High hydrogen sulfide emissions from subtropical forest soils based on field measurements in south China

The present estimate of global hydrogen sulfide (H₂S) emission from natural sources has large uncertainty mainly due to the lack of valid field data, particularly for subtropical forest soil in China with elevated atmospheric S deposition. In this study, the field observation of H₂S fluxes over subtropical forest soil was conducted for the first time in south China to measure the magnitude of emissions of H₂S, and evaluate its contribution to the large S sink, using the Dynamic Flux Chamber (DFC) method. Daily variations of H₂S fluxes showed an increasing emission with the increasing air temperature in the morning, a peak at the middle of the day, and a decreasing emission thereafter, then approximated to zero at night. The H₂S flux had positive values in all seasons, with the highest in summer, followed by spring, and relatively lower values in fall and winter. The H₂S flux measurements showed relatively large emission with annual average value of 0.028 g S m^-2 yr^-1, possibly due to the elevated sulfate concentration in the soil solution by S deposition, the hot and humid climate, as well as the lower soil pH in subtropical China. Thus, not only tropical, subtropical soils need to be included as significant H₂S sources to accurately portray the global H₂S budget. Although the soil in subtropical forests acted as a strong source for H₂S to atmosphere, H₂S emission from soil had limited contribution (about 0.2%) to the large S sink in this forest catchment.