Prominent Role of Sulfate Reduction in Considerable Sulfur Retention in a Subtropical Soil

Abstract

The sulfur biogeochemical cycle controls the sulfur dynamics in the soil. In contrast to almost all deposited sulfur leaching out in temperate catchments, approximately 80% of the deposited S is retained in the catchments in subtropical regions. However, the mechanisms for sulfur retention were unclear, hindering the understanding of potential threats of legacy sulfur with environmental changes. Here, we demonstrated that sulfate reduction (as sulfide fixed in soil) was a prominent yet overlooked mechanism for sulfur retention in addition to the widely recognized sulfate adsorption in soil, based on a study on soil sulfur storage and stable isotope signatures within entire soil profiles in a typical subtropical catchment in China. Using a dual‐isotopic model, the sulfate reduction flux was further determined to be 30% of the S deposition. Due to a lot of deposited sulfur fixed via sulfate reduction, the release of soil legacy sulfur would be less in response to decreasing sulfur deposition compared to the projections only considering adsorption. However, the remobilization of large amounts of reduced S should be regarded as a threat to the environment under climate change in the future.

Long-term Trends of Dissolved Organic Carbon Dynamics in a Subtropical Forest Responding to Environmental Changes

“…The relatively lower DOC concentration illustrated that there was a strong sorption or mineralization of DOC in the mineral soil in the subtropical forests. Actually, the old and deeply weathered soils typical of subtropical regions often exhibit low pH (3.7–4.6) and high iron/aluminum (Fe/Al) oxide contents, which contribute to have a large adsorption capacity. , Additionally, the subtropical TSP catchment has been approved to have a stronger biogeochemical cycle of sulfur and nitrogen in the mineral soil, which might lead to the stronger DOC mineralization in the soil. The significant decline of soil organic carbon content from top soil to the subsoil might provide further evidence for the organic carbon mineralization in the mineral soil (Table S1).…”