Changes in labile C supply from root exudates are known to stimulate microbial activity and nutrient transformations in soil, but the consequences of such changes have rarely been evaluated in situ. We developed a novel method for adding model exudate solutions to forest soils at ecologically-relevant rates via a sustained, low-volume drip from an artificial root (i.e. rhizosphere simulator). Based on a preliminary research at the Duke FACE site, we hypothesized that C supply rate would mediate rhizosphere microbial activity, and that the magnitude of such effects would be greater in low fertility soils. Exudate solutions (five simple sugars, five amino acids and five low-molecular weight organic acids) varying in concentration were added to fertilized and unfertilized soils in a loblolly pine forest in North Carolina. After eight weeks of addition, rhizosphere (< 1 cm from the simulator) and bulk soil were harvested and analyzed for active microbial biomass, net N mineralization potential and enzyme activity. 

Results/Conclusions

In non-fertilized soils, greater C supply increased microbial biomass by 59% and 13% in rhizosphere and bulk soils, respectively. In fertilized soils, microbial biomass was unaffected by C supply addition rate, and was 91% lower than in unfertilized soils. In unfertilized soils, rhizosphere N availability was mediated by C supply with net N immobilization at low C and net N mineralization at high C supply. These results suggest that microbial activity and rhizosphere N availability are tightly coupled to C supply from roots, and that factors which influence the rates of such inputs (e.g. elevated CO2, N deposition, herbivory, etc.) may result in feedbacks to ecosystem productivity.