Rhizosphere Microorganisms as a Factor in Temperature Sensitivity of SOC Decomposition.

Elevated temperature, caused by elevated atmospheric CO₂, could cause an important positive feedback to global warming by increasing soil organic carbon (SOC) decomposition and an associated release of CO₂ to the atmosphere. There are many potentially important factors involved in the temperature sensitivity of SOC decomposition such as plant species, rhizosphere processes, soil type, and especially the role of soil microbial communities. Soil microbial communities are the essential link between ecosystem function, plants, and soil, yet we lack a detailed understanding of microbial response to elevated temperature. In this study we simultaneously measured many aspects of the microbial community (biomass, composition, turnover, and activity) in a unique ¹³CO₂ continuous labeling facility to elucidate the relationships between elevated temperature, plants, microorganisms, and SOC decomposition. Our treatments included elevated temperature (unheated or +5 °C using soil heating cables), two soil types (organic farm soil versus grassland soil), and three planting treatments (soybean, sunflower, and an unplanted control). After six and ten weeks of heating and plant growth, we destructively harvested pots to examine many aspects of microbial community dynamics including: activity (soil respiration, rhizosphere respiration, nitrogen mineralization and extra-cellular enzyme activity), community composition (¹³C lipid analysis), biomass (lipid biomass and chloroform fumigation), and turnover rates (using ¹⁵N pool dilution). A major finding of our study, from preliminary data, was that microbial community dynamics and treatment effects varied greatly between the two soil types.  Microbial biomass was higher, and enzyme activity lower, in grassland soil than organic farm soil. The effect of heating also appeared to be more significant in grassland soil than organic farm soil. In conclusion, we suggest that soil characteristics, and the associated microbial community, are important factors in the temperature sensitivity of SOC decomposition.