Fraction-specific pathways of sulfur cycling in agroecosystems with highly weathered soils

Understanding the biogeochemical cycle of sulfur (S) is important for global food production, but much information remains unknown regarding the transformations of S species in strongly weathered soils. This study investigated the chemical speciation of S in bulk soil and its fractions of native and cropped soils from Kingaroy, Australia. Bulk soil was fractionated into free particulate organic matter (fPOM), occluded POM (oPOM), fine mineral fraction (f-MF, < 53 µm) and coarse MF (c-MF, > 53 µm). The speciation of S was examined using traditional chemical methods and synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy. It was found that cropping decreased the total S of native soil (by 14% at 0–10 cm) in a manner similar to soil organic carbon (SOC), reflecting the interconnection between S and SOC. The chemical composition of S was inherently different between the soil fractions, with fPOM and oPOM having markedly higher proportions of C-bonded S (73–90%) than f-MF and bulk soil (51–72%). Furthermore, long-term cropping profoundly increased the proportions of oxidised S (sulfate and ester sulfate) in the f-MF and bulk soil, but the S species in POM fractions remained largely unchanged. Soil pH and chemical weathering strongly influenced extractable S, with lower pH enhancing sulfate retention in acid soil. Our study demonstrates the varied roles that different soil fractions play in the biogeochemical cycling of S, with these findings highlighting that monitoring S chemical composition in MF is critical for sustaining soil S fertility in agroecosystems. • Long-term cropping decreased total sulfur in bulk soil. • Cropping shifted S toward oxidised forms in the mineral fraction (MF) and bulk soil. • Particulate organic matter had up to 90% C-bonded S, considerably more than the MF. • Changes in soil S speciation were connected to changes in organic C. • Chemical weathering and soil pH play a critical role in the bioavailability of S.