Bringing soil sulfur to the forefront: How long-term cropping impacts sulfur speciation in soil organic matter fractions

Soil sulfur (S) cycling is of increasing interest for crop S supply, given that S inputs to soils have declined due to the application of high-analysis (low-S) fertilisers and decreased atmospheric SO₂ emissions. This study uses three paired sites [undisturbed (native) and cropped (up to 82 y)] from subtropical Australia to examine the impact of land use change on soil S composition. Soil organic matter (SOM) was separated into three fractions: free particulate organic matter (fPOM), occluded POM (oPOM), and fine mineral-associated organic matter (fine-MAOM), before using X-ray absorption near-edge structure (XANES) spectroscopy for analyses of S speciation. In the native soils, S speciation across the SOM fractions was distinct. Both POM fractions were dominated by reduced thiol and thio-ether groups (∼33 %), with an average S oxidation state of +2.4. In contrast, the S in the fine-MAOM was mostly oxidised sulfate ester and sulfate (44 %), with an average oxidation state of +3.9. The loss of SOM during long-term cropping caused a concomitant loss in S – up to 99 % of S was lost from the less protected fPOM fraction, while only 44 % of S was lost from the fine-MAOM. Under cropping, the bulk soil shifted to higher proportions of oxidised S. However, this was not caused by a change in S speciation within any SOM fraction but was due to the decreased contributions of POM fractions to the bulk soil. This study develops our understanding of the influence of cropping on S speciation and the role of SOM fractions in S dynamics.