Experimental warming across a natural climate gradient reverses soil nutrient effects on arbuscular mycorrhizal abundance in prairie plants

Background/Question/Methods

Arbuscular mycorrhizal fungi (AMF) form a symbiotic relationship with the majority of terrestrial plants, for which the fungi provide enhanced nutrient uptake and various other services to the plant in exchange for photosynthate. Despite the well known importance of this ubiquitous and ancient relationship, relatively little is known about how these relationships will be affected by climate change. We examined how the abundance of AMF in the roots of four native prairie forbs responded to experimental warming along a 520-km natural climate gradient from southern Oregon to central Washington. At each location, degree of colonization (a measure of AMF abundance) was quantified for each plant species in five heated plots (3°C above ambient) and five control plots.  We were also interested in how differential treatment effects on AMF abundance may affect plant performance, so we measured the aboveground plant biomass of each plant sample.  Additionally, to control for the different soil types of each site, especially with regard to nutrient availability, we performed a greenhouse experiment in which we measured AMF abundance for each plant species grown in soil collected from each site.

Results/Conclusions

In the field experiment, there was a significant decrease in AMF abundance in the heated plots compared to the control plots across all plant species and sites. Additionally, the southern Oregon site had significantly higher AMF abundance than the two northern sites, despite the higher soil nutrient availability in the southern site. We also found there to be a weak, but significant, positive correlation between AMF abundance and plant biomass. Surprisingly, in our greenhouse study, we found the opposite trend. Plants grown in soil from the southern site had significantly lower AMF abundance than those grown in soils from the two more northern sites. Plants grown in the southern soil were also significantly larger than those grown in the northern site soils despite their low colonization, suggesting that higher AMF colonization in the northern soils does not fully compensate for their lower nutrient availability.  Our results indicate the intriguing possibility that climate may counteract or reverse soil effects on AMF abundance. We will use structural equation modeling to deconvolve the direct and indirect (nutrient and water availability) affects of warming on the AMF-plant relationship.