Phosphorus (P) for carbon (C) exchange is the pivotal function of arbuscular mycorrhiza (AM), but how this exchange varies with soil P availability and among co-occurring plants in complex communities is still largely unknown. We collected intact plant communities in two regions differing c. 10-fold in labile inorganic P. After a 2-month glasshouse incubation, we measured ³²P transfer from AM fungi (AMF) to shoots and ¹³C transfer from shoots to AMF using an AMF-specific fatty acid. AMF communities were assessed using molecular methods. AMF delivered a larger proportion of total shoot P in communities from high-P soils despite similar ¹³C allocation to AMF in roots and soil. Within communities, ¹³C concentration in AMF was consistently higher in grass than in blanketflower (Gaillardia aristata Pursh) roots, that is P appeared more costly for grasses. This coincided with differences in AMF taxa composition and a trend of more vesicles (storage structures) but fewer arbuscules (exchange structures) in grass roots. Additionally, ³²P-for-¹³C exchange ratios increased with soil P for blanketflower but not grasses. Contrary to predictions, AMF transferred proportionally more P to plants in communities from high-P soils. However, the ³²P-for-¹³C exchange differed among co-occurring plants, suggesting differential regulation of the AM symbiosis.