Despite the major role microbial communities play in ecosystem functions such as soil respiration or nitrogen transformations, most climate research has targeted macro indicators or chemical-physical parameters. In addition, few studies have addressed the role of microbial soil communities and their response to multiple climatic stressors. To better understand climate effects on soil we used rRNA cloning libraries to examine microbial communities in a multifactor climate field experiment. DNA was extracted from soils at the Old-field Community Climate and Atmospheric Manipulation (OCCAM) at ORNL, where CO2 (+300 ppm), warming (+3.5 °C), and moisture are manipulated in a constructed ecosystem with seven plants typical of old-field systems. Cloned libraries of the PCR-amplified 16S rRNA genes were constructed; sequenced and phylogenetic analyses were conducted. 
Results/Conclusions

The bacterial 16S rRNA cloned libraries revealed great diversity, with the majority of sequences related to the Proteobacteria phylum (ca. 50%), followed by Firmicutes, Bacteriodetes, Actinobacteria, Acidobacteria and Planctomycetes. The patterns of response in abundance for the different phylogenetic phylum were highly complex. Acidobacteria and Actinobacteria exhibited a positive response to elevated temperature (ET). Firmicutes responded positively to elevated CO2 (EC) only in the ambient temperature (AT) treatments, while Verrucomicrobia exhibited a negative response to EC. Acidobacteria abundance increased in low soil moisture while Proteobacteria abundance decreased. The dominant phyla, Proteobacteria increased in the ET treatments at ambient CO2 (AC) but did not respond to temperature in the presence of EC. These results indicate that the different climate drivers and their interactions with one another select for distinct microbial populations, however the microbial response to these climatic drivers is very complex.