Fluxes of nitrous oxide and methane in different coastal Suaeda salsa marshes of the Yellow River estuary, China

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−2h−1 to 0.0982mgN2Om−2h−1 and −0.7421mgCH4m−2h−1 to 0.4242mgCH4m−2h−1, respectively. The mean N2O fluxes in spring, summer, autumn and winter were 0.0325mgN2Om−2h−1, 0.0089mgN2Om−2h−1, 0.0119mgN2Om−2h−1 and 0.0140mgN2Om−2h−1, and the average CH4 fluxes were −0.0109mgCH4m−2h−1, −0.0174mgCH4m−2h−1, −0.0141mgCH4m−2h−1 and −0.0089mgCH4m−2h−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.