Mycorrhiza: Saprotroph interactions and carbon cycling in the rhizosphere

Labile carbon (C) inputs in soils are expected to increase in the future
due to global change drivers such as elevated atmospheric
CO2 concentrations or warming and potential increases in plant
primary productivity. However, the role of mycorrhizal association in
modulating microbial activity and soil organic matter (SOM)
biogeochemistry responses to increasing below-ground C inputs remains
unclear. We employed 18O–H2O quantitative stable isotope probing to
investigate the effects of synthetic root exudate addition (0, 250, 500,
and 1000 µg C g soil-1) on bacterial growth traits and SOM biogeochemistry
in rhizosphere soils of trees associated with arbuscular mycorrhizal (AM)
and ectomycorrhizal (ECM) fungi. Soil respiration increased proportionally
to the amount of exudate addition in both AM and ECM soils. However,
microbial biomass C (MBC) responses differed, increasing in AM and
decreasing in ECM soils. In AM soils, exudate addition increased
taxon-specific and community-wide relative growth rates of bacteria
leading to enhanced biomass production. Conversely, in ECM soils, relative
growth rates were less responsive to exudate addition, and estimates of
MBC mortality increased with increasing exudate addition. In the AM soils,
aggregated bacterial growth traits were predictive of soil respiration but
this relationship was not observed in ECM soils, perhaps due to
substantial MBC mortality. These findings highlight the distinct responses
of bacterial communities in AM and ECM rhizosphere soils to exudate
addition. Considering that microbial products contribute to the formation
of stable soil organic carbon (SOC) pools, future increases in labile
exudate release in response to global change may consequently lead to
greater SOC gains in AM soils compared ECM soils.