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To investigate the spatial and seasonal variations of nitrous oxide (N2O) fluxes and understand the key controlling factors, we explored N2O 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 N2O differed significantly between sampling periods as well as between sampling positions. During all times of day and the seasons measured, N2O fluxes ranged from -0.0051 to 0.0805 mg N2O m(-2) h(-1), and high N2O emissions occurred during spring (0.0278 mg N2O m(-2) h(-1)) and winter (0.0139 mg N2O m(-2) h(-1)) while low fluxes were observed during summer (0.0065 mg N2O m(-2) h(-1)) and autumn (0.0060 mg N2O m(-2) h(-1)). The annual average N2O flux from the intertidal zone was 0.0117 mg N2O m(-2) h(-1), and the cumulative N2O emission throughout a year was 113.03 mg N2O m(-2), indicating that coastal marsh acted as N2O source. Over all seasons, N2O fluxes from the four marshes were significantly different (p < 0.05), in the order of HM (0.0256 ± 0.0040 mg N2O m(-2) h(-1)) > MF (0.0107 ± 0.0027 mg N2O m(-2) h(-1)) > LM (0.0073 ± 0.0020 mg N2O m(-2) h(-1)) > MM (0.0026 ± 0.0011 mg N2O m(-2) h(-1)). Temporal variations of N2O emissions were related to the vegetations (Suaeda salsa, Phragmites australis, and Tamarix chinensis) 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 N2O contributions (particularly during non-growing season) to the estimation of local N2O inventory, and highlighted both the large spatial variation of N2O fluxes across the coastal marsh (CV = 158.31%) and the potential effect of exogenous nitrogen loading to the Yellow River estuary on N2O emission should be considered before the annual or local N2O inventory was evaluated accurately.

Abstract: The spatial and temporal variations of the fluxes of nitrous oxide (N2O) and methane (CH4) and associated abiotic sediment parameters were quantified for the first time across the coastal marsh dominated by Suaeda salsa in the Yellow River estuary during 2009/2010. During all times of day and the seasons measured, N2O and CH4 fluxes from coastal marsh ranged from −0.0147mgN2Om−2 h−1 to 0.0982mgN2Om−2 h−1 and −0.7421mgCH4 m−2 h−1 to 0.4242mgCH4 m−2 h−1, respectively. The mean N2O fluxes in spring, summer, autumn and winter were 0.0325mgN2Om−2 h−1, 0.0089mgN2Om−2 h−1, 0.0119mgN2Om−2 h−1 and 0.0140mgN2Om−2 h−1, and the average CH4 fluxes were −0.0109mgCH4 m−2 h−1, −0.0174mgCH4 m−2 h−1, −0.0141mgCH4 m−2 h−1 and −0.0089mgCH4 m−2 h−1, respectively, indicating that the coastal marsh acted as N2O source and CH4 sink. Both N2O and CH4 fluxes differed significantly between times of day of sampling. N2O fluxes differed significantly between sampling seasons as well as between sampling positions, while CH4 fluxes had no significant differences between seasons or positions. Temporal variations of N2O emissions were probably related to the effects of vegetation (S. salsa) during summer and autumn and the frequent freeze/thaw cycle of sediment during spring and winter, while those of CH4 fluxes were controlled by the interactions of thermal conditions and other abiotic factors (soil moisture and salinity). Spatial variations of N2O and CH4 fluxes were primarily affected by soil moisture fluctuation derived from astronomic tide, sediment substrate and vegetation composition. N2O and CH4 fluxes, expressed as CO2-equivaltent (CO2-e) emissions, showed that N2O comprised the principal part of total calculated CO2-e emissions during spring and winter, while the contributions of CH4 could not be ignored during summer and autumn. This study highlights the importance of seasonal N2O and CH4 contributions, particularly during times of significant CH4 consumption. For the accurate up-scaling of N2O and CH4 fluxes to annual rates, a careful sampling design at site-level is required to capture the potentially considerable temporal and spatial variations of N2O and CH4 emissions. [Copyright &y& Elsevier]