Sulfur dynamics in forest soil profiles developed on granite under contrasting climate conditions

Highlights: • S in soils formed from granite is derived mainly from decomposing litter. • Cold/dry climate results in S accumulation at the surface. • Warm/wet climate results in S retention in the subsurface. • Pedogenic Fe/Al minerals play a key role in retaining soil S. • A conceptual model of S dynamics in soil profiles of different climate is proposed. Sulfur (S) dynamics in soils formed from granite remain poorly understood despite its importance as an essential plant macronutrient and component of soil organic matter. We used stable S isotope ratios to trace the sources and biogeochemical processes of S in four forest soil profiles developed on granite under contrasting climate conditions. The soil S is derived mainly from decomposing litter; no significant geogenic contribution to its content is noted as a result of the low S concentration of the granite (~ 5 μg/g). Colder/drier climate results in high organic S retention at the surface due to weak mineralization of organic S. Although warmer/wetter climate increases the S mineralization and leaching loss, SO₄²⁻ adsorption is an important S retention process in the subsurface. The vertical distribution of S isotope compositions in the soil profiles across the four sites indicates (i) a downward increase in δ³⁴S values in the upper profiles due to continuous mineralization of organic S with an occasional decrease in δ³⁴S values in the subsurface due to dissimilatory sulfate reduction (DSR), (ii) constantly high δ³⁴S values in the middle profiles due to the low water permeability, and (iii) a downward decrease in δ³⁴S values in the low profiles due to increased contribution of bedrock with depth. Regardless of the variation in soil depth and climate, the total S concentration is proportional to the pedogenic Fe/Al minerals, suggesting the important role of secondary Fe/Al minerals in retaining S in soils. This study provides an integration and synthesis of controls of climatic and edaphic variables on S dynamics in forest soil profiles developed on granite.