Increasing atmospheric CO₂ has been shown to significantly affect terrestrial ecosystems through increased primary production. This response is thought to be mitigated by changes to the soil microbial community, which can alter nutrient availability in these systems. In this study we examine the effects of a unique field gradient of atmospheric CO₂, ranging form 250 ppm to 500 ppm, on soil microbial communities and function in a model grassland system. Within this experimental set-up there are three distinct soils from the following three series: two silty-clay soils, Austin (mollisol) and Houston (vertisol), and a sandy loam, Bastrop (alfisol). Each of these plots was planted with identical grassland communities. To assess how microbial function and communities differed in response to changing atmospheric CO₂, we measured extracellular enzyme activity of several enzymes associated with biogeochemical function, as well as fungal-to-bacterial ratios with qPCR, in 2 soil types at several points along the CO₂ gradient. These measurements in combination with studies of soil respiration and organic matter decomposition provide insight into how this ecosystem responds to changing CO₂ concentrations.
Results/Conclusions

A variety of processes within our model ecosystem respond to the CO₂ gradient. Soil respiration tends to increase with CO₂ concentration, although the response varies by soil type. To understand the role the microbes have in this response, we found that in some soils, the microbial community shifted towards a higher fungi:bacteria ratio. Microbially-produced enzymes also responded to increase CO₂ concentration, with decreased cellulase and glucosidase activity, and decreases increased chitinase activity, indicating a shift in the function of the microbial community response to changing CO₂ concentrations. These data demonstrate that changes in CO₂ concentrations affect the entire ecosystem, including microbial nutrient cycling.