What is the Fate of Belowground Carbon? Grass Crown and Root Traits, and Soil Organic Matter Priming Following Seven Years of Prairie Heating and CO2 Enrichment (PHACE), WY, USA

Grasslands contain more than 10% of the global carbon (C) stock, 98% of which is in the soil. This large C pool may be attributed to a large fraction of NPP allocated belowground to crowns and roots and a high root:shoot ratio. Analyzing climate change effects on grass crowns, roots, and soil organic C (SOC) may aid in understanding and predicting the fate of grassland soil C, which is critical, as grassland soils may shift from C sinks to C sources with climate change. To assess climate change effects on belowground grassland C we analyzed species- and community-level 1) crown and root biomass, 2) root chemistry, morphology, and decomposition, and 3) root-induced priming of SOC following seven years of Prairie Heating and CO2 Enrichment (PHACE) in Wyoming, USA. We sampled plants and soil from five replicate plots of four field treatments; ambient, warmed, elevated CO2, and combined warmed and elevated CO2. We found the sedge, Carex eleocharis, had greater root biomass under elevated CO2 and greater crown biomass under warming with elevated CO2, suggesting it may become a more prominent species in this semi-arid grassland. Furthermore, for Pascopyrum smithii we found longer and thinner roots under elevated CO2 and positive, linear relationships between root length, surface area, and root C, indicating a resource-acquisition strategy under elevated CO2 for this species. Across field treatments, adding P. smithii roots to soil induced short-term, negative SOC priming and reduced cumulative SOC decomposition by 10.7%. This indicates potential, future SOC sequestration; however, applying laboratory results to the field is multifaceted because of other factors, such as fluctuations in precipitation, temperature, and plant and microbial composition, in the field. The unique belowground species- and community-level trait responses to climate change suggest shifts in plant community composition may be important for understanding the fate of C in this semi-arid grassland.