Precipitation-Driven Shifts in Organic Sulfur Decomposition and Oxidation State along Rainfall Gradients

The decomposition of organic sulfur (S) influences S availability to plants and trace metal dynamics in soils. While temperature effects on decomposition rate are known, the influence of precipitation remains less understood. We examined organic S decomposition and oxidation states in volcanic soils across two rainfall gradients on the Hawaiian Islands (mean annual precipitation [MAP]: 285-5066 mm). Higher MAP increased atmospheric sulfate deposition, which was largely converted to organic S by biological processes. However, the impact of precipitation on decomposition of organic S varied by rainfall regime. In wetter regions (MAP > 1500 mm), higher soil moisture promoted reducing conditions, resulting in markedly less oxidized and less decomposed organic S, with average oxidation state (AOS) of organic S decreasing from 4.5 to 2. In drier regions (MAP < 1500 mm), the decomposition of organic S (AOS = 4.2 ± 0.1) was high but did not correlate with MAP, likely because the soils remained sufficiently oxic for decomposition regardless of MAP and soil moisture. These trends reflect thermodynamic constraints on organic matter decomposition and suggest that soil organic matter enriched in reduced carbon tends to accumulate reduced sulfur, underscoring a strong coupling between carbon and sulfur cycles. Our findings suggest that climate change-induced shifts in precipitation and temperature could alter soil S cycling, with important consequences for nutrient availability and trace metal dynamics in terrestrial ecosystems.