Quantifying competition between ectomycorrhizal and saprotrophic fungi, and implications for soil carbon and nitrogen cycling

Background/Question/Methods

Recent theoretical and empirical work suggests the presence of ectomycorrhizal (ECM) fungi allows plants to compete directly with decomposers for soil nitrogen (N) via exo-enzyme synthesis. Experimental ECM exclusion often results in a release from competition of saprotrophic decomposers, allowing for increased C-degrading enzyme production, microbial biomass and eventually declines in soil C stocks. Our knowledge of this phenomenon is limited, however, to the presence or absence of ECM fungi. It remains unknown if competitive repression of saprotrophic microbes and soil C-cycling by ECM fungi varies with ECM abundance. This is particularly relevant to global change experiments when manipulations are thought to alter plant C-allocation to ECM symbionts. To test if variation in ECM abundance alters the competitive inhibition of saprotrophic soil microbes (quantitative inhibition) we established experimental ECM exclusion treatments along an age gradient of ECM trees. To test for competitive release, we dug trenches in these stands to exclude roots and ECM fungi. To control for disturbance we placed root in-growth bags both inside and outside of trenches. ECM abundance was quantified by measuring ergosterol concentrations in a separate set of in-growth bags filled with sand (organic matter-free), which have been shown to exclude saprotrophic, but not ECM fungi.

Results/Conclusions

Ergosterol concentrations in sand in-growth bags increased from younger to older stands, suggesting the gradient does reflect variation in ECM abundance. Consistent with the quantitative inhibition hypothesis, older sites have significantly less microbial biomass per unit soil C and lower rates of N mineralization. Consistent with a release from competition, we found C-degrading enzyme activities to be higher and gross proteolytic rates to be lower per unit microbial biomass inside compared to outside trenches. We interpret this to reflect increased microbial investment in C-acquisition and decreased investment in N-acquisition in the absence of ECM fungi. Furthermore, the increase in C-degrading enzymes per unit microbial biomass is significantly greater in sites with the most ECM fungi. Based on these results, ECM-saprotroph competition does appear to slow soil C-cycling and the effect is quantitative. More ECM fungi leads to more ECM-saprotroph competition for N and in turn slower cycling of C in soil. Environmental change that alters ECM abundance may thus alter soil C stocks by ameliorating or exacerbating plant-decomposer competition.