Agricultural Sulfur Applications Alter the Quantity and Composition of Dissolved Organic Matter from Field-to-Watershed Scales
Anna L. Hermes, Merritt Logan, Brett A. Poulin, Amy M. McKenna, Todd E. Dawson, Thomas Borch, Eve‐Lyn S. Hinckley
Abstract
Over the past several decades, agricultural sulfur (S) use has dramatically increased. Excess S in the environment can cause several biogeochemical and ecologic consequences, including methylmercury production. This study investigated agriculturally associated changes to organic S─the most dominant form of S within soils─from field-to-watershed scales. Using a novel complementary suite of analytical methods, we combined Fourier transform ion cyclotron resonance mass spectrometry, δ 34 S-DOS, and S X-ray absorption spectroscopy to characterize dissolved organic S (DOS) in soil porewater and surface water samples from vineyard agriculture (S addition) and forest/grassland areas (no S addition) within the Napa River watershed (California, U.S.). Vineyard soil porewater dissolved organic matter samples had two-fold higher S content compared to forest/grasslands and had unique CHOS 2 chemical formulas─the latter also found in tributary and Napa River surface water. The isotopic difference between δ 34 S-DOS and δ 34 S–SO 4 2– values provided insights into the likely dominant microbial S processes by land use/land cover (LULC), whereas the S oxidation state did not strongly differ by LULC. The results add to our understanding of the modern S cycle and point to upland agricultural areas as S sources with the potential for rapid S transformations in downgradient environments.