Spatial and temporal variations of nitrous oxide flux between coastal marsh and the atmosphere in the Yellow River estuary of China

摘要

To investigate the spatial and seasonal variations of nitrous oxide (N_2O) fluxes and understand the key controlling factors, we explored N_2O fluxes and environmental variables in high marsh (HM), middle marsh (MM), low marsh (LM), and mudflat (MF) in the Yellow River estuary throughout a year. Fluxes of N_2O differed significantly between sampling periods as well as between sampling positions. During all times of day and the seasons measured, N_2O fluxes ranged from -0.0051 to 0.0805 mg N_2O m~(-2)h~(-1), and high N_2O emissions occurred during spring (0.0278 mg N_2O m~(-2) h~(-1)) and winter (0.0139 mg N_2O m~(-2) h~((-1)) while low fluxes were observed during summer (0.0065 mg N_2O m~(-2) h~(-1)) and autumn (0.0060 mg N_2O m~(-2) h~(-1)). The annual average N_2O flux from the intertidal zone was 0.0117 mg N_2O m~(-2) h~(-1), and the cumulative N_2O emission throughout a year was 113.03 mg N_2O m~(-2), indicating that coastal marsh acted as N_2O source. Over all seasons, N_2O fluxes from the four marshes were significantly different (p<0.05), in the order of HM (0.0256±0.0040 mg N_2O m~(-2) h~(-1))>MF(0.0107±0.0027 mg N_2O m~(-2) h~(-1))>LM (0.0073±0.0020 mg N_2O m~(-2)h~(-1))>MM (0.0026±0.0011 mg N_2O m~(-2) h~(-1)). Temporal variations of N_2O emissions were related to the vege tations {Suaeda salsa, Phragmites australis, and Tamarix chinen-sis) and the limited C and mineral N in soils during summer and autumn and the frequent freeze/thaw cycles in soils during spring and winter, while spatial variations were mainly affected by tidal fluctuation and plant composition at spatial scale. This study indicated the importance of seasonal N_2O contributions (particu larly during non-growing season) to the estimation of local N_2O inventory, and highlighted both the large spatial variation of N_2O fluxes across the coastal marsh (CV= 158.31 %) and the potential effect of exogenous nitrogen loading to the Yellow River estuary on N_2O emission should be considered before the annual or local N_2O inventory was evaluated accurately.