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