Toward approaches for disentangling root from microbial contributions to total soil respiration measurements using molecular genetic approaches

Climate change impacts on ecosystems have received a great deal of attention; but most research has targeted macro indicators or chemical-physical parameters. Few studies have addressed the role of microbial soil communities responses to multiple climatic stressors and how shifts in these communities may impact ecosystem functions such as soil respiration.  To better understand climate effects, DNA and RNA were extracted from soils at the climatic multifactor site Old-field Community Climate and Atmospheric Manipulation (OCCAM) at ORNL, where CO₂ (+300 ppm) and warming (+3.5 ⁰C) are manipulated in a constructed ecosystem with seven plants typical of an old-field systems. Cloned libraries of the PCR-amplified 16S rRNA and citrate synthase genes were constructed; sequenced and phylogenetic analyses were conducted.  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. The dominant Proteobacteria phyla increased in the ET treatments at ambient CO₂ (AC) but did not respond to temperature in the presence of EC. Within the Proteobacteria phyla, the subdivision Deltaproteobacteria exhibited a large response to all the treatments with one or more of the factors elevated.  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.  Currently we are analyzing RNA based data to try to understand how activity, in addition to abundance, may influence overall system response such as soil respiration to climate change drivers.