The Role of Phosphorus Availability in the Response of Soil Nitrogen Cycling,Understory Vegetation and Arbuscular Mycorrhizal Inoculum Potential to Elevated Nitrogen Inputs

The impacts of increased nitrogen (N) inputs into temperate ecosystems via atmospheric nitrogen deposition on nitrogen cycling and nitrogen retention have been described in a variety of ecosystem types. The role of secondary nutrients such as phosphorus (P) in ecosystem responses to increased N inputs is less well-understood. N and P availability are likely to interact to influence ecosystem productivity and N cycling rates, and this interaction would be expected to vary as N inputs increase. Furthermore, N and P inputs may affect plant-mycorrhizal associations and the ability of arbuscular mycorrhizae (AM) to colonize roots. We added nitrogen (97 kg ha^-1 yr^-1) and phosphorus (30 kg ha^-1 yr^-1) to an oak-maple forest in southwestern Virginia (U.S.A.) from 1994 through 1996. Inorganic nitrogen concentrations, net nitrogen mineralization, net nitrification rates and arbuscular mycorrhizal inoculum potential (MIP) were assessed during the growing season in 1996. Responses of the understory vegetation and soil N cycling to N addition suggested that the ecosystem was strongly N-limited. Nitrogen cycling rates were not affected by P inputs, though P addition increased P availability and decreased MIP. It was hypothesized that P availability may have more significant influences on N cycling and the plant-mycorrhizal association in ecosystems showing stronger symptoms of nitrogen saturation. Results suggest that the use of P fertilization would be effective in alleviating P-deficiency in vegetation receiving elevated atmospheric N deposition, but perhaps at the cost of benefits that associations with arbuscular mycorrhizae provide.