Data from: Interactions among roots, mycorrhizae and free-living microbial communities differentially impact soil carbon processes

Plant roots, their associated microbial community and free-living soil
microbes interact to regulate the movement of carbon from the soil to the
atmosphere, one of the most important and least understood fluxes of
terrestrial carbon. Our inadequate understanding of how plant–microbial
interactions alter soil carbon decomposition may lead to poor model
predictions of terrestrial carbon feedbacks to the atmosphere. Roots,
mycorrhizal fungi and free-living soil microbes can alter soil carbon
decomposition through exudation of carbon into soil. Exudates of simple
carbon compounds can increase microbial activity because microbes are
typically carbon limited. When both roots and mycorrhizal fungi are
present in the soil, they may additively increase carbon decomposition.
However, when mycorrhizas are isolated from roots, they may limit soil
carbon decomposition by competing with free-living decomposers for
resources. We manipulated the access of roots and mycorrhizal fungi to
soil in situ in a temperate mixed deciduous forest. We added 13C-labelled
substrate to trace metabolized carbon in respiration and measured
carbon-degrading microbial extracellular enzyme activity and soil carbon
pools. We used our data in a mechanistic soil carbon decomposition model
to simulate and compare the effects of root and mycorrhizal fungal
presence on soil carbon dynamics over longer time periods. Contrary to
what we predicted, root and mycorrhizal biomass did not interact to
additively increase microbial activity and soil carbon degradation. The
metabolism of 13C-labelled starch was highest when root biomass was high
and mycorrhizal biomass was low. These results suggest that mycorrhizas
may negatively interact with the free-living microbial community to
influence soil carbon dynamics, a hypothesis supported by our enzyme
results. Our steady-state model simulations suggested that root presence
increased mineral-associated and particulate organic carbon pools, while
mycorrhizal fungal presence had a greater influence on particulate than
mineral-associated organic carbon pools. Synthesis. Our results suggest
that the activity of enzymes involved in organic matter decomposition was
contingent upon root–mycorrhizal–microbial interactions. Using our
experimental data in a decomposition simulation model, we show that
root–mycorrhizal–microbial interactions may have longer-term legacy
effects on soil carbon sequestration. Overall, our study suggests that
roots stimulate microbial activity in the short term, but contribute to
soil carbon storage over longer periods of time.