Sulfur isotopes reveal agricultural changes to the modern sulfur cycle

Abstract

The environmental fates and consequences of intensive sulfur (S) applications to croplands are largely unknown. In this study, we used S stable isotopes to identify and trace agricultural S from field-to-watershed scales, an initial and timely step toward constraining the modern S cycle. We conducted our research within the Napa River Watershed, California, US, where vineyards receive frequent fungicidal S sprays. We measured soil and surface water sulfate concentrations ([SO42−]) and stable isotopes (δ34S–SO42−), which we refer to in combination as the ‘S fingerprint’. We compared samples collected from vineyards and surrounding forests/grasslands, which receive background atmospheric and geologic S sources. Vineyard δ34S–SO42− values were 9.9 ± 5.9％(median ± interquartile range), enriched by ∼10％ relative to forests/grasslands (−0.28 ± 5.7％). Vineyards also had roughly three-fold higher [SO42−] than forests/grasslands (13.6 and 5.0 mg SO42−–S l−1, respectively). Napa River δ34S–SO42− values, reflecting the watershed scale, were similar to those from vineyards (10.5 ± 7.0％), despite vineyard agriculture constituting only ∼11% of the watershed area. Combined, our results provide important evidence that agricultural S is traceable at field-to-watershed scales, a critical step toward determining the consequences of agricultural alterations to the modern S cycle.