Nitrogen deposition: The up and down side for production agriculture
Deposition of nitrogen-containing air pollutants contributes as much as 80 kg-N yr-1 to rural lands in proximity to large urban centers. Even in areas distant from pollution sources atmospheric deposition in the US, and much of Europe, is 5 to 10 times higher than natural background levels. Wet deposition in rain, snow, and fog is relatively easy to measure and has known fertility and acidification consequences. Dry deposition, on the other hand, is much more difficult to quantify, but the effects may be significantly more deleterious. In the semi-arid regions of the US, dry deposition accounts for 75% to 95% of the total deposition, and although rainfall is heavier in the southeast part of the country, dry deposition may still account for as much as 50% of the total. Deposition of both ammonia/ammonium and nitrate/nitric acid contributes to increase ecosystem fertility. The effects of increased fertility on natural ecosystems have been studied for over 20 years. By and large, the consequences are negative, with changes in plant community composition, increases in weedy exotic species, and decline in health and sustainability of forests. The consequences for managed ecosystems are understudied. Two areas of concern are: Nitrogen management activities that do not take into account the additions sources from atmospheric deposition and direct foliar damage to perennial crops by dry deposition of nitric acid.