Location: Rangeland Resources ResearchTitle: Plant rhizosphere influence on microbial C metabolism: the role of elevated CO2, N availability and root stoichiometry) Author
Submitted to: Biogeochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/5/2014
Publication Date: 2/1/2014
Citation: Carrillo, Y., Dijkstra, F.A., Pendall, E., Lecain, D.R., Colin Tucker 2014. Plant rhizosphere influence on microbial C metabolism: the role of elevated CO2, N availability and root stoichiometry. Biogeochemistry. doi 10.1007/510533-014-9954-5. Interpretive Summary: We took advantage of continuous, homogeneous 13C labeling of plant C under growth chamber conditions to distinguish C sources so that SOM derived-C could be quantified in all pools and fluxes. Our case study offered strong experimental evidence demonstrating that the plant rhizosphere can play a role in regulating the metabolism of SOM-C and further, that elevated CO2 can alter this role. Specifically, by increasing SOM-C assimilation into biomass while reducing specific respiration, elevated CO2 can increase microbial efficiency. Increased efficiency in SOM use was associated with negative rhizosphere SOM priming and we hypothesize that the magnitude and direction of the widely observed phenomenon of rhizosphere priming may result, at least in part, from changes in the metabolic efficiency of microbial populations. Further, our findings suggest that elevated CO2 modified the rhizosphere environment so that N availability was a stronger controlling factor of microbial SOM efficiency than under ambient conditions. Notably, we demonstrated that under elevated CO2 conditions, SOM decomposition is responsive to changes in the stoichiometry of roots and that increases in root tissue N concentration can lead to increased microbial SOM efficiency, likely via the chemistry of rhizodeposition. Thus, plants are able to influence the metabolism of soil C via their chemical traits and for the case of this grass elevated CO2 can alter the degree of this influence. Our results supported that changes in the structure of the microbial community contributed to the observed metabolic shifts with elevated CO2 and increased N availability/concentration.
Technical Abstract: Microbial decomposer C metabolism is considered a factor controlling soil C stability, a key regulator of global climate. The plant rhizosphere is now recognized as a crucial driver of soil C dynamics but specific mechanisms are unclear. Climate change could affect microbial C metabolism via impacts on the plant rhizosphere. Using continuous 13C labelling under controlled conditions to allow us to quantify SOM derived-C all pools and fluxes, we evaluated the microbial metabolism of soil C in the rhizosphere of a C4 native grass exposed to elevated CO2 and under variation in N concentrations in soil and in plant root C:N stoichiometry. Our results demonstrated that this plant can influence soil C metabolism and further, that elevated CO2 conditions can alter this role by increasing microbial C efficiency as indicated by a reduction in mass specific respiration of soil-derived C by soil-derived microbial biomass. Moreover, under elevated CO2 increases in soil N, and notably, root tissue N concentration increased C efficiency, suggesting elevated CO2 shifted the stoichiometric balance so N availability was a more critical factor regulating efficiency than under ambient conditions. That microbial C efficiency was altered by root C:N stoichiometry indicates that plant chemical traits such as root N concentration are able to influence the metabolism of soil C and that elevated CO2 conditions can modulate this role. Increased efficiency in soil C use was associated with negative rhizosphere priming and we hypothesize that the widely observed phenomenon of rhizosphere priming may result, at least in part, from changes in the metabolic efficiency of microbial populations. Changes in the microbial community support they were a contributing factor to the observed metabolic responses. For our case study findings point at lower mass specific respiration (greater efficiency) of the SOM-degrading populations in a high CO2, high N world, potentially leading to greater C storage of microbially assimilated C in soil.