Increases in atmospheric CO₂ concentrations will impact future ecosystem productivity and carbon storage. Predicting the effect of increasing CO₂ on ecosystems, however, requires that we understand how excess atmospheric CO₂ alters the strength and nature of multiple, co-occurring species interactions. In particular, both microbial mutualists and pathogens of plants can modulate plant-carbon cycles.Thus, the growth, fecundity and population dynamics of these two groups of widespread microbes may modify plant performance in response to elevated CO₂. Previous studies indicated that the interaction between mycorrhizal fungi and plant enemies cause changes in overall plant performance. However, there have been no experimental studies explicitly studying the interactions between mutualistic mycorrhizal fungi and viral pathogens.

Here we investigate the impact of arbuscular mycorrhizal fungi (AMF) and barley yellow dwarf virus PAV (BYDV) on plant performance in response to abiotic changes in atmospheric CO₂ and phosphorous addition. While AMF provide a nutritional benefit to the host plant in the form of nitrogen and phosphorous, BYDV causes necrosis of phloem tissue and disrupts carbohydrate translocation. We grew the invasive species Avena fatua (wild oats) and Bromus hordeaceus (soft brome) in individual pots of sterilized sandy loam soil in replicated continuous flow chambers.  We factorially manipulated atmospheric CO₂ (585 PPM), soil phosphorous, AMF association, and BYDV infection. Each pot was either inoculated with live spores of AMF, or mock inoculated with sterilized inoculum. Additionally, each plant was either inoculated or mock inoculated with the aphid vector Rhopalosiphum padi. To assess plant performance, we measured survivorship, above and belowground biomass, tiller number, BYDV infection, aphid reproduction, AMF colonization, seed production, photosynthetic capacity and longest leaf length.

Results/Conclusions

AMF and phosphorous addition interacted to alter photosynthetic capacity of both host species, however, the direction of change was different for each plant species.While A.fatua exhibited decreased photosynethetic capacity, B. hordeaceus exhibited increased photosynthetic capacity. BYDV inoculation reduced B. hordeaceus photosynthetic capacity by 30% compared to mock inoculated plants. Additionally, rates of transpiration for B. hordeaceus were 34% lower when plants were both infected with BYDV and phosphorous addition while A. fatua rates of transpiration were 19% lower when plants were infected with BYDV. Thus, both BYDV infection and AMF colonization impacted plant performance.  Their effects depended on phosphorus supply, but did not interact together, or with atmospheric CO₂ concentration.