Anthropogenic nitrogen (N) deposition is exposing plants and their arbuscular mycorrhizal (AM) symbionts to elevated N availability, often leading to shifts in communities of AM fungi. However, physiological trade-offs among AM taxa in their response to N enrichment versus the ability to acquire other soil nutrients could have negative effects on plant and ecosystem productivity. AM fungal taxa that produce extensive networks of external hyphae should forage for N and phosphorus (P) more effectively, but these services incur greater carbon (C) costs to the plant. If N enrichment ameliorates plant nutrient limitation, then plants could reduce C available for AM fungi, which in turn could eliminate AM fungal taxa with large extensive external hyphae from the soil community. As a result, the remaining AM fungal taxa may confer less P benefit to their host plants.
Using a synthesis of data from the literature, we found that the ability of a taxon to persist in the face of increasing soil N availability was particularly high in isolates from the genus Glomus, but especially low among the Gigasporaceae. Across AM fungal genera, we found support for the prediction that association with high N availability was negatively related to external hyphal lengths and the ability of plants to obtain P from the mycorrhizal partner. If this physiological trade-off is widespread, then shifts in AM fungal communities associated with N deposition could have negative consequences for the ability of plants to acquire other nutrients such as P via a mycorrhizal pathway. This trade-off could constrain net primary productivity in P-limited ecosystems exposed to N enrichment.