Decomposition by ectomycorrhizal fungi alters soil carbon storage and efflux

Background/Question/Methods

Interactions between plants and their symbiotic mycorrhizal fungi are at the interface of above and belowground systems and their interactions can influence soil carbon (C) dynamics. Plants allocate a portion of their C to mycorrhizae, where it becomes hyphal biomass. In return, mycorrhizal fungi increase plant water and nutrient uptake. However, when plants become stressed the symbiosis can breakdown as they reduce C allocation to mycorrhizae. Mycorrhizae then shift to gain C via soil organic matter (SOM) decomposition. The plant-mycorrhizal interaction could become a source of C to the atmosphere instead of a sink. The shift from sink to source is two-fold: with plant stress hyphal biomass no longer stores plant C, and mycorrhizal fungi begin to acquire C from soil. The effect magnitude of ectomycorrhizal C acquisition, whether from plants or SOM, is crucial to predicting soil C efflux to the atmosphere and storage in soils. In this study we asked (1) if soil C is reduced and efflux is increased when plants are defoliated, and (2) if there is a threshold of plant defoliation that causes a change in the magnitude of soil C storage and efflux. We addressed this question using mathematical theory and a greenhouse experiment.

Results/Conclusions

Decomposition by ectomycorrhizal fungi reduces soil C storage and increases efflux compared to ectomycorrhizal fungi that do not decompose SOM. Our results demonstrate that when plants are defoliated soil C storage decreases by 10-40% due to ectomycorrhizal decomposition compared to when ectomycorrhizae do not decompose SOM. There is a threshold, according to our mathematical model, between 30 and 60% defoliation where the effect of mycorrhizae on soil C storage is minute. This is likely due to reduced total enzyme production associated with low mycorrhizal abundance when plants are defoliated. We manipulated plant defoliation in a regression design from 0 – 50% in a greenhouse experiment to validate our theoretical findings. In all, our study suggests that belowground interactions play a key role in C storage and release from soil.