The objective of this study was to investigate the interactive effects of elevated atmospheric carbon dioxide concentrations [CO₂], increased temperature and altered precipitation regimes on soil microbial community composition in an old-field ecosystem.  Differential effects of [CO₂], temperature and precipitation on bacteria and fungi will likely alter competition dynamics and relative abundances of bacteria and fungi, affecting the decomposition and nutrient cycling functions of these communities.  While the direct effects of these factors on ecosystem processes have been well-studied individually, there is little information on the ecological implications of these climate change factors applied simultaneously, based on future climate predictions.    
Single and interactive effects of climate change factors on the soil microbial community were examined in constructed old-field ecosystems within open-top chambers. In each of three blocks, four chambers are exposed to ambient or elevated [CO₂], ambient or elevated temperature, and two levels of soil moisture, maintained as split plots within each chamber.  We tested the response of the microbial community using phospholipid fatty acid (PLFA) analysis.  Total PLFA was used as a proxy for live microbial biomass, and specific PLFA biomarkers were used to determine the relative contributions of gram negative, gram positive and actino bacteria, as well as arbuscular mycorrhizal and saprophytic fungi.

Results/Conclusions

Water availability and [CO₂] had significant interactive effects on the composition of the microbial community.  Under ambient [CO₂], wet treatments had decreased total PLFA, decreased actino bacteria, and increased fungi and arbuscular mycorrhizal fungi compared to dry treatments; however there were no significant differences between dry and wet treatments under elevated [CO₂].  The differential responses to watering treatments under elevated and ambient [CO₂] may represent altered competition dynamics due to increased carbon inputs under elevated [CO₂]. 
Water availability and temperature also had significant interactive effects on microbial community composition.  Under ambient temperature, wet plots had lower total PLFA, higher fungi and a higher fungi: bacteria ratio than dry plots.  Under elevated temperature, however, there were no significant differences between wet and dry treatments. 
Elevated temperature directly affected microbial community composition by decreasing the fungi: bacteria ratio.  The dry treatment directly affected microbial community composition by increasing gram positive bacteria, decreasing gram negative bacteria and decreasing arbuscular mycorrhizal fungi. 
These data indicate that elevated [CO₂], increased temperature, and altered precipitation regimes, singly and in combination, will alter microbial community composition which may affect pathways and rates of soil organic matter decomposition and carbon cycling.