Seasonal, biogeochemical, and microbial response of soils to simultaneous warming and nitrogen additions

Climate warming and nitrogen deposition are global environmental threats that could alter soil microbial communities and the biogeochemical processes they perform. Few studies have examined interactive effects of elevated temperatures and nitrogen inputs. Many studies have also not considered the role that season plays in mediating the response of soils to warming and nitrogen. Finally, most research has not linked changes in the soil microbial community with ecosystem-scale dynamics. One objective of this dissertation was to examine season-specific microbial and biogeochemical responses to simultaneous warming and nitrogen additions. Another aim was to investigate whether warming and nitrogen can restructure microbial communities in such a way as to alter ecosystem processes. The research occurred at the Soil Warming x Nitrogen Addition study at the Harvard Forest, and included four treatments: control, warming, warming x nitrogen, and nitrogen additions. Soil respiration and nitrogen mineralization were measured continuously for two years. During winter, spring, summer, and fall of a single year, labile carbon, enzyme activity, microbial biomass, and microbial community structure were quantified. Finally, a wood decomposition study was conducted at the field site to examine changes in both wood decay and the fungi performing the decay. Results indicated season-specific responses of soil respiration, nitrogen mineralization, and microbial biomass to the experimental manipulations. Soil respiration and nitrogen mineralization increased with warming and nitrogen additions, even during winter. Soil respiration in the warming treatment also displayed heightened temperature sensitivity during winter months. By contrast, microbial biomass declined with warming and nitrogen and this decline primarily occurred in autumn. Where warming x nitrogen occurred together, warming appeared to moderate the negative effect of nitrogen additions on soil respiration and microbial biomass. Regarding the decomposition experiment, nitrogen additions suppressed wood decay while warming had no effect. The combination of warming x nitrogen was synergistic, accelerating wood decay beyond either treatment on its own. Lower decay rates in fertilized plots were not associated with a concomitant change in the structure of the fungal community colonizing the wood. Overall, the findings suggest that anthropogenic stressors and seasonal changes can interact to affect soil microbial communities and biogeochemical cycles