A shift in sulfur-cycle manipulation from atmospheric emissions to agricultural additions

Burning fossil fuels has resulted in a prominent yet unintended manipulation of the global sulfur cycle. Emissions of sulfur dioxide and reactive sulfur to the atmosphere have caused widespread health and environmental impacts and have led, ultimately, to calls to decrease sulfur emissions. However, anthropogenic modification of the sulfur cycle is far from over. Using four contrasting case studies from across the United States, we show how high levels of sulfur are added to croplands as fertilizers and pesticides and constitute a major yet under-studied environmental perturbation. Long-term sulfur additions to crops probably cause similar consequences for the health of soil and downstream aquatic ecosystems as those observed in regions historically impacted by acid rain, yet the cascade of effects has not been broadly explored. A new wave of research on the sulfur cycle will require studies that examine the integrated roles of climate, hydrology and other element cycles in modifying sulfur processes and flows within and downgradient of agricultural source areas. Such research must include not only scientists, but also farmers, regulating authorities and land managers who are engaged in developing approaches to monitor and mitigate environmental and human health impacts.