Root and arbuscular mycorrhizal exudation under different global change scenarios

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In addition to their influences on plant growth, arbuscular mycorrhizae (AM) also add to soil carbon (C) through their own growth and turnover. However, whether AM release soluble C into the soil (i.e. exudation), as has been shown for roots and ectomyrrhizae, is poorly understood and could represent an important pathway for inputs of C to soil. Since AM could increase in abundance under elevated atmospheric CO₂ (eCO₂), particularly with low nitrogen (N) availability, it is important to understand this potential pathway of C. Our objectives were to test 1) whether AM contribute to exudation and 2) whether roots and AM hyphae release more exudates at eCO₂ and low N (i.e. when plants are especially nutrient-limited) compared to conditions when plants are less nutrient-limited. We conducted a pot study within the Biodiversity, CO₂ and N Experiment (BioCON) at the Cedar Creek Ecosystem Science Reserve in Minnesota. Briefly, plants were grown from seed with or without an AM inoculum in sterilized sand and exposed to a full factorial of CO₂ and N treatments. Exudation was measured after one growing season. Exudate samples were analyzed for DOC and plants were harvested to assess root and shoot biomass. After accounting for shoot biomass, plants with AM added produced significantly higher DOC than non-inoculated controls (ANCOVA, P < 0.05), suggesting AM increase exudation and C release into soil. There was no effect of eCO₂, N, or their interaction on shoot biomass or DOC content, despite other work from BioCON showing effects of eCO₂, N, and their interaction on plant growth. Overall, this work suggests that AM contribute to soil C inputs via exudation. More work is needed to determine whether the effect is via an AM-induced increase in root exudate production or from AM exudation.