Genotype and plant trait effects on soil CO2 efflux responses to altered precipitation in switchgrass

Background/Question/Methods

Global climate change models predict increasing drought during the growing season, which will alter many ecosystem processes including soil CO₂ efflux (J_CO2), with potential consequences for carbon retention in soils. Soil moisture, soil temperature and plant traits such as leaf nitrogen content, leaf photosynthesis (A_CO2), leaf area index (LAI), and above ground biomass (AGB) are typically good predictors of J_CO2, and often differ among plant genotypes adapted to different climates. We examined how changes in amount of annual precipitation affected J_CO2 in switchgrass, Panicum virgatum. We hypothesized that soil carbon flux will differ among genotypes of P. virgatum and that this response will differ depending on annual precipitation amount received. Ten genotypes of P. virgatum from divergent climatic origins were established under a rainout shelter in Temple, Texas, USA. Genotypes received five precipitation scenarios, representing the driest 10 percent to the wettest 10 percent of the historic rainfall record, in a completely randomized block design. J_CO2 was measured monthly in low, mean, and high precipitation treatments.  In June, J_CO2 was measured in all genotypes and treatments, and leaf nitrogen, LAI, A_CO2, and AGB in four representative genotypes.

Results/Conclusions

We found evidence for precipitation, genotype, and plant trait effects on J_CO2 of switchgrass.  J_CO2 increased 1.41-fold from low to high precipitation (P=0.0084), and 1.39-fold among genotypes (P<0.0001), with similar genotype differences in all precipitation treatments (P = 0.25). J_CO2 peaked in June, and peak J_CO2 increased 1.47-fold with precipitation amount (P = 0.02). J_CO2 increased with soil temperature (P<0.0001), but decreased with soil moisture (P<0.0001), and the highest J_CO2 rates occurred during drier summer months, indicating that temperature was the main control on J_CO2.  A significant precipitation treatment x soil temperature interaction (P<0.0001) showed that J_CO2 increased more strongly with soil temperature in the high precipitation treatment. Plant traits explained part of the genotype and precipitation effects on J_CO2.  J_CO2 increased with leaf nitrogen content (P=0.01) and J_CO2 varied in genotype interactions with leaf N and AGB (P=0.01, P=0.05). There were no three way interactions with precipitation treatment, which indicate that genotypic variation in biomass and leaf N affected J_CO2 similarly across precipitation treatments. A_CO2 and LAI were unrelated to J_CO2. Thus plant traits related to nitrogen investment and growth contributed to precipitation and genotype effects on J_CO2 in switchgrass, and may have consequences for soil carbon dynamics.