Carbon allocation and N acquisition by plants following defoliation may be linked through plant-microbe interactions in the rhizosphere.  Feedbacks between herbivory and plant-microbe interactions may also be affected by increasing atmospheric CO₂, through plant responses to changes in carbon and nitrogen availability.  We studied two widespread perennial grasses native to rangelands of western North America to examine the hypotheses that (1) defoliation-induced enhancement of rhizodeposition would stimulate microbial activity and N availability in the rhizosphere, contributing to enhanced N uptake and regrowth following defoliation, and (2) defoliation-induced enhancement of rhizodeposition would increase under elevated CO₂, thus allowing grazed grasses to overcome CO₂-induced reductions in soil N availability.  We grew both species in greenhouses under ambient and elevated CO2, and used 15N and 13C pulse labelling to examine carbon allocation, changes in microbial N content, and N uptake by the plants in response to defoliation. 

Results/Conclusions Contrary to our hypotheses, we found that defoliation reduced carbon inputs to the rhizosphere by both  Bouteloua gracilis and Pascopyrum smithii.  However, both species also exhibited increased N allocation to shoots of defoliated versus non-defoliated plants 8 days after treatment.  The magnitude of this response was greatest for P. smithii, and was associated with negative defoliation effects on root biomass and N content and no change in allocation of post-defoliation assimilate to roots versus shoots.  In contrast, B. gracilis increased the relative allocation of post-defoliation assimilate to roots versus shoots, and did not exhibit defoliation-induced reductions in root biomass or N content.  Our findings highlight key differences between these species in their response to grazing, and indicate that defoliation-induced enhancement of shoot N concentration and shoot N yield is not necessarily mediated by rhizosphere processes.