Industrialization has greatly affected the biogeochemistry of northern forests by increasing the atmospheric deposition of acid and nitrogen (N). In 1990, the US Congress amended the Clean Air Act to include tighter emissions regulations; this reduced acid deposition (by >50% in this study), but did not effectively lower N deposition. Here, we demonstrate that since this legislation was enacted, there have been marked decreases in sulfur (-16%), calcium (-17%), and aluminum (-42%) concentrations in sugar maple (Acer saccharum) foliage across the Upper Great Lakes region of the US, signaling a declining influence of acid deposition. In contrast, N deposition has persistently been over 75% greater than the amount of N needed to offset annual plant N sequestration, creating increases in N availability and soil N leaching. Recent emissions regulations will reduce N deposition somewhat, but further increases in soil N availability and leaching are likely. Policy decisions regarding N deposition will have to weigh increased carbon storage against negative impacts on water quality and species diversity.