Global Change Effects on Alpine Microbial Communities and Nitrogen Cycling

Anthropogenic pollution is predicted to cause significant environmental change throughout the world’s ecosystems. Alpine ecosystems, which historically have been isolated from direct human impact, are particularly susceptible to atmospheric changes. Increasing emissions of greenhouse gases and nitrogenous compounds into the atmosphere may have serious consequences for alpine communities. Here we examine the effects of elevated temperature, elevated snowpack, and elevated soil nitrogen on a moist meadow alpine tundra ecosystem. This study compares the effects of individual and combined factorial treatments of these three factors on soil nitrogen accumulation and microbial communities, as well as community composition and productivity. Research was conducted during summer 2010 on 48 plots that were established in 2006 on Niwot Ridge, Colorado, USA, in the Front Range of the Rocky Mountains. Microbial biomass nitrogen was measured to determine microbial response to the treatments. Resin beads were used to measure rates of nitrogen accumulation in each plot throughout the growing season. Plant community composition within each plot was monitored for four consecutive years of treatment. Plant above-ground biomass was collected from subplots within each treatment, and dry mass was obtained.

Under conditions of elevated temperature, microbial biomass N decreased relative to the control, possibly due to drying of the soil. However, the combination of elevated temperature with elevated N or snowpack yielded no significant changes from the control in microbial biomass. As predicted, nitrogen accumulation rates increased with N fertilization, but not under any other treatment. Shifts in plant community composition were observed, with some graminoid species increasing in relative abundance under elevated N but decreasing with elevated temperature. Above-ground plant productivity increased in response to elevated N, but decreased in response to elevated snowpack. No interactive effects of the combination of global change factors were observed, suggesting that these factors may offset one another. Furthermore, our results suggest that single factor global change manipulations may yield valid predictions of shifts in community structure and ecosystem function. We conclude that individual global change factors contribute to change in the functionality and species composition of alpine tundra moist-meadow ecosystems, but that interactive effects may suppress these responses. Further study of ecosystem response to single and interactive global change treatments is necessary.