Carbon and phosphorus exchange rates in arbuscular mycorrhizas depend on environmental context and differ among...

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.