Sulfur isotopes reveal agricultural changes to the modern sulfur cycle

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 ([SO4 (2-)]) and stable isotopes (delta S-34-SO4 (2-)), 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 delta S-34-SO4 (2-) values were 9.9 +/- 5.9 parts per thousand (median +/- interquartile range), enriched by similar to 10 parts per thousand relative to forests/grasslands (-0.28 +/- 5.7 parts per thousand). Vineyards also had roughly three-fold higher [SO4 (2-)] than forests/grasslands (13.6 and 5.0 mg SO4 (2-)-S l(-1), respectively). Napa River delta S-34-SO4 (2-) values, reflecting the watershed scale, were similar to those from vineyards (10.5 +/- 7.0 parts per thousand), despite vineyard agriculture constituting only similar to 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.

Automated summary

In this study, the researchers used sulfur stable isotopes to trace and identify agricultural sulfur in the Napa River Watershed, California, US. The results showed that vineyards, which receive frequent sulfur sprays, had higher sulfur concentrations and isotopic values compared to surrounding forests/grasslands. Additionally, the sulfur fingerprint in the Napa River reflected the contribution of vineyard agriculture, indicating that agricultural sulfur can be traced at a larger scale.