COS 153-4 - Exchange ratios in arbuscular mycorrhizas depend on resource availability and differ among co-occurring plants in unexpected ways

Abstract: Biological market theory has emerged as a useful tool to predict variations in the phosphorus (P) for carbon (C) exchange in the arbuscular mycorrhizal (AM) symbiosis. However, our knowledge of AM function stems largely from controlled experiments, and whether findings represent resource exchange in natural communities where many plants and fungal taxa co-occur is largely unknown. We collected intact communities harboring the forb blanketflower (Gaillardia aristata) and various grasses from sites in two regions that differed ten-fold in soil P-availability. We installed hyphal ingrowth cores, and maintained communities in the greenhouse for two months, then labeled with ¹³CO₂ and ³²P. We measured ³²P in shoot tissue to estimate P delivery from AM fungi to plants, and ¹³C concentration in the AM-fungal specific fatty acid 16:1ω5 to assess reciprocal C delivery from plants to AM fungi. We predicted that AM fungi would deliver proportionally more P to plant communities growing in low-P than high-P soils, and that this would result in greater C allocation to AM fungi and AM fungal biomass. We also predicted that the more coarse-rooted forb would exchange proportionally more resources with AM fungi than the grasses, especially in the low-P communities.


Contrary to predictions, plants received proportionally more ³²P from AM fungi where soil P availability was higher (R=0.63, P< 0.001) but this did not coincide with greater C allocation to AM fungi or AM fungal biomass. This implies AM fungal-derived P became cheaper where availability was higher. Low-P soils had higher phosphatase activity, which—if released from plants or other soil biota—may signal a lesser reliance on AM fungi for P acquisition. Regardless of soil P availability, larger plants contained more ³²P, but grasses always allocated more C to AM fungi than blanketflower, which was unexpected. This resulted in less favorable P-for-C exchange in grasses and coincided with more vesicles in grasses and more arbuscules in blanketflower. Blanketflower also appeared to regulate C-for-P exchange ratios based on soil P-availability, whereas no such relationship was observed in grasses. Thus, biological market theory predicted resource exchange in some—but not all—co-occurring plants, which suggests C allocation to AM fungi is not solely driven by reciprocal P delivery. Combined, our results highlight the value of assessing AM function in more complex systems to complement common reductionist experiments.