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.