Seasonal variations of nitrous oxide fluxes and soil denitrification rates in subtropical freshwater and brackish tidal marshes of the Min River estuary

Estuarine tidal marshes provide favorable conditions for nitrous oxide (N2O) production. Saltwater intrusion caused by sea-level rise would exert complex effects on the production and emission of N2O in estuarine tidal marshes; however, few studies have been conducted on its effects on N2O emissions. Salinity gradients are a common occurrence in estuarine tidal marshes. Studies on production and emission of N2O in tidal marshes with different salinities may elucidate the impact of saltwater intrusion on the emission of greenhouse gases. This study explores the seasonal variations of N2O fluxes and soil denitrification rates in freshwater (Daoqingzhou wetland) and brackish (Shanyutan wetland) tidal marshes dominated by Cyperus malaccensis var. brevifolius (shichito matgrass) in the Min River estuary, southeastern China. N2O fluxes in both marshes showed strong temporal variability. The highest N2O fluxes were observed in the hot and wet summer months, whereas the lowest fluxes were observed in the cold winter and autumn months. N2O fluxes from the freshwater marsh (48.81 +/- 9.01 mu g m(-2) h(-1)) were significantly higher (p < 0.05) than those from the brackish-water marsh (27.69 +/- 4.01 mu g m(-2) h(-1)). Soil denitrification rates showed a similar temporal pattern, with the highest rates observed in summer and the lowest in winter. Similarly, soil denitrification rates were significantly higher (p < 0.05) in the freshwater marsh (32.72 +/- 19.15 mu mol N m(-2) h(-1)) than in the brackish-water marsh (4.97 +/- 2.64 mu mol N m(-2) h(-1)). Temperature and the salinity, sulfate (SO42-), and ammonia nitrogen (NH4+-N) concentrations of the overlying water were key factors affecting soil denitrification rates. N2O fluxes and soil denitrification rates demonstrated negative correlations with salinity and SO42- concentrations in both marshes. The results indicate that increased seawater intrusion would reduce N2O emissions from estuarine tidal wetlands and exert a negative feedback on the climate system. (c) 2017 Elsevier B.V. All rights reserved.