Modelling plant-mycorrhizae interactions - a review

Symbiotic associations between plants and soil microbes, especially mycorrhizal fungi, are fundamental for plant nutrition and belowground processes associated with carbon (C) transfer from plants to the rhizosphere and mycorrhizae are key components of the global carbon cycle. Plants sacrifice photosynthetically acquired C in exchange for nutrients from their symbiotic partner. This exchange can be advantageous when mycorrhizae can access nutrient pools that plants cannot reach—either because chemically recalcitrant (e.g., nutrients in organic matter), or physically isolated (hyphae explore soils more effectively than roots). Additionally, the mycorrhizal network can extend into great distance and allows plants to share C and nutrients. Therefore, understanding this relationship and the interactions between plants and soil microbes are vital for creating realistic predictions of C and nutrient cycling in forests.

In this contribution, we review current modelling approaches to plant-mycorrhizae processes and pathways, focusing on C and nutrient cycling, to highlight ongoing trends and knowledge gaps. It is evident that further model-development is needed in order to get accurate predictions. Some models include C and nutrient exchanges between plants and mycorrhizae via empirical factors, lacking a process-based description of these exchanges. Other models describe C-nutrient exchanges based on stoichiometric demand and supply of C and nutrients, possibly resulting in excessively constrained exchanges. The approaches that quantify costs and benefits of symbiosis in an eco-evolutionary framework are promising as they capture adaptation mechanisms. In general, models tend to focus more on stoichiometry than on temperature and soil moisture effects on plant-mycorrhizae interactions. Information about how the soil-plant system reacts to changes with climate dependent environmental conditions are also underrepresented. Therefore, while coupled plant-mycorrhiza models have been tremendously improved in recent years, they might still not fully capture the role of mycorrhizae in the C and nutrient cycling in terrestrial ecosystems.