Air pollution, emissions regulations, and long-term changes in regional forest biogeochemistry

Background/Question/Methods

In the early 1990s, emissions regulations were developed in the U.S. to address widespread concerns about the ecological impacts of two forms of atmospheric pollution: acid deposition (principally H₂SO₄) and nitrogen (N) deposition. These regulations caused sustained reductions in acid deposition and stopped growth in N deposition, with N deposition levels remained relatively constant. However, there are few studies that have assessed the ecological consequences of these emissions regulations. In terrestrial systems, H₂SO₄ deposition increases S availability and mobilizes both Al and Ca from the soil. For plants, high concentrations of Al are toxic and increased mobilization of Al leads to increased uptake. Although Ca is an essential nutrient and increased mobilization can cause greater plant uptake, mobilization ultimately leads to the loss of this essential nutrient from ecosystems. Nitrogen deposition is hypothesized to increase N availability, with potential impacts on forest growth and water quality. Here we use two decades of measurements from forest sites spread across 500 km in the north-central U.S. to show that acid deposition impacts on S, Ca, and Al chemistry have mirrored reductions in emissions, while the persistent levels of N deposition have caused clear increases in the availability of N.

Results/Conclusions

Beginning in the early 1990s, foliar concentrations of both Ca and Al have declined at each of the four study sites. These changes are evidence of declining Ca and Al mobility in the soil. Declines in foliar concentrations of sulfur (S) have also been observed at three of the four study sites. Cumulatively, this is strong evidence that emissions regulations have significantly reduced the burden of S-dominated acid deposition on these forests. Conversely, several metrics of N cycling have shown changes consistent with steady increases in N availability. Foliar N concentrations in have significantly increased at two sites, while foliar d¹⁵N, a metric of N availability, has increased at the three sites with the highest levels of N deposition. Most convincingly, leaching losses of N have increased at all four study sites, with the largest increases at the sites with the largest levels of N deposition. This suggests that although N deposition has not increased, the accumulation of this N has led to increasing impacts on terrestrial N cycling.