Up to 40% of photosynthetically fixed carbon (C) enters the soil via plant roots as labile root exudates. Root exudates control the microbial community structure and its activity and as a result, could regulate the turnover of more recalcitrant plant detritus. Yet, the extent by which root exudates mediate the microbial community structure, its activity and ultimately decomposition rates of more recalcitrant plant material is uncertain. With this study we aimed to 1) determine whether turnover of plant material is regulated by root exudation rates, and 2) identify how changes in root exudation rates impact the soil microbial community structure. In a 14-day laboratory incubation study, we added a synthetic exudate cocktail at 5 different rates of C release (0, 0.72, 1.4, 3.6, 7.2 and 14.4 mg C g^-1 dry soil) to soils amended with ¹³C labeled plant material. We also included soils that received the exudates, but no plant material.

Results/Conclusions Each increase in exudate application rates to soil significantly enhanced total microbial C respiration (i.e. C derived from soil + exudates + roots). After partitioning between soil- and plant-derived C respiration, we found that addition of the low amounts of exudates (i.e. 0.72 mg, 1.4 mg, 3.6 mg C g^-1 soil) significantly stimulated decomposition of plant residue by 29%, 13% and 10% respectively, whereas adding exudates at rates exceeding 7.2 mg C g^-1 significantly reduced decomposition rates of the plant material by 30% to 50%. These changes were accompanied by a shift in the microbial community structure as determined by Q-PCR. Exudate concentrations in excess of 3.6 mg C g^-1 significantly promoted r-strategist fungal and bacterial groups, leading to preferential substrate utilization and a negative priming effect. In contrast, exudate additions at concentrations smaller than 3.6 mg C g^-1 soil day^-1 did not significantly enhance bacterial or fungal biomass, but triggered metabolic activity of the microbial community resulting in a positive priming effect. These results highlight the importance of root exudation for controlling decomposition rates of plant detritus and determining the structure of microbial communities. Efforts to quantify how labile root derived C inputs affect overall decomposition rates for better estimates of soil C cycling are warranted.