Fungi decrease the sequestration of root-derived C under elevated CO2

Background/Question/Methods

The lack of suitable techniques for tracking recently-fixed C into belowground pools has hindered efforts to elucidate the role of soil processes in influencing feedbacks to plant productivity under global change.  We sought to quantify the relative contribution of root and mycorrhizal-derived C exposed to CO₂ and N enrichment at the Duke Forest FACE site, NC.  Using a modification of the ¹³C natural abundance method, we transplanted soils with pre-existing differences in the ¹³C of SOC (~7 per mil) into field plots using a reciprocal transplant design (i.e. ambient CO₂ soils into elevated CO₂ plots, elevated CO₂ soils into ambient CO₂ plots).  Because roots and mycorrhizal fungi in these plots have unique isotopic signatures relative to the transplanted soils, the sequestration of root-derived C can be calculated using a two end-member mixing model.  Three types of in-growth cores were placed in each plot (n = 4 replicates): one with a mesh size permitting the penetration of roots and mycorrhizal fungi, one with a fine mesh size allowing for penetration by fungal mycelium only, and one with a fine mesh that was “turned” bi-weekly to reduce mycelial inputs into the core.  All soils were mixed with sand (1:1 v/v) to promote colonization of the substrate by mycorrhizal rather than saptrophic fungi.

Results/Conclusions

Across all three core treatments, CO₂ enrichment reduced the amount of root-derived C sequestered in soil (P < 0.0001), with the largest reductions (~60%) occurring in the fine mesh (fungi only) cores.  Given previous reports of greater fine root and mycorrhizal productivity under elevated CO₂ at this site, reduced storage of root- and mycorrhizal-derived C suggests an enhanced turnover of fungal necromass under elevated CO₂.  Coincident with decreases in root-derived C, elevated CO₂ decreased the N content of soils in the cores (P = 0.0007), indicating that the enhanced fungal turnover accelerate soil N turnover and uptake.  Collectively, our findings suggest that ectomycorrhizal roots may be delaying the onset of progressive N limitation at this site by mining N from a fast-cycling pool of fungal biomass.