Anna Hermes* and Eve-Lyn Hinckley
*University of Colorado Boulder, 4001 Discovery Dr., Boulder, CO 80303 (anna.hermes@colorado.edu)

In October 2017, the North Bay wildfires swept through hillsides of California’s Napa and Sonoma winegrowing regions. We asked how the wildfires affected storage and transport of organic carbon (C), an important soil stabilizer and energy source in stream nutrient cycles, and sulfur (S), which is applied to vineyards to combat powdery mildew. Prior research showed that S transport in vineyards is controlled by dormant season rainstorm events. Thus, we designed a laboratory soil leaching experiment to measure rain event-based and seasonal losses of S and C from soils. We collected soil cores (0 to 10 cm) from three site types: non-burned grassland, non-burned vineyard, and burned vineyard. We simulated 10 rain events from the 2017 to 2018 wet season and collected soil leachates. We also sampled streams throughout the Napa River watershed during one rain event to measure post-fire watershed-scale C and S fluxes. Soil leachates and stream samples were analyzed for S species and dissolved organic carbon (DOC) concentrations. We found that while burned and unburned vineyard soils leached more S than grassland soils, unburned soils leached less DOC than burned soils. Burned vineyard soils leached on average three times less S, but 2.5 times more DOC than unburned vineyard soils. For all soil cores, leachates from rain events over 50 mm had ~3 times greater DOC and S concentrations than smaller events. In contrast to what we expected, DOC concentrations of stream samples from burned and unburned sampling locations were not significantly different. Streams draining vineyard-dominated landscapes, however, had significantly more S than burned or unburned forest landscapes, suggesting that vineyard soils export more S than grasslands, likely from fungicidal applications during the growing season. The increase in DOC losses from burned vineyard soils suggests that winegrowers may need to amend soils with organic carbon (e.g., cover crops) when replanting burned vineyards.

Funding Support: National Science Foundation, University of Colorado Boulder Environmental Studies Program, University of Colorado Boulder Center for Water, Earth Science, and Technology