Shifts from methyl chloride sink to source functions within a coastal salt marsh in eastern China: an examination of the effects of biomass burning prohibition policies

Abstract

Our previous study found that a salt marsh in eastern China can act as a large CH₃Cl sink. One striking finding of this previous study was a strong relationship between high-ambient CH₃Cl concentrations and fluxes during the growing season. Moreover, the high-ambient CH₃Cl concentration was likely to be related to local biomass burning. However, implementation of biomass burning prohibition policies has effectively reduced biomass burning. Therefore, we predicted that the prohibition of biomass burning would alter CH₃Cl concentration and flux within the eastern Chinese coastal salt marsh. In this study, we used static flux chambers to measure CH₃Cl fluxes in the early (July of 2004 and January of 2005) and middle-late stages (August and December of 2013) of biomass burning prohibition of along a creek and vegetation transects of the salt marsh. After implementation of the biomass burning prohibition, the concentration and flux of CH₃Cl directly related to biomass burning changed remarkably. During the middle-late stage of prohibition, the initial CH₃Cl concentration was significantly reduced compared to during the early stage of prohibition. Reductions in atmospheric CH₃Cl concentration were especially apparent during the growing season, when biomass burning was prohibited and atmospheric CH₃Cl concentration dropped to levels nearly as low as the Northern Hemisphere background concentration. Atmospheric CH₃Cl concentration significantly varied throughout the salt marsh, with the highest concentrations appearing over the inland areas and mudflat and lower values occurring over the middle locations. This spatial distribution of CH₃Cl may have been directly related to the existence and distribution of potential CH₃Cl sources, such as coastal seawater, terrestrial biomass burning, and senescent and decaying aboveground biomass. These changes in initial CH₃Cl concentration caused by the biomass burning prohibition may eventually lead to shift in the salt marsh from the tendency to act as a CH₃Cl sink to the tendency to act as a CH₃Cl source. When the initial atmospheric CH₃Cl concentration was high, the vegetation stands acted as CH₃Cl sinks. Conversely, they became CH₃Cl sources. Therefore, we conclude that the biomass burning prohibition altered the ecosystem–atmosphere exchange of CH₃Cl within the studied eastern Chinese coastal salt marsh.