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

Abstract:

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.