Porter, J.H. 1990. Experimental evaluation of nitrogen saturation in a northern hardwood forest. M.S. Thesis, State University of New York, College of Environmental Science and Forestry, Syracuse, 106 pp.

Abstract: The goal of this research was to induce nitrogen saturation in a forest stand, and to evaluate saturation-related changes in biogeochemistry. Of particular concern was the possibility that leaching of nutrient cations would be accelerated under conditions of elevated nitrate loss. Following two years of nitrogen loading at rates up to twelve times ambient deposition, there was a small increase in nitrogen leaching below the root zone, but no increase in nutrient cation leaching. Treatment plots remained a net sink for both ammonium and nitrate. These results suggest that atmospheric deposition of nitrogen will not accelerate the loss of nutrient cations from this northern hardwood forest. However, nitrogen saturation was not achieved (plots remained net sinks for N), hence any conclusions regarding cation loss may be premature. Continued addition of N might eventually cause N saturation, with associated changes in cation leaching rates.

The fact that these plots remained net sinks for nitrogen despite increased in put of N suggests the conclusion that this site was "nitrogen-deficient." This does not necessarily mean forest productivity would increase upon addition of nitrogen. Changes in the C:N ratio of microbes and soil organic matter potentially could account for much of the immobilization of nitrogen input to the system. It might be better to consider this system as having a high "nitrogen retention capacity," as this avoids the implication that productivity is limited by nitrogen.

Although net flux of ions did not change, nitrogen addition at rates 3-12 times the ambient deposition rate did cause some changes in soil solution chemistry. The nature of the changes varied with the rate of N addition. At 15 cm depth under low treatment, Ca2+ concentration was lower than on reference plots and NO3 concentration was unchanged, while under high treatment Ca2+ and NO3 concentrations were significantly and substantially higher than on reference plots. Microbial processes were also affected differentially: low treatment caused substantial inhibition of nitrification but had little impact on ammonification, while high treatment resulted in negative rates for both processes. These results indicate that biogeochemical changes associated with nitrogen loading are not simply threshold dependent, with changes beginning above some critical load and increasing in magnitude with increasing loading. Rather, the amount of N available influences both the nature and the magnitude of biogeochemical processing of N and, consequently, affects the dynamics of other elements in the soil.

As with all experiments, it is important to realize this experiment necessarily involved somewhat unrealistic conditions, hence all conclusions must be considered captiously. The use of a single site, a single treatment (albeit at two levels), and a short time period relative to natural successional time frames preclude making definitive statements regarding the effects of atmospheric deposition of nitrogen on northern hardwood ecosystems.