Abstract
The transformation characteristics of mineral-associated soil components have profound impacts on their physical, biological, and chemical properties in drying-affected soils, whereas their mechanisms of sequestration and transformation remain elusive. To elucidate these phenomena, the solid-phase, water extracts (labile state, LS) and alkali-extracts (complexed state, CS) of four drying-affected soil types were examined. On average, the contents of soil organic carbon (SOC), soil total nitrogen (STN), and soil total hydrogen (STH) decreased in the order: forest > grassland > agriculture > desert. The extracted dissolved organic matter (DOM)(LS), DOM(CS) and nutrients varied greatly among soil types, which indicated the occurrence of mineralization, sequestration, neoformation, and either export or emission. In particular, the relatively high levels of dissolved inorganic carbon (DIC)(LS) and relatively low levels of DIC(CS) in agricultural soils could be ascribed to the impact of human activities, i.e., tilling and cultivation, on mineral-bound DIC, leading to its export in LS forms. The stable isotopes of δ(13)C-SOC and their significant relationships with DIC(LS) and SO(4)(2‒)(LS+CS) suggest the occurrence of carbon and sulfur sequestration through the uptake of CO(2), DIC, or carbonyl sulfide (COS) following their generation from SOC or DOM mineralization. In forested and agricultural soils, the humic substances (HS) components in LS forms were subjected to a substantial degradation, whereas HS(CS) components remained mostly unaffected, implying their occurrence in organo-mineral protection. Overall, low soil total sulfur (STS) and sulfate (SO(4)(2‒))(LS+CS) contents were correlated with high amounts of soil components in both the solid and liquid phases, and vice versa. These findings suggest that microbial SO(4)(2‒) might operate in the dissolution and mineralization of HS-bound organo-minerals, which would potentially generate soil inorganic carbon (SIC) or DIC, leading to either their subsequent sequestration as carbonate minerals or their exports and emissions as DIC and CO(2).