The relative abundance of limiting resources, particularly light and soil nutrients, may be a key predictor of plant interactions with arbuscular mycorrhizal fungi (AMF). AMF are typically plant mutualists, increasing access to limiting soil nutrients though their extensive network of soil hyphae in exchange for plant carbon. However, when soil nutrients are abundant, relative to light, AMF are less beneficial to plants. In those situations, as plants shift towards light limitation, stoichiometric theory predicts decreases in four metrics of plant-mycorrhizal interactions: plant benefit, fungal benefit, plant root colonization by AMF, and plant carbon allocation to AMF. Indeed, fertilization with soil nutrients does decrease at least some of those metrics of plant-mycorrhizal interactions, but many questions remain. First, do conceptual models of stoichiometry adequately capture negotiation between plants and AMF? With Chris Klausmeier and Todd Robinson, I developed a mathematical model that points to two key features of trade between mutualists that have previously been ignored: the negotiated exchange ratio of one resource for another, and allocation to self-provisioning of those resources by each partner. Second, why do AMF sometimes parasitize plants in high nutrient environments? In theory, plants should be able to impose "sanctions" to avoid parasitic carbon drains by "cheating" AMF. In a greenhouse experiment, I show that two C3 grasses (quackgrass, Elymus repens, and smooth brome, Bromus inermis) avoided parasitism by effectively reducing carbon allocation to AMF in high phosphorus environments while one C4 grass (big bluestem, Andropogon gerardii) did not. Third, why do plant allocation to AMF and AMF abundance not always decrease with increases in nutrient availability? Some field studies have shown no change or even increases in those metrics of plant-mycorrhizal interactions with nitrogen or phosphorus fertilization. In a field fertilization experiment with Todd Robinson, I found that AMF increased in response to nitrogen addition in very nitrogen-poor soils, consistent with AMF nitrogen limitation. In an additional field experiment across a natural productivity gradient, I showed that increases in productivity do not necessarily lead to increases in plant light limitation, calling into question the expectation that increases in fertility should change plant-mycorrhizal interactions. Finally, do differences among plant species affect how shifts in stoichiometry alter plant-mycorrhizal interactions? In nutrient poor soils, AMF benefit different plant species differentially, but how those species differences affect mycorrhizal response to fertilization is unclear. In a greenhouse experiment, I found that two C3 grasses did differ from two C4 grasses in terms of how plant benefit, fungal benefit, and plant root colonization responded to increases in phosphorus availability. In a field fertilization experiment, I again found that a C3 grass (B. inermis) differed consistently from a C4 grass (A. gerardii) in how strongly nitrogen and phosphorus fertilization affected plant-mycorrhizal interactions. Taken together, these studies show that stoichiometric theory is a powerful tool for understanding plant-mycorrhizal interactions. However, relationships are complex, and differences among species as well as aspects of negotiation and trade also play important roles.
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