Phase shifts in plant-microbe interactions with stoichiometric modeling of above- and belowground processes

Plant and soil microbial populations are dynamically linked by the exchange of nutrients that have specific elemental compositions. The composition of these nutrients varies between both plant and microbial species, and has the capacity to greatly impact ecosystem structure, composition, and dynamic functioning. Here we present a stoichiometrically explicit (C and N) model for a simplified system that includes plants, microbes, organic, and inorganic soil matter, to gain a more fundamental understanding of the rich dynamics that emerge from the inclusion of this intuitively simple property. As expected, the dynamic behavior of the system contains phase changes across a gradient from high to low C:N plant inputs, as the plant-microbial interaction shifts from one of competition for a co-limiting resource (N), to one of co-facilitation which alleviates limitation for different resources (N for plants, C for microbes). Additionally, we find that kinetic uptake parameters and fundamental growth rates, more than tissue stoichiometry, determine the population dynamics for both plants and microbes. Finally, we use the model to determine combinations of plant and microbial parameters that optimize total production, and explore the potential disruption of these communities by N addition and plant species invasion. This model highlights the need to explicitly incorporate stoichiometry into plant-microbial population models, and suggests critical values for kinetic uptake, growth, and tissue stoichiometry parameters that influence the behavior of interaction between these populations.