Genotypic variation in traits controlling carbon uptake responses to precipitation in switchgrass

Background/Question/Methods

Fluxes of carbon in terrestrial ecosystems are key indicators of their productivity and carbon storage potential. Ecosystem fluxes will be impacted by climate change, especially changes in rainfall amount. Fluxes may also be affected by plant traits, including aboveground biomass (AGB), leaf photosynthesis (A_CO2), leaf area index (LAI), leaf nitrogen (N) and chlorophyll (Chl) contents. Plant traits differ among genotypes adapted to different climate regimes, hence ecosystem fluxes may differ among genotypes.  Here we demonstrate genotypic variation in trait-based control of net ecosystem exchange (NEE) in the native C₄ tallgrass species Panicum virgatum L. (switchgrass), a widespread, dominant component of tallgrass prairie, and a potential bioenergy crop. Nine genotypes of P. virgatumoriginating from 27 to 35° N latitude were established under a rainfall exclusion shelter in central Texas, USA. The genotypes received rainfall treatments representing dry, average and wet years in a randomized complete blocks design. NEE [and its components, gross primary production (GPP), ecosystem respiration (Re)], plant traits, and normalized difference vegetation index (NDVI) were measured during rapid tiller growth (June) and near peak growth (August), and AGB was measured at the end of the growing season.

Results/Conclusions

NEE increased 22-83% with increasing rainfall (0.003<p<0.08) and varied 80-300% among genotypes (0.004<p<0.0001), because of strong responses in both GPP and Re. Genotypes varied up to 5-fold in NEE, GPP, and Re at high rainfall, compared to ~ 2-fold at low rainfall, indicating that genotypic differences in ecosystem carbon fluxes were magnified at high rainfall (0.04 < p < 0.08). NEE, GPP, and Re were strongly correlated with AGB, A_CO2, and LAI (0.0001<p<0.04). Significant AGB x genotype, A_CO2 x genotype, and NDVI x genotype interactions (0.001<p<0.04) indicated that AGB, A_CO2 , and NDVI relationships with fluxes differed among the genotypes. Leaf N and Chl contents and NDVI were mostly unrelated to ecosystem fluxes and did not interact with genotype or treatment. These results indicate that P. virgatum genotypes varied in the control of ecosystem fluxes by plant traits related to biomass and photosynthetic carbon uptake. These results extend previous research by demonstrating genotypic variation in traits controlling ecosystem carbon fluxes in a widespread dominant native grassland species which is responsive to precipitation amount and may become more prevalent in bioenergy cropping systems.