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 NHNO-N m y) on the methane (CH) and carbon dioxide (CO) emissions from freshwater and oligohaline tidal marshes in the Min River estuary, southeast China. On an annual scale, the oligohaline marsh has significantly higher CO emissions, while it has slightly lower CH emissions relative to freshwater marsh. The addition of N increased CH emission from the freshwater marsh and decreased CH emission from the oligohaline marsh, although there was no statistically significant difference in CH emission between either of the two marshes and the control. The addition of 96 g NHNO-N m y significantly increased CO emission from the freshwater marsh, while it did not significantly influence CO emission from the oligohaline marsh. CH and CO emission levels were positively correlated with soil temperature under all conditions. The CH flux resulting from both the control and the addition of N was negatively correlated with porewater SO 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 CH and CO 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.