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.