Belowground carbon cycling after six years of prairie heating and CO2 enrichment: Decomposition of C3 and C4 grass roots and soil organic matter

Background/Question/Methods

In grassland ecosystems, roots play a key role in carbon sequestration and root decomposition is a notable, though often not quantified, source of CO₂ to the atmosphere. Within a large field manipulation experiment in a mixed-grass prairie, we sought to 1) understand the impact of elevated CO₂ and temperature on fine root decomposition rates of dominant native grass species with differing photosynthetic pathways and 2) assess the potential for root priming of soil organic matter (SOM) decomposition.  Using two lab-incubation experiments with soils collected from the field manipulation, we quantified the rate of SOM decomposition with and without Bouteloua gracilis (C₄), Pascopyrum smithii (C₃), and community (combination of C₄ and C₃ grass species) fine roots from plants grown under elevated CO₂ only, warming only, elevated CO₂ and warming, and ambient CO₂ and temperature conditions near Cheyenne, WY.  We measured CO₂ efflux over a three-month period and analyzed a subset of gas efflux samples for carbon isotopes to distinguish sources of respired carbon and assess the role of root decomposition in SOM priming.   

Results/Conclusions

Contrary to our expectations, root decomposition rates did not differ between soils from the climate change treatments under laboratory conditions.  Root decomposition rates also did not differ between the grass species.  After correcting for initial differences in soil organic carbon, SOM decomposition rates in the laboratory were not significantly different among the four field treatments.  Because decomposition rates were similar across soil from the climate change treatments and between species, our findings suggest acclimation by the microbial community, changes in grass root morphology in response to climate change treatments, or a combination of both. Stable carbon isotope analyses further enabled us to differentiate the impact of roots on SOM decomposition rates as influenced by climate change treatments.  Our results indicate that despite differences in root biomass reported elsewhere, elevated CO₂ and temperature conditions may not affect their rate of decay. Our findings can be used to better predict the role of grass roots in belowground carbon cycling under future climate conditions.