Effects of inorganic nitrogen enrichment on soil CH4 and CO2 production in freshwater and mesohaline marshes across six estuaries in China.

• Total nitrogen was a key driver in soil CH 4 and CO 2 production in tidal marshes. • Nitrate enrichments tended to decrease CH 4 and CO 2 production. • Nitrate but not ammonium addition decreased mcr A gene abundance. • Salinity was not a mitigating factor in either nitrogen eutrophication scenarios. Eutrophication is an important environmental stressor in coastal wetlands that alters ecosystem processes including primary production and the carbon cycle. However, how different nitrogen eutrophication scenarios may affect CH 4 and CO 2 production along a salinity gradient in coastal wetlands is poorly known. We collected the surface soil samples from both tidal freshwater and mesohaline Phragmites australis marshes in six main estuaries in China and conducted inorganic nitrogen enrichment anaerobic incubation experiments. Background soil CH 4 and CO 2 production rates were strongly correlated with total nitrogen but not salinity. On average, nitrate enrichment decreased soil CH 4 and CO 2 production rates by ca. 20 % in freshwater marsh and 16–43 % in mesohaline marshes, whereas ammonium enrichment decreased CO 2 production by ca. 26–30 % but had no effect on CH 4 production. Salinity was not an important mitigating factor in either eutrophication scenario. In most cases, nitrogen addition decreased the extracellular enzyme activities of β-1,4-glucosidase and cellobiohydrolase, but increased the activity of β-N-acetyl glucosaminidase, phenol oxidase and peroxidase. Nitrate addition decreased the mcr A gene abundance on average by 34–35 % but ammonium addition had insignificant effect. Total nitrogen availability was an important driver of CH 4 and CO 2 production rates in these tidally influenced coastal wetlands. Our results showed that tidal marshes with salinity < 15 ppt had similar soil CH 4 production rates, and increasing inorganic nitrogen eutrophication might lower soil microbial carbon gas production in both tidal freshwater and mesohaline marshes. [ABSTRACT FROM AUTHOR]