundances of most of bacterial and fungal carbon degradation genes, implying higher carbon degradation potentials. As the most abundant inorganic nitrogenous species in mangrove sediment, ammonium was a key factor in shaping microbial functional communities. Collectively, our findings showed that microbial community compositions in the mangrove sediment were highly sensitive to short-term N and W treatments, giving rise to ecological consequences such as higher N₂O efflux.