Fluxes of H2S and SO2 above a subtropical forest under natural and disturbed conditions induced by temporal land-use change

Abstract

Hydrogen sulfide (H2S) is one of predominant biogenic sulfur gases, influencing aerosol formation and climate change. There is considerable uncertainty of the global budget of H2S due to limited field data, especially in subtropical forests. In addition, an interaction between soil-emitted H2S and ambient sulfur dioxide (SO2) might exist within forest ecosystems. In this study, the aerodynamic gradient method was applied to consecutively measure H2S and SO2 fluxes above a subtropical forest canopy in Southwest China under natural and disturbed conditions induced by temporal land-use changes. The average H2S concentration and flux under natural conditions were 0.79 ± 0.07 ppbv and 0.04 ± 0.01 g S m−2 yr−1, respectively. The emission was larger than that in most croplands and freshwater wetlands. Vegetation emissions might account for about 26% of the total forest H2S emissions at this site. The deposition of SO2 was likely balanced by H2S oxidization under the forest canopy, with the mean concentration and net flux as 1.23 ± 0.11 ppbv and −0.03 ± 0.10 g S m−2 yr−1, respectively. Under disturbed conditions with soils excavation and scattering on the forest floor, simultaneously high emissions of H2S and SO2 were observed above the canopy, reaching 5.78 ± 0.16 and 1.60 ± 0.87 g S m−2 yr−1, respectively. This suggested that land-use change in subtropical forests might lead to release of legacy S in subsoils to the atmosphere in the form of H2S and SO2. Regarding the widely documented large S accumulation and expanding deforestation across subtropical forests, potentially high emissions of H2S and SO2 from subtropical forests should be carefully considered in regional air quality control and forest management.