Arbuscular mycorrhizal fungi (AMF) form mutualistic associations with about 80% of terrestrial plant species and may play a key role in the maintenance of plant diversity. However, AMF are sensitive to changes in soil nutrient availability, in particular phosphorus (P) and nitrogen (N). Deposition of both P and N are increasing worldwide largely due to intensive fertilizer use and fossil fuel combustion. Shifts in nutrient regimes, especially in nutrient-poor systems such as tropical montane forests, will therefore likely result in changes in the abundance of mutualistic microorganisms such as AMF. However, the ecological impacts of the effect of nutrient enrichment on AMF are unclear, particularly in tropical montane forests, because previous work has shown conflicting results. In this study, we used a greenhouse mean infection percentage (MIP) method to estimate AMF abundance under long-term (8 years) N and P addition in a tropical montane forest in southern Ecuador. In this context, we also examined the relationship between AMF abundance and woody plant diversity.
AMF abundance was negatively correlated with soil P (intercept=-3.106; slope=-0.337) and positively correlated with soil N (intercept=-3.106; slope=0.047) concentrations, and was positively correlated with tree diversity (intercept=-3.106; slope=0.056). These results point to an overall net reduction in AMF abundance in response to nutrient enrichment, with the negative effect of P addition likely outweighing the positive effect of N. The long-term effects of changes in AMF abundance on plant communities remain uncertain, but some positive feedback seems probable, with plant diversity responding slowly to the decrease in AMF. AMF reduction may ultimately contribute to a decrease in woody plant diversity, potentially resulting in a reduction in ecosystem resilience to further changes. In addition, it will be important for future research to understand the impacts of AMF reduction on ecosystem processes not related to nutrients, including soil aggregation, pathogen resistance, and interplant signaling.