Nutrient-driven dynamics of competition between mycorrhizal fungi: The effects of nitrogen solubility on fungal dominance

Background/Question/Methods

Mycorrhizal associations enable plants to acquire essential nutrients by exchanging carbon with mycorrhizal fungi.  The factors determining fungal traits and the diversity of fungal communities therefore should have great importance to nutrient cycling dynamics and to the function and diversity of aboveground food webs.   Many quantitative models, including game theoretic and market models, reasonably focus on plant-fungal trade strategies to address these questions.  But when plant growth is limited by nutrient availability, fungal competition to acquire those nutrients should shape fungal communities, rather than negotiation with plants for relatively available carbon.  Here we present a model to describe nutrient competition between fungi, which trade away some of the nutrients they acquire to plants.  Because nitrogen predominantly limits terrestrial ecosystems, and because major fungal groups differ in their ability to take up soluble and insoluble nitrogen, we include two types of nutrients, and also incorporate ecosystem feedbacks which can convert each form into the other.  

Results/Conclusions

We show that, when nitrogen is limiting, fungal trade of nitrogen for carbon can be represented as a relatively minor modification to established population models for resource competition, which have hitherto not been widely employed for fungi.   The effects of ecosystem processes that interconvert soluble and insoluble nitrogen can then be understood by comparing predictions from the model including ecosystem feedbacks with well-known results for independent nutrients.  These comparisons suggest that empirical quantification of rates of fungal turnover and lifespan could clarify the extent to which results from models for independent nutrients apply to fungi.  We use simulation to show that the relative abundances of soluble and insoluble nitrogen could play a straightforward and important role in determining turnover of fungal communities over large spatial scales or with changes in anthropogenic nutrient additions.