Source Identification of Sulfur in Uncultivated Surface Soils from Four Chinese Provinces

The analysis of stable isotopes of sulfur (δ34S) is a useful tool for identifying sources of sulfur in soils. Concentrations and sulfur (S) isotopes of water-soluble sulfate (WSS), adsorbed sulfate (AS), residual sulfur (RS), and total sulfur (TS) in uncultivated surface soils of four Chinese provinces were systematically analyzed for identifying sources of S in the soils. Green and healthy mosses (Haplocladium microphyllum) were sampled as bioindicators. The mean WSS concentration (27.8 ± 23.4 mg kg−1) in the surface soils was lower than those of AS (101.4 ± 57.0 mg kg−1) and RS (381.5 ± 256.7 mg kg−1). The mean δ34S values of WSS and AS were very similar (about 2.0%), lower than those of RS (8.0%) and TS (6.1%). A significant linear correlation was found between the δ34S values of AS and WWS (y = 1.0002x – 0.0557, P < 0.0001), indicating that sulfate adsorption in the soils did not markedly fractionate S. All S species in the soils of Guizhou Province were characterized by the lowest δ34S values, consistent with the most 34S-depleted rainwater sulfate reported at Guiyang of Guizhou Province. The δ34S values of sulfate in mosses and rainwater previously reported were significantly linearly correlated with those of both WWS and AS in surface soils, suggesting that atmospheric S input was an important source for soil WSS and AS. However, there were no significant correlations between isotopic composition of rainwater sulfate and RS or TS. The slopes of all these significant linear correlations (soil/rainwater or soil/moss isotopic ratio) were 0.4–0.6, indicating that inorganic sulfate in the surface soils should be a result of mixing of deposited atmospheric sulfate with a more 34S-depleted sulfate component possibly from mineralization of RS.