World Congress of Soil Science Logo 18th World Congress of Soil Science
July 9-15, 2006 - Philadelphia, Pennsylvania, USA
International Union of Soil Sciences

Friday, 14 July 2006

This presentation is part of 99: 1.2B Soil System Behavior in Time - Theater

High Sulfur Inputs in Northern California Vineyards: Short-term Fates and Long-term Implications.

Eve-Lyn S. Hinckley, Scott Fendorf, and Pamela A. Matson. Stanford University, Braun Hall Building 320, 450 Serra Mall, Stanford, CA 94305

The widespread application of elemental sulfur (S0) to vineyards, a predominant agricultural system in Northern California, creates the opportunity to study the transformations, fates, and effects of this reactive nutrient at multiple scales. We used a variety of measurement techniques to assess S dynamics in a two-part study to (1) evaluate the short-term response of the soil environment to regular S additions, and (2) measure changes in S transformations and transfers under two hydrologic regimes: the growing (dry) season (April-October), when vintners use drip irrigation to sustain vines, and dormant (wet) season (November-March), when rains saturate the soils. We consider these patterns and the underlying mechanisms that drive them using a spatially-explicit approach to identify the potential long-term implications of sustained elevated S inputs to this region. In the first part of the study, we evaluated the short-term fate of applied S0 using X-ray absorption near edge structure (XANES) spectroscopy, a direct method that determines S speciation using the relationship with energy required for core electron transitions and correlation with additional spectral features. Surficial soil samples (0-2 cm) were collected immediately prior to and following two applications of S0 (11 kg S ha-1), with weekly collections in the two weeks between applications and following the last application. XANES spectra revealed that S0 oxidizes rapidly to sulfur dioxide or remains in the soil as sulfate (SO42-), the single form of S abundant at the soil surface. Measurements of the SO42- pool, 34S/32S isotopic ratios, and pH of these soils also indicate that rapid oxidation of S0 occurs, and when compared with results from oak woodland and grassland soils (the pre-vineyard state), indicate that vineyard soils are ~11-fold greater in S content. We found that S pools and 34S/ 32S isotopic ratios were highly variable at the soil surface, indicating that factors such as moisture content and S mineralization rates likely control SO42- availability. In the second part of our study, we investigated hydrologic losses of S species to determine whether secondary effects, such as elevated base cation losses, occur in response to S additions, and to assess the spatial variability of S transformations and transfers under different hydrologic conditions. We instrumented a 3.75 acre vineyard block with zero-tension and tension lysimeters to measure dissolved SO42- and other major anions (chloride, nitrate, and phosphate) and cations (potassium, sodium, aluminum, iron, magnesium, and calcium). We collected samples during irrigation events when soil remains unsaturated but a fraction of the irrigation inputs are transported to deeper depths via preferential or crack flow, and during extended irrigation and rain events when the soil is saturated. Solution losses were highest in crack flow during irrigation events in the growing (dry) season, and declined over the course of the dormant (wet) season, indicating a “raining out” effect in the system. Sulfate constituted the majority of anion losses below the rooting zone of the vines, and losses of major cations scaled with the magnitude of SO42- lost. Our study illuminated the significance of crack flow in clay-rich systems; not only did we measure as high as 25% of irrigation inputs lost via this pathway, but elevated concentrations of major anions and cations in these solutions indicated that rapid delivery of reactive nutrients to subsurface clay layers during the dry season may lead to hotspots of biogeochemical activity at the onset of the wet season. These data suggest that oxidation of S0 inputs and subsequent mobilization of SO42- may be a major control on losses of other key nutrients, and rapid delivery of SO42- via preferential flow paths to subsurface clay layers may greatly influence redox chemistry. Such effects of elevated S inputs may impact the long-term soil and water quality in California's premier winegrowing regions, which is a concern to both winegrowers interested in sustaining yield quantity and quality, and environmental decision-makers seeking to manage limited resources.

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