Historical changes in atmospheric nitrogen deposition to Cape Cod, Massachusetts, USA

We reconstructed the historical trends in atmospheric deposition of nitrogen to Cape Cod, Massachusetts, from 1910 to 1995 by compiling data from literature sources, and adjusting the data for geographical and methodological differences. The reconstructed data suggest that NO3-N wet deposition to this region increased from a low of 0.9 kg N ha−1 yr−1 in 1925 to a high of approximately 4 kg N ha−1 yr−1 around 1980. The trend in NO3-N deposition has remained since the early 1980s at around 3.6 kg N ha−1 yr−1. In contrast, NH4-N wet deposition decreased from more than 4 kg N ha−1 yr−1 in the mid 1920s to about 1.5 kg N ha−1 yr−1 from the late-1940s until today. Emissions of NOx-N in the Cape Cod airshed increased at a rate of 2.1 kg N ha−1 per decade since 1910, a rate that is an order of magnitude higher than NO3-N deposition. Estimates of NH3 emissions to the northeast United States and Canada have decreased slightly throughout the century, but the decrease in reconstructed N-NH4+ deposition rates does not parallel emissions estimates. The trend in reconstructed total nitrogen deposition suggests an overall increase through the century at a rate of 0.26 kg N ha−1 per decade. This overall increase in deposition may expose coastal forests to rates of nitrogen addition that, if exceeded, could induce nitrogen saturation and increase nitrogen loads to adjoining estuaries.