Integrating hydrological and biogeochemical approaches for tracing the fates of applied water and sulfur in vineyards

Background/Question/Methods
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 S pools, allowing the estimation of water and S budgets. The objectives of this two and a half year study were to (1) determine the immediate fates of applied elemental S in a Napa Valley vineyard, (2) characterize the near-surface hydrological flow paths during irrigation and storm events, and (3) identify how those flow paths affect the fate and transport of S across seasons. Integrating hydrological theory with measurements of S species and sulfate-S isotopic ratios (expressed as δ³⁴S) in inputs, soil, soil water, and leachate provided a means of determining S pools and flow paths.

Results/Conclusions

Low [SO₄^2-] and δ³⁴S in leachate during four-hour 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 [SO₄^2-] and δ³⁴S values during eight-hour irrigation and storm events reflect near complete engagement of the soil matrix, indicating that lateral flow was the dominant pathway. These results suggest that (1) preferential flow is an important loss pathway to consider in managing water and nutrients in vineyards, and (2) over the long-term, continued application of reactive S may have potentially negative consequences for soil and water quality both on- and off-site, due to high SO₄^2- export.