The effects of experimental nitrogen and phosphorus addition on a temperate deciduous forest ecosystem

As human activities continue to increase nitrogen inputs into temperate ecosystems via atmospheric nitrogen deposition, understanding the impact of increased nitrogen levels on plant community composition and ecosystem functioning is of vital importance. I added nitrogen and phosphorus to a forest near Mountain Lake Biological Station (VA) from 1994 through 1996. I then tested whether nitrogen addition reduced the species diversity of the community or altered the rates of nutrient cycling and nitrogen uptake in the ecosystem. I also tested whether phosphorus availability influenced the community's or the ecosystem's response to elevated nitrogen inputs.; Nitrogen addition increased the abundance and biomass of the dominant understory species, Aster acuminatus, and decreased the species diversity of the plant community. However, contrary to expectations based on observations following N addition in other ecosystems, the number of species in the community was not affected by nitrogen inputs. Phosphorus addition did not affect the response of species diversity or the species richness of the community to nitrogen addition.; Nitrogen inputs did not saturate the capacity of the Mountain Lake ecosystem to retain nitrogen. Soil N and, to a lesser extent, canopy vegetation were the ecosystem components that appeared most responsible for N retention. Nitrogen content of microbial populations did not increase. Phosphorus inputs did not affect the ecosystem's ability to retain nitrogen, as neither nitrogen cycling nor nitrogen contained in ecosystem pools were significantly different in plots receiving phosphorus addition.; Dramatic changes in composition and diversity of European plant communities indicate that atmospheric nitrogen deposition may impact North American communities in the future. Regions in North America receiving high rates of atmospheric nitrogen deposition, such as high-elevation forests in the northeastern United States, are likely to face substantial changes in community composition and ecosystem function. Grasslands, prairies, and savannas existing in areas of moderate-high levels of nitrogen deposition are also especially vulnerable to future community changes because grass species have been especially able to increase dominance following increased N inputs in other regions. Greater monitoring of community composition and diversity in areas of high-nitrogen inputs will be necessary to track future community changes.