From Acid Rain to Agriculture: Our Changing Manipulation of the Sulfur Cycle

Human manipulation of the sulfur (S) biogeochemical cycle has shifted dramatically in recent decades. Once dominated by fossil fuel emissions that caused inadvertent inputs of reactive S as acid deposition to forested ecosystems, our major pathway of S manipulation is now the multiple forms used in agriculture. While many decades of research have documented the consequences of atmospheric S deposition to acid-sensitive forest and aquatic ecosystems, comparatively little is known about the unintended consequences of its use in large-scale agricultural systems. This synthesis examines the inputs, transport, and ecological effects of S applications in agriculture. We include case studies of three regional crops in the United States: grapes in California where S is used as a fungicide, corn in the Ohio River Valley where it is used as fertilizer, and sugarcane in Florida where it is used to regulate soil pH and liberate phosphorus. Data from these areas demonstrate that as atmospheric S deposition has declined over time in response to federal regulation of emissions, agricultural S inputs have increased with expansion and intensification of crop systems, or in response to a decline in "free" atmospheric S deposition. Surface water sulfate export has declined about 2-5% per year over the past 30 years in areas of the northeastern U.S. historically impacted by acid rain and where crop systems are dispersed. Conversely, streamwater sulfate concentrations have increased or remained elevated in the large agricultural areas represented by our case studies. Analysis of agricultural S in the context of a changing climate - particularly during the last decade of drought in CA - demonstrates the tight link between hydrology and the residence time of S. That is, whether it remains on-site and causes negative effects like soil acidification, or moves off-site and stimulates production of methylmercury, a neurotoxin, in adjacent aquatic ecosystems. Ultimately, it is important to quantify S budgets in major agricultural areas where it is used heavily and adjacent downstream areas that can be impacted by these inputs. In addition, science-stakeholder partnerships must develop approaches for sustainable S management that reduce negative effects on local and adjacent ecosystems.