The influence of 3-years of warming and N-deposition on ecosystem dynamics is small compared to past land use in subalpine meadows

Abstract

Drivers of ecosystem dynamics that are under human influence range from local, land-management decisions to global processes such as warming temperatures and N deposition. The goal of this study was to understand how multiple, potentially interacting factors influence net primary production, N mineralization, and water and soil CO2 fluxes. Here I report on a three-year experiment that manipulated air temperature using ITEX passive warming cones and N deposition in a mountain meadow ecosystems that were historically grazed or protected from grazing. The strongest and most consistent effect was due to the legacy of grazing, with previously grazed sites having lower primary production, lower soil respiration rates, lower soil moisture, and lower soil C and N stocks than historically ungrazed sites. Warming increased soil respiration, but the effect was transient, and decreased over the 3-year study. Nitrogen addition increased primary production in the second and third year of the experiment but had no significant effect on soil respiration. The effect of historical grazing on primary production was approximately double the effect of N addition. Temperature and N deposition rarely interacted except for increasing N availability during the warm, wet growing season of 2004. These findings indicate that the legacies of land use, with their influence on plant community composition and hydrologic processes, are locally more important than short-term step changes in temperature and nutrient availability.