Nitrogen input weakens the control of inundation frequency on soil organic carbon loss in a tidal salt marsh

Abstract

The soil carbon (C) sequestration capacity of salt marshes is of considerable importance with respect to the mitigatiing the potentially detrimental consequences of global climate change. Given that tidal salt marshes are subjected to periodic cycles of inundation, it is assumed that soil C cycle in these marshes is expected to be controlled to varing extent by changes in soil moisture and salinity induced by the alternating patterns of drying and rewetting. In addition, with increases in the extent and severity of nitrogen (N) eutrophication becomes serious, we predicted that soil organic carbon (SOC) loss in tidal salt marshes is likely to be highly responsive to increased N loading. In this study, we conducted a two-factor mesocosm experiment (simulated inundation and N input in a tidal salt marsh) to determine the interactive effects of N eutrophication and inundation frequency on CO2 and CH4 emissions and dissolved organic carbon (DOC) contents. Our results showed that increased inundation frequency led to lower levels of CO2 emission but greater CH4 emission, whereas N input weakened the effects of changes in inundation frequency on CO2 and CH4 emissions. Moreover, N input was found to modify CO2 and CH4 emission in response to variations in soil moisture. We also observed an enhancement of soil DOC loss in response to increasing inundation frequency, and that DOC loss in the surface soil was considerably greater than that in subsurface soil. Further, as inundation frequency increased, we detected changes in the relationship between soil DOC and CO2 and CH4. Our findings highlighted that vertical variation in soil moisture induced by inundation is a key factor controlling SOC loss in tidal salt marshes, although N input can weaken this control.