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Keywords:

  • preferential flow;
  • sulfate;
  • sulfur isotopes;
  • precision irrigation;
  • vineyards;
  • Napa Valley

[1] California's widespread and economically important vineyards offer substantial opportunities to understand the interface between hydrology and biogeochemistry in agricultural soils. The common use of native sulfur (S) as a fumigant or soil additive provides a novel way to isotopically differentiate among sulfate (SO42−) pools, allowing the estimation of water and SO42− budgets. The objectives of this study were (1) to characterize the near-surface hydrological flow paths in a vineyard during irrigation and storm events and (2) to determine how those flow paths affect the fate and transport of SO42− across seasons. Integrating hydrological theory with measurements of SO42− concentration and sulfate-S isotopic ratios (expressed as [SO42−] and δ34S, respectively) in inputs, soil water, and leachate provided a means of determining flow paths. Low [SO42−] and δ34S in leachate during 4-h irrigation events reflect minimal engagement of the soil matrix, indicating that preferential flow was the dominant path for water in the near surface. In contrast, high [SO42−] and δ34S values during 8-h irrigation and storm events reflect near-complete engagement of the soil matrix, indicating that lateral flow was the dominant pathway. Because hydrologic response and SO42− mobility are tightly coupled in these soils, the magnitude of water fluxes through the near surface controls S cycling both on and off site. These results indicate that preferential flow is an important loss pathway to consider in managing both water resources and water quality (reactive elements) in vineyard land use systems.