Intensive smooth cordgrass removal strengthens tidal and temperature impacts on methane emission

The world’s largest ecosystem restoration via intensive removals of invasive smooth cordgrass (Spartina alterniflora) is being implemented in coastal China, potentially exerting a large impact on soil biogeochemical cycles of greenhouse gases including methane (CH4). However, the degree to which CH4 emission and its environmental controls change with such anthropogenic disturbances has been rarely assessed with direct empirical evidence. To quantify these disturbance effects, we utilized the eddy covariance (EC) approach to continuously measure net CH4 exchange from Jul. 2022 to Oct. 2023, covering both pre- and post-removal periods, in a disturbed coastal wetland of Southeast China experiencing an intensive cordgrass removal in late Oct. 2022. Our analyses, based on this unique EC dataset of high-frequency (30-min) time series CH4 fluxes, revealed that (a) the removal caused a pulse of CH4 emission peaking one month later up to 0.76 g CH4 m−2 d-1, with the mean post-removal emission over ten times that of the pre-removal level (0.03 g CH4 m−2 d-1); (b) the removal intensified the controls of tidal inundation and pumping on CH4 fluxes, showing much stronger pumping effects within two months following the disturbances; (c) the removal also enlarged the temperature sensitivity of CH4 emission, leading to larger daytime emission especially at afternoon hours; (d) the combination of enhanced tidal impacts and temperature dependence thus promoted the diel variability of CH4 fluxes during the post-removal period. These results suggest that coastal restoration via intensive cordgrass removals boosts both the magnitude and the diel variability of CH4 emission, highlighting the necessity of better understanding the climate impact of restoration activities. Future longer flux data with extended years are needed to further assess potential regime shift in soil CH4 biogeochemistry and long-term evolution of such unintended environmental costs of the restoration.