Fates and fingerprints of sulfur and carbon following wildfire in economically important croplands of California, US

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 bio-geochemical 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 similar to 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 delta S-34 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 similar to 2-5 fold higher S concentrations and similar to 10% enriched delta S-34-SO42- values relative to streams draining non-agricultural areas. However, streams draining a higher fraction of burned non-agricultural areas also had enriched delta S-34 values relative to unburned non-agricultural areas, which we attribute to loss of S-32 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. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The study investigated the bio-geochemical fingerprints of sulfur and carbon in the Napa River Watershed in California, showing the interacting effects of wildfire and land use on watershed sulfur and carbon cycling.