Sulfur's Long Game: 145 Years of Soil Sulfur Speciation in the World's Oldest Agricultural Experiments

Sulfur (S) is an essential plant nutrient, but ongoing decreases in inorganic S inputs to soil continue to reduce S availability in agricultural soils globally. This study investigated long-term trends in soil S speciation after land use change and the application of different soil amendments using the world's longest-running agricultural experiments at the Rothamsted Research Centre, UK. Soil samples spanning 145 years were obtained from the Broadbalk Wheat Experiment (continuous cropping with different amendments) and two Wilderness sites, Broadbalk Wilderness and Geescroft Wilderness (cropping land left to rewild) and analysed using synchrotron-based x-ray absorption near-edge structure (XANES) spectroscopy. It was found that changes in S speciation were linked to changes in soil organic carbon (SOC). In the Broadbalk Winter Wheat experiment, farmyard manure applications increased the proportion of reduced C-bonded S by 40% over 145 years, while the S speciation in the inorganic fertiliser (NPKMgS) and Control treatments remained unchanged and was comprised of ~48% oxidised S. In the Wilderness sites (cropping ceased 143-146 years from present), SOC accumulation during rewilding generally increased the proportions of reduced organic S. However, soil acidification at the Geescroft site initially increased the average oxidation state of S (from +3.7 in 1883 to +4.4 in 1965) despite increasing SOC. Thus, whilst SOC is important in controlling S speciation, soil pH also has a central effect. These findings provide new insights into the long-term dynamics of S speciation under different agricultural practices and land-use changes and contribute to our understanding of S and its availability in cropping systems.