Carbon and phosphorus exchange rates in arbuscular mycorrhizas depend on environmental context and differ among co-occurring plants

Biological market theory predicts variation in phosphorus (P) for carbon (C) exchange between arbuscular mycorrhizal fungi (AMF) and their host plants as P availability varies. To determine if this also occurs in more natural communities and if P-for-C exchange differs among co-occurring plants, we collected intact soil monoliths from three grasslands in each of two regions differing ~10-fold in labile inorganic P. Because the composition of AMF communities can vary regionally due to dispersal limitations and differences in soil N and P availability, we characterized AMF communities by amplifying the small subunit rRNA (SSU rRNA) region using the primer pair WANDA-AML2. Grasslands within the high-P and low-P grasslands were all dominated by AMF belonging to Glomeraceae, but the composition of taxa within families differed. Additionally, we observed clear differences in AMF composition between blanketflower and grasses in high-P but not in low-P grasslands. Overall, biological market theory predicted shifts in resource exchange ratios for some, but not all co-occurring plants in this study. This calls into question P-for-C exchange as the sole driver in the AM symbioses in natural communities.