Effects of nitrogen loading on emission of carbon gases from estuarine tidal marshes with varying salinity.

Estuarine tidal marshes sequester significant quantities of carbon and are suffering from anthropogenic nitrogen (N) enhancement. However, the effects of this N loading on carbon gas emissions from freshwater-oligohaline tidal marshes are unknown. In this paper, we report on our evaluation of the effects of a N gradient (0, 24, 48 and 96 g NH4NO3-N m-2 y-1) on the methane (CH4) and carbon dioxide (CO2) emissions from freshwater and oligohaline tidal marshes in the Min River estuary, southeast China. On an annual scale, the oligohaline marsh has significantly higher CO2 emissions, while it has slightly lower CH4 emissions relative to freshwater marsh. The addition of N increased CH4 emission from the freshwater marsh and decreased CH4 emission from the oligohaline marsh, although there was no statistically significant difference in CH4 emission between either of the two marshes and the control. The addition of 96 g NH4NO3-N m-2 y-1 significantly increased CO2 emission from the freshwater marsh, while it did not significantly influence CO2 emission from the oligohaline marsh. CH4 and CO2 emission levels were positively correlated with soil temperature under all conditions. The CH4 flux resulting from both the control and the addition of N was negatively correlated with porewater SO42- and Cl- concentrations and soil EC in the oligohaline marsh. Overall, N addition significantly increased carbon gas emissions under freshwater conditions while slightly inhibiting carbon gas emissions from the oligohaline marsh. Our findings suggested that even under low salinity conditions, the effects of N loading on CH4 and CO2 emissions from freshwater and oligohaline tidal marshes can vary. We propose that the addition of N to estuarine tidal marshes has a significant effect on the carbon cycle and promotes soil carbon loss, phenomena which may be influenced by salinity.