Interactive effects of global change on soil microbial community composition and function.

Abstract

Anthropogenic activity has altered biogeochemical cycles and reduced plant species richness on a global basis. Atmospheric CO₂ and O₃ enrichment, atmospheric N deposition, and plant species loss alter plant production and litter biochemistry, which could modify heterotrophic soil microbial activity. The objective of my dissertation research was to determine the interactive effects of these global change components on microbial community composition and metabolism. To achieve this goal, I studied changes in composition and function of soil microbial communities in two distinct experiments in which levels of atmospheric CO₂, O₃, N deposition, and plant species richness were manipulated to simulate aforementioned global change. I analyzed the combined impacts of CO₂ and O₃ enrichment on fungal community composition and function in an experiment in which northern hardwood trees were grown under elevated levels of CO₂ and O₃. Elevated CO₂ enhanced fungal metabolism through greater plant litter input as evidenced by higher cellulolytic and chitinolytic activity, and elevated O₃ dampened this effect via reduced substrate input; however, the interactive effects of CO₂ and O₃ were not statistically significant. Repressed fungal metabolism under elevated O₃ was accompanied by a change in fungal community composition. My results suggest that elevated CO₂ will stimulate fungal metabolism and hasten belowground C cycling, and that repressed fungal activity and changes in fungal community composition under elevated O₃ will lead to slower soil C cycling. In a different experiment in which grassland plant communities of increasing species richness (1, 4, 9, and 16 species) were subjected to factorial CO₂ and N deposition treatments, interactions among declining plant species richness, elevated CO₂, and N deposition on soil microbial communities were examined. Interactive effects of plant species richness, elevated CO₂, and N deposition significantly altered microbial community composition, but microbial degradative potential was affected little by the interactions. Plant species richness increased microbial biomass, fungal abundance, cellulolytic potential, and microbial incorporation of new photosynthate, which suggest that greater plant species richness promotes faster rates of microbial metabolism, and that microbial activity may decrease with plant species loss. Nitrogen amendment lowered total microbial biomass, which could be due to the inhibitory effect of N addition on lignin degradation by white-rot basidiomycetes or mycorrhizal infection. Altogether, my results demonstrate that individual effects of global change components more strongly influence microbial community composition and function than their interactive effects.