AMF effects on diversity vary with relative nitrogen, water, and light availability, but not phosphorus availability

Background/Question/Methods

Arbuscular mycorrhizal fungi (AMF) are generally thought to enhance plant diversity, although these effects are predicted to vary with phosphorous availability and the mycorrhizal dependence of the dominant and subdominant species.  However, the outcome of individual plant-AMF interactions depends on the relative availability of many resources, suggesting that a similar reliance on multiple resources could determine AMF effects on diversity, regardless of phosphorous availability.  When communities contain no dominant species the predictions are less clear, although some studies have shown that AMF can help maintain diversity in such communities following nitrogen enrichment.  To test whether co-limitation among multiple resources determine AMF effects on diversity, we established a factorial experiment applying fungicide (Rovral^®), adding NPK fertilizer, and removing plant litter in spatially heterogeneous native grassland limited by nitrogen and water availability.  After 5 years, we tested to see if AMF effects on plant diversity were contingent on the other treatments.  We then used measurements of nitrogen, phosphorus, water, and light availability taken in the plots to determine which resources influenced AMF effects on plant diversity using both absolute resource availability and pairwise resource ratios. 

Results/Conclusions

The relationship between AMF, resources, and plant diversity was complex.  Although nutrient enrichment influenced AMF controls on plant diversity, these effects were contingent upon the presence of plant litter.  Further, resource ratios, rather than absolute resource quantities, best explained AMF effects on plant diversity.  Specifically, AMF increased the number of coexisting plant species under three sets of conditions: low water relative to light, low water relative to nitrogen, and low light relative to nitrogen.  Interestingly, we found very little evidence that available phosphorus influenced AMF effects on plant diversity.  Overall, we suggest that water dynamics have been underexplored in plant-AMF interactions, at least in regard to AMF effects on diversity.  In drier environments, AMF likely increase access to nutrients for plants that would otherwise be resource limited.  Similarly, when nitrogen is abundant, diffusion is unlikely to limit uptake, and plants are more likely to be phosphorus limited without AMF.  Further, the importance of AMF for plant diversity under low light and high nitrogen, although surprising, suggests that light does not limit diversity in this system as long as AMF are intact and there is sufficient nitrogen.