Declining Atmospheric Sulfate Deposition in an Agricultural Watershed in Central Pennsylvania, USA

Sulfur emissions in the northeastern United States are only 20% of levels measured in 1987 due to the enactment of the US federal Clean Air Act. While there are numerous reports of forested ecosystems recovering from acidification as a result of the decline in sulfur deposition, few studies describe such recovery in agricultural watersheds. We used long-term (30+ yr) atmospheric and watershed data from a USDA experimental watershed to investigate whether daily agricultural practices masked the declining sulfur (as sulfate-sulfur) trends seen in mainly forested watersheds. Over the study period, atmospheric wet deposition of sulfate-sulfur decreased 75% while sulfate-sulfur at the watershed decreased by approximately 30%. While the deposition of sulfur is detrimental to stream quality, the reduction of sulfur deposition in recent years has caused many soils in the watershed to develop sulfur deficiencies. Long-term declines in watershed sulfur export reveal emerging concerns about reducing atmospheric sulfur levels.