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

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.