Suillus mycelia under elevated atmospheric CO2 support increased bacterial communities and scarce nifH gene activity in contrast to Hebeloma mycelia

Bacterial communities associated with mycorrhizal roots are likely to respond to rising atmospheric CO2 levels in terms of biomass, community composition and activity since they are supported by the carbon (C) flow outside the root tips, especially by exudation of low molecular weight organic compounds. We studied how general bacterial and diazotrophic communities associated with ectomycorrhizal (ECM) fungi respond to different belowground C supply conditions, mediated by elevated atmospheric CO2 concentration under nitrogen (N) limited conditions. Microcosm systems were constructed using forest soil and Scots pine seedlings, which were either pre-inoculated with one of the ECM fungal species Hebeloma velutipes or Suillus variegatus, or non-inoculated. These fungal species differ in C allocation and exudation patterns. Seedlings were maintained under ambient (380 ppm) or elevated (700 ppm) CO2 levels for 6 months. Quantitative polymerase chain reaction (PCR) showed a significant increase in 16S rRNA gene copy numbers for Suillus-inoculated microcosms under elevated CO2 compared to ambient CO2. The copy numbers of the nitrogenase reductase (nifH) gene were under the detection limit in all samples regardless the CO2 treatments. Denaturing gradient gel electrophoresis analysis of PCR-amplified nifH genes revealed simple and consistent communities in all samples throughout the incubation period. A nested reverse transcription PCR approach revealed that expression of nifH genes were detected in some microcosms. Our findings suggest that the effect of mycorrhizal fungi on soil bacteria may vary depending on C supply and fungal species.