Abstract: Nitrous oxide is an atmospheric trace gas that contributes to global warming and the destruction of stratospheric ozone. This gas is produced by both aerobic and anaerobic nitrogen transformations in soil. Forests can be a significant source of this gas and its flux is tightly linked to internal nitrogen cycling processes and to inputs of reactive nitrogen from the atmosphere. We have been monitoring soil:atmosphere nitrous oxide fluxes from forest soils at two long-term ecological research (LTER) sites in the eastern U.S. monthly for more than 20 years. At the Hubbard Brook Experimental Forest in New Hampshire, in situ flux chambers are situated along elevation gradients in two watersheds, one of which has been treated with calcium to replace that lost due to 50 years of acid rain. In the Baltimore urban LTER site, chambers are deployed in urban and rural forest stands, as well as in fertilized urban grasslands (lawns).
Over the past 20 years, fluxes have declined markedly at all sites. At Hubbard Brook, these declines are consistent with an overall pattern of nitrogen oligotrophication, where declines in nitrogen availability have been driven by declines in atmospheric deposition and increases in plant demand for nitrogen driven by increases in atmospheric carbon dioxide levels, a longer growing season, and deacificication. The declines in nitrous oxide have been more marked than other symptoms of nitrogen oligotrophication at the Hubbard Brook site. Atmospheric deposition has also declined in the Baltimore region, but declines in nitrous oxide flux have occurred even in urban forests with high nitrogen availability, and in fertilized lawns. These results suggest that nitrous oxide fluxes may be more tightly tied to atmospheric deposition than to internal transformations of nitrogen. If so, this raises questions about how deposition interacts with the soil ecosystem which is important for understanding and managing the ecological effects of deposition on air and water quality and biodiversity. The results also support that idea that low intensity (monthly) but long-term monitoring can effectively show trends in important environmental fluxes.
