Soil sulphur speciation in two glacier forefield soil chronosequences assessed by S K-edge XANES spectroscopy

In regions with little atmospheric input of sulphur (S) and S-poor parent material, the bio-availability of S, which is dependent on its speciation, may limit ecosystem production and succession. In our study, soil S speciation in two glacier forefield soil chronosequences (Hailuogou Glacier, Gongga Shan, China; Damma Glacier, Swiss Alps) was investigated for the first time. Different S species were quantified by synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy at the S K-edge. Both chronosequences show similar patterns and pedogenetic trends of their topsoil S status. Topsoil concentrations of total S were correlated with the concentrations of organic carbon and pedogenic Fe/Al oxyhydroxides. Both moraine materials contained inorganic sulphides, which in the topsoil were oxidized within 30 (Hailuogou) or 75years (Damma) of soil development after deglaciation. About 50% of total S in the fresh moraine material at Hailuogou and 75% of that in the 15year-old soil at Damma was organically-bound. During initial soil development, the contribution of organic S to total S increased at the expense of inorganic sulphide and sulphate, resulting in organic S percentages>90% of total topsoil S after 30 (Hailuogou) and 75(Damma) years of pedogenesis. Organic S compounds with electronic oxidation states of the S atom>+1.5 (sulphoxides, sulphones, sulphonates and ester sulphates) dominated the organic S pool in all soils. Hence, microbial degradation of non-sulphide organic S (sulphonates and ester sulphates) is probably important to mitigate S scarcity caused by limited availability of SO₄^2--S in these soils. Changes in topsoil S speciation during initial stages of pedogenesis and ecosystem succession in glacier forefields under a cool, humid climate appear to be governed by combined effects of mineral weathering (oxidation of inorganic sulphides and formation of S-adsorbing sesquioxides), accumulation and microbial turnover of soil organic matter and the type of vegetation succession.