Mycorrhizal fungi enhance plant productivity but reduce soil organic matter stocks

Mycorrhizal fungi and plants form symbiotic relationships that are essential for plant nutrition and carbon (C) storage in soil. Plants invest photosynthetically fixed C in their fungal partners in exchange for nutrients, especially nitrogen (N), which the fungi obtain from inorganic sources or by breaking down organic matter. This exchange also helps to stabilize root-derived C, as mycorrhizal necromass can persist in the soil, but it can also promote C loss when mycorrhizal fungi act as decomposers. Capturing these relationships in process-based models is crucial for quantifying C and N cycles and understanding how mycorrhizae influence ecosystem processes.

In this study, we utilized an ecosystem model calibrated with field data from a boreal forest in northern Sweden to compare ecosystem functions with and without ectomycorrhizal fungi (EMF) and to investigate how variations in parameters encoding microbial traits affect model outcomes. Through simulations involving different scenarios of elevated CO₂ and N deposition, both individually and in combination, we assessed how the presence or absence of EMF influences ecosystem responses.

We found clear differences between ecosystems with and without ectomycorrhizal fungi. Plant productivity and saprotrophic biomass were generally higher and soil C more stabile when EMF were present in the ecosystem model. But, EMF also increased decomposition resulting in higher plant growth at the cost of reduced soil C storage. Increasing CO₂ and N deposition had similar effects in most of the cases. However, N addition had little effect on soil organic N suggesting that plants and microbes together control the soil organic N pool.

These findings demonstrate the significance of ectomycorrhizal fungi in influencing ecosystem responses to changing environmental conditions and highlight the benefit of including microbial interactions in ecosystem models to improve predictions of C and N dynamics.