Rhizosphere interactions, carbon allocation, and nitrogen acquisition of two perennial North American grasses in response to defoliation and elevated atmospheric CO2

Authors: Augustine, David; Dijkstra, Feike; HAMILTON, E. WILLIAM - Washington And Lee University; Morgan, Jack

Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/2010
Publication Date: 3/10/2011
Citation: Augustine, D.J., Dijkstra, F.A., Hamilton, E., Morgan, J.A. 2011. Rhizosphere interactions, carbon allocation, and nitrogen acquisition of two perennial North American grasses in response to defoliation and elevated atmospheric CO2. Oecologia. 165:755-770.

Interpretive Summary: Carbon allocation and N acquisition by plants following grazing may be linked through interactions between plant roots and organisms found in the soil surrounding the roots. Feedbacks between grazers, plants, and organisms in the soil may also be affected by increasing amounts of CO2 in the atmosphere. We studied two widespread perennial grasses native to rangelands of western North America to examine the idea that (1) after a plant is defoliated, it increases the inputs of carbon to the soil to stimulate the release of nutrients by soil organisms, and (2) this response would increase under elevated CO2, thus allowing grazed grasses to overcome potential declines in soil nutrient availability due to climate change. Contrary to our hypotheses, we found that defoliation reduced carbon inputs to the soil surrounding the roots by both Bouteloua gracilis and Pascopyrum smithii. Both species also exhibited increased the allocation of nutrients to shoots following defoliation. This response was greatest for P. smithii, and was associated with a negative effect of defoliation on roots. In contrast, B. gracilis increased the relative allocation of carbon to roots after defoliation, and did not exhibit a negative response in root biomass. Our findings highlight key differences between these species in their response to grazing, and indicate that defoliation-induced enhancement of shoot protein concentration and yield is not necessarily caused by interactions between plants and soil organisms.

Technical Abstract: 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. 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.