Agricultural Sulfur Applications Alter the Quantity and Composition of Dissolved Organic Matter from Field-to-Watershed Scales.

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, δ34S-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 CHOS2 chemical formulas─the latter also found in tributary and Napa River surface water. The isotopic difference between δ34S-DOS and δ34S-SO42- 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.

Fates and fingerprints of sulfur and carbon following wildfire in economically important croplands of California, U.S.

Sulfur (S) is widely used in agriculture, yet little is known about its fates within upland watersheds, particularly in combination with disturbances like wildfire. Our study examined the effects of land use and wildfire on the biogeochemical "fingerprints," or the quantity and chemical composition, of S and carbon (C). We conducted our research within the Napa River Watershed, California, U.S., where high S applications to vineyards are common, and ~ 20% of the watershed burned in October 2017, introducing a disturbance now common across the warmer, drier Western U.S. We used a laboratory rainfall experiment to compare unburned and low severity burned vineyard and grassland soils. We then sampled streams draining sub-catchments with differing land use and degrees of burn and burn severity to understand combined effects at broader spatial scales. Before the laboratory experiment, vineyard soils had 2-3.5 times more S than grassland soils, while burned soils-regardless of land use-had 1.5-2 times more C than unburned soils. During the laboratory experiment, vineyard soil leachates had 16-20 times more S than grassland leachates, whereas leachate C was more variable across land use and burn soil types. Unburned and burned vineyard soils leached S with δ34S values enriched 6-15‰ relative to grassland soils, likely due to microbial S processes within vineyard soils. Streams draining vineyards also had the fingerprint of agricultural S, with ~2-5 fold higher S concentrations and ~ 10‰ enriched δ34S-SO42- values relative to streams draining non-agricultural areas. However, streams draining a higher fraction of burned non-agricultural areas also had enriched δ34S values relative to unburned non-agricultural areas, which we attribute to loss of 32S during combustion. Our findings illustrate the interacting effects of wildfire and land use on watershed S and C cycling-a new consideration under a changing climate, with significant implications for ecosystem function and human health.