| 1. 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.
2. 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.
3. 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.
4. 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.
5. 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 longerterm
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.
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.
3. 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.
4. 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.
5. 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 longerterm
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.
WHETHER IN LIFE OR IN DEATH: FRESH PERSPECTIVES ON HOW PLANTS AFFECT
BIOGEOCHEMICAL CYCLING
Interactions among roots, mycorrhizas and free-living
microbial communities differentially impact soil carbon
processes
Jessica A. M. Moore
1
*, Jiang Jiang
1
, Courtney M. Patterson
1
, Melanie A. Mayes
2
,
Gangsheng Wang
2
and Aim
?
ee T. Classen
1,3
1
Ecology & Evolutionary Biology, University of Tennessee, 569 Dabney Hall, 1416 Circle Dr., Knoxville, TN 37996,
USA;
2
Climate Change Science Institute and Environmental Sciences Division, Oak Ridge National Laboratory, Oak
Ridge, TN 37831, USA; and
3
The Natural History Museum of Denmark, University of Copenhagen, Universitetsparken
15, 2100, København Ø, Denmark
Summary
1. 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 under-
stood ?uxes 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.
2. Roots, mycorrhiza l fungi and free-living soil microbes can alter soil carbon decomposition
through exudation of carbon into soil. Exudates of simple carbon compounds can increase mic robial
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-livi ng
decomposers for resources.
3. We manipulated the access of roots and mycorrhizal fungi to soil in situ in a temperate mixed
deciduous forest. We added
13
C-labelled substrate to trace met abolized 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.
4. Contrary to what we predicted, root and mycorrhizal biomass did not interact to additively increase
microbial activity and soil carbon degradation. The metabolism of
13
C-labelled starch was highest
when root biomass was high and mycorrhizal biomass was low. These results suggest that mycorrhiz as
may negatively interact with the free-living microbial community to in?uence 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 in?uence on particulate than mineral-associated organic carbon pools.
5. Synthesis. Our resul ts suggest that the activity of enzymes involved in organic matter decomposition
was contingent up on root–mycorrhizal–microbial interactions. Using our experimental data in a decom-
position 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 micro-
bial activity in the short term, but contribute to soil carbon storage over longer periods of time | |