Thursday, August 5, 2010: 1:50 PM
317-318, David L Lawrence Convention Center
Background/Question/Methods 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 Bouteloua gracilis (C4) and Pascopyrum smithii (C3), two widespread perennial grasses native to rangelands of western North America to examine the hypotheses that (1) defoliation-induced enhancement of rhizodeposition would change prokaryotic community composition in the rhizosphere and (2) defoliation-induced enhancement of rhizodeposition would increase under elevated CO2, thus increasing the abundance of N–cycling prokaryotes. To quantify diversity and abundance we used polymerase chain reaction-based methods utilizing universal prokaryotic, ammonia oxidizing bacteria and archaea (AOB and AOA respectively) , and nitrite oxidizing bacteria (NOB) specific primers. Composition was measured using operational taxonomic units (OTU’s) determined by terminal restriction fragment length polymorphism analysis and abundance was based on gene copy number using quantitative PCR.
Results/Conclusions Rhizospheric soil taxonomic composition differed between the two plant species with the C3 grass having average of 46 OTU’s and the C4 grass only 33 of which 22 were shared between both species. Elevated CO2 increased the number of OTU’s by 5 but only with the C3 grass and each represented a unique OTU. Quantitative PCR analysis of the N-cycling genes for ammonia and nitrite oxidation showed that defoliation increased both AOA, AOB, and NOB. In C4 rhizosphere soils there was no effect of elevated CO2 and in defoliated soils, AOA were numerically greater than AOB. In C3 soils elevated CO2 increased AOB and had no effect on AOA while defoliation increased AOA. Defoliation and elevated CO2 increased abundance of nxrB abundance irrespective of species.
Our findings show that rhizospheric prokaryotic populations and communities, especially ammonia oxidizers, are influenced by the plant species with which they interact, even over relatively short time scales. The influence of defoliation and elevated CO2 on soil prokaryotes differed between the rhizospheres of C3 and C4 grass species, suggesting that N-cycling prokaryotes will be influenced by climate change.
Results/Conclusions Rhizospheric soil taxonomic composition differed between the two plant species with the C3 grass having average of 46 OTU’s and the C4 grass only 33 of which 22 were shared between both species. Elevated CO2 increased the number of OTU’s by 5 but only with the C3 grass and each represented a unique OTU. Quantitative PCR analysis of the N-cycling genes for ammonia and nitrite oxidation showed that defoliation increased both AOA, AOB, and NOB. In C4 rhizosphere soils there was no effect of elevated CO2 and in defoliated soils, AOA were numerically greater than AOB. In C3 soils elevated CO2 increased AOB and had no effect on AOA while defoliation increased AOA. Defoliation and elevated CO2 increased abundance of nxrB abundance irrespective of species.
Our findings show that rhizospheric prokaryotic populations and communities, especially ammonia oxidizers, are influenced by the plant species with which they interact, even over relatively short time scales. The influence of defoliation and elevated CO2 on soil prokaryotes differed between the rhizospheres of C3 and C4 grass species, suggesting that N-cycling prokaryotes will be influenced by climate change.