Short-term versus long-term effects of nitrogen addition and warming on ecosystem N dynamics in a grass-dominated temperate old field

Benjamin F. A. Souriol, Western University

Abstract

Increased atmospheric nitrogen (N) deposition and climate warming are both anticipated to influence the ecosystem N dynamics of northern temperate ecosystems substantially over the next century. Nevertheless, in field experiments with N addition and warming treatments, temporal scale can play an important role in determining the extent of treatment effects on N dynamics, and it is unclear to what extent the results of short-term studies can be extrapolated to responses over longer time scales. I compared the short-term versus long-term effects of N addition and warming on net N mineralization, N leaching, and N retention in a grass-dominated old field. I examined new N addition and warming plots (3 years old) added to an existing field experiment (16 years old) to compare the treatment effects on the N responses over the two time scales while controlling for variation in background environmental conditions. I used in situ soil cores to quantify net N mineralization and leaching, and ¹⁵N tracer to estimate ecosystem N retention. Nitrogen addition significantly increased total N mineralization over the growing season and there was a significant interaction between plot age and N addition over this period. Similarly, N leaching significantly increased under N addition over winter. However, there were no significant interactions between either of the treatments and plot age for ¹⁵N retention. The latter result might be attributed to the lack of plant community composition shifts over the 16 years of the field experiment in response to the treatments, and it also indicates the new N addition plots may have already reached the point of N saturation.