Importance of denitrification driven by the relative abundances of microbial communities in coastal wetlands

Excessive nitrogen (N) loadings from human activities have led to increased eutrophication and associated water quality impacts in China's coastal wetlands. Denitrification accounts for significant reduction of inorganic N to nitrous oxide (N₂O) or dinitrogen gas (N₂), and thereby curtails harmful effects of N pollution in coastal and marine ecosystems. However, the molecular drivers and limiting steps of denitrification in coastal wetlands are not well understood. Here, we quantified the abundances of functional genes involved in N cycling and determined denitrification rates using ¹⁵N paring technique in the coastal wetland sediments of Bohai Economic Rim in eastern China. Denitrification accounting for 80.7 ± 12.6% of N removal was the dominant pathway for N removal in the coastal wetlands. In comparison, anaerobic ammonium oxidation (ANAMMOX) removed up to 36.9 ± 7.3% of inorganic N. Structural equation modeling analysis indicated that the effects of ammonium on denitrification potential were mainly mediated by the relative abundances of nosZ/nirS, nirS/(narG + napA) and amoA/nirK. Denitrification was limited by the relative strength of two steps, namely N₂O reduction to N₂ and nitrite (NO₂^-) reduction to nitric oxide (NO). Our results suggest that the relative abundances of functional genes which are more stable than sediment chemical compounds in the context of environmental changes are indictive of denitrification potential in coastal wetlands.