Disproportionate variations in denitrification and anaerobic ammonium oxidation across freshwater-oligohaline wetlands in Min River Estuary, Southeast China

Spatial and temporal variations in soil denitrification and anaerobic ammonium oxidation (anammox) across the freshwater-oligohaline wetlands in subtropical estuary have not been well understood. In this study, continuous-flow soil core incubation combined with nitrogen isotope tracer was used to determine denitrification and anammox rates across freshwater-oligohaline tidal wetlands in Min River Estuary, Southeast China. Areal rates of denitrification and anammox varied from 3.89 to 19.0 μmol m⁻² h⁻¹ and from 0.15 to 1.11 μmol m⁻² h⁻¹, respectively, across these wetlands and throughout sampling months. Denitrification rates were higher in warm months (July, September) than in cool months (November, January), whereas anammox did not vary significantly across the sampling months. Average denitrification rates throughout the sampling months were higher in freshwater than in oligohaline wetlands, while anammox rates did not vary among the wetlands. Relative contribution of anammox (R_a) to N₂ production (including denitrification and anammox) varied from 1.03 to 18.3% across the sampling months and wetlands. Denitrification rates differed significantly across the wetlands and sampling months. Anammox rates and R_a did not vary significantly among the sampling months. Denitrification rates were positively correlated with water content, total organic carbon (TOC), total nitrogen, dissolved organic nitrogen, NH₄⁺, NO_x^–, Fe²⁺, and Fe²⁺/Fe³⁺, but negatively related to pH. Anammox rates showed negative relationships with water content and TOC. Water content, temperature, and pH were crucial for organic carbon and Fe²⁺ availability with important implications on denitrification and anammox. Therefore, denitrification rates vary significantly, whereas anammox rates do not vary significantly across freshwater-oligohaline wetlands in the Min River Estuary.