Carbon cycling by Douglas-fir and fungal communities in forest soil

Background/Question/Methods

Interactions between host plants, symbiotic fungi, and soil microbes are central to understanding carbon cycling in forest soils.  The degree to which recent photosynthate is stored, transferred to symbionts, and/or returned to the environment remains a key question.  Here we used stable isotope pulse-chase labeling to investigate the fate of photosynthesised ¹³C in a Douglas-fir donor-recipient system mediated by ectomycorrhizas.  A kin selection experiment was initiated using paired interior Douglas-fir seedlings in field soils.  Pairs were either full siblings or had different parents.  One of two mesh bag treatments separated the donor seedling from the recipient seedling to either restrict hyphal growth (0.5 μm mesh) or allow it (35 μm mesh).  Donor seedlings were sealed inside a gas-labeling bag into which a stable isotope was added, in the form of ¹³CO₂, and photosynthesised over 11 hours.  Following a 6-day chase period we harvested all plant, ectomycorrhiza, and soil biomass fractions.  EA-IRMS compared the ¹³C content of each fraction from each sample against unlabeled controls.  A fractionation technique allowed application of phospholipid fatty acid (PLFA) analysis to ¹³C enriched biomass only.  Finally metatranscriptomic analysis of soil RNA was used to determine which fungi were active during the isotope chase period.

Results/Conclusions

Full sibling recipients exhibited significantly greater ¹³C in their root tissue than unrelated recipients, potentially indicating a degree of kin selection.  Approximately 20% of the ¹³C added during labeling was detected following the 6-day chase period.  Of the ¹³C that remained in the system the majority remained in donor plant tissue.  However, significant amounts were also recovered in donor and recipient ectomycorrhizas and most soil and recipient biomass fractions.  As a proportion of their total biomass, ectomycorrhizas contained the most labeled carbon in both donor and recipient systems, regardless of the mesh treatment used.  PLFA analysis revealed that > 80% of the labeled carbon detected after the chase period was contained in fungal biomass, and this was true in donor roots, donor ectomycorrhizas, donor and recipient soil fractions, and recipient ectomycorrhizas.  Metatranscriptome analysis showed that the ectomycorrhizal genus Rhizopogon was active during the chase period as was the arbuscular mycorrhizal genus Glomus, but that ‘unknown fungi’ displayed the highest levels of activity in the soil fungal community.  These data demonstrate that fungi appear to be the most important component of the microbial system in regards to uptake and transfer of recent photosynthate in the forest soil environment during carbon cycling.