|HECKMAN, ROBERT - University Of Texas|
|KHASANOVA, ALBINA - University Of Texas|
|JOHNSON, NICHOLAS - Kent State University|
|WEBER, SOREN - University Of Texas|
|BONNETTE, JASON - University Of Texas|
|ASPINWALL, MICHAEL - University Of North Florida|
|REICHMANN, LARA - University Of San Francisco|
|JUENGER, THOMAS - University Of Texas|
|HAWKES, CHRISTINE - North Carolina State University|
Submitted to: Journal of Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2020
Publication Date: 9/9/2020
Citation: Heckman, R.W., Khasanova, A.R., Johnson, N.S., Weber, S., Bonnette, J.E., Aspinwall, M.J., Reichmann, L.G., Juenger, T.E., Fay, P.A., Hawkes, C.V. 2020. Plant biomass, not plant economics traits, determines responses of soil CO2 efflux to precipitation in the C4 grass Panicum virgatum. Journal of Ecology. 108:2095-2106. https://doi.org/10.1111/1365-2745.13382.
Interpretive Summary: Understanding controls on the gains and losses of soil carbon is critical to predicting and managing agroecosystem responses to future climate change. Potential bioenergy feedstock crops such as switchgrass, Panicum virgatum are poorly studied in this regard, and also exhibit wide genetic variability in traits that may predict soil carbon loss through soil respiration. We measured rates of soil respiration from genotypes of switchgrass maintained under extremely dry to average to extremely wet watering treatments. Switchgrass genotypes with high leaf nitrogen content increased soil respiration, but these traits changed little with precipitation; genotype biomass increased with precipitation, and larger genotypes had higher soil respiration, indicating that genotype biomass was a key mechanism by which precipitation influenced soil respiration. Switchgrass aboveground biomass and leaf nutrient content thus are key traits that may predict soil carbon losses through soil respiration in switchgrass stands. These results can aid predictions of soil carbon cycling, carbon sequestration, other ecosystem services from switchgrass, and therefore in degree of agroecosystem benefit switchgrass may provide compared to other biomass feedstock crops.
Technical Abstract: Plant responses to major environmental drivers like precipitation can influence soil CO2 efflux (JCO2) and other aspects of carbon (C) cycling. These responses may be predicted by two independent classes of drivers: plant size-related traits alter the quantity of inputs to C cycling—larger plants respire more and produce a larger quantity of labile C, while plant economics traits alter the quality of inputs to C cycling—plants possessing more acquisitive strategies (i.e., fast-growing) produce higher-quality tissue with higher metabolic requirements and faster decomposition. At two sites in central Texas, USA, we examined the response of eight Panicum virgatum genotypes to three precipitation levels, ranging from extremely dry to average to extremely wet. We used confirmatory path analysis to determine how plant economics and plant size traits mediated the effect of precipitation on two aspects of soil carbon cycling—JCO2 and soil organic C (SOC) accumulation. These genotypes generated variation in plant economics and plant size traits, but direct effects of precipitation or trait-mediated indirect effects of precipitation primarily drove changes in JCO2 and SOC content. At each site, plant size traits were stronger predictors of JCO2 than plant economics traits. Moreover, the influence of traits on JCO2 differed between sites. In Austin, on coarse, shallow soils, the effect of precipitation was fully mediated, primarily by plant size traits (i.e., there was a significant indirect effect of precipitation, but no significant direct effect). In Temple, on deeper, finer textured soils, there was a significant direct effect of precipitation in addition to precipitation effects mediated by plant size traits. Plant economics and plant size traits may help explain how precipitation alters ecosystem function. Plant size traits responded to changes in precipitation and influenced JCO2; plant economics traits influenced JCO2 and responded less strongly to precipitation. Thus, increasing JCO2 can result from growing larger (i.e., increasing quantity of inputs) or possessing a more acquisitive resource allocation strategy (i.e., increasing quality of inputs); yet only the former depends on precipitation regime.