Mineral association drives divergent chemical trajectories of organic sulfur during dryland soil development

Hai-Ruo Mao , Zhuojun Zhang, Stephen C. Hart, Jianchao Zhang, Mohsen Shakouri, Mengqiang Zhu

Sulfur (S) deficiency is becoming more widespread as atmospheric S deposition declines, increasing ecosystem reliance on mineralization of soil organic S as a source of plant-available sulfate. Yet how organic S chemistry evolves during soil development, and how this evolution is mediated by microbial processing and mineral association, remains poorly understood, particularly in drylands. Here, we used S K-edge X-ray absorption near-edge structure spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry to examine bulk, water-extractable, mineral-associated, and light particulate organic S across a semi-arid soil chronosequence characterized by progressive texture development and increasing microbial activity. Total soil S increased along the chronosequence, with organic S accounting for more than 90% of total S, suggesting that long-term accumulation of atmospherically derived S reflects biological assimilation and stabilization in organic matter, particularly mineral-associated organic matter, rather than passive accumulation of inorganic sulfate alone. Organic S was reorganized into pools with divergent chemical trajectories: water-extractable organic S became less aromatic, more oxidized, and more microbial-derived, reflecting microbial processing and adsorption-induced fractionation; mineral-associated organic S remained enriched in oxidized S species, consistent with retention of oxidized groups by fine minerals; and light particulate organic S became progressively more reduced, consistent with preferential loss of oxidized S from the weakly protected particulate pool. Despite increases in microbial activity and total organic S, the proportion of phosphate-extractable sulfate remained stable, indicating that immediate S availability was buffered rather than simply increasing with microbial processing. These patterns support a mineral-filtering model in which increasing silt- and clay-sized mineral surfaces during soil development progressively reorganize organic S between weakly protected particulate matter and stabilized mineral-associated organic matter. By stabilizing organic S and retaining released sulfate in an exchangeable, plant-available form, these mineral surfaces buffer sulfate supply.