Root decomposition and their mediation of soil C stability under warming and elevated CO2

Background/Question/Methods

Atmospheric changes are expected to alter how roots grow, how they decompose, and how they influence the dynamics of existing soil organic matter. To understand the role of roots in an ecosystem’s carbon cycle a multi-faceted approach is needed. We explored single and combined impacts of elevated CO₂ and warming on root C and N dynamics in a temperate, semiarid, native grassland at the Prairie Heating and CO₂ Enrichment experiment (PHACE). We assessed root standing mass, morphology, residence time, seasonal appearance/disappearance and mass loss of community-aggregated roots, as well as mass and N losses during decomposition of two dominant grass species (a C₃ and a C₄). We also evaluated whether living roots or dead roots could prime the decomposition of soil organic matter.  

Results/Conclusions

Greater root standing mass under elevated CO₂ resulted from increased production, unmatched by disappearance. Elevated CO₂ plus warming produced roots that were longer, thinner and had greater surface area, which together with greater standing biomass could potentially alter root function and dynamics. Decomposition increased under environmental conditions generated by elevated CO₂, but not those generated by warming, likely due to soil desiccation with warming. Elevated CO₂, particularly under warming, slowed N release from C₄ –but not C₃-  roots, and consequently could indirectly affect N availability through treatment effects on species composition. Dead roots enhanced soil organic matter decomposition and this effect was enhanced in elevated CO₂ plots.  Living roots also enhanced the decomposition of existing organic matter but this effect was ameliorated by elevated CO₂. We integrate these findings into a future ecosystem scenario.