Investigating how arbuscular mycorrhizal fungi (AMF)-plant interactions vary with edaphic conditions provides an opportunity to test the context-dependency of interspecific interactions, which is currently recognized as a major avenue of future research.  We study plant-mycorrhiza symbiotic relationships along a gradient of heavy metal contamination at a recently revegetated “Superfund” site on Blue Mountain, in Palmerton, Pennsylvania.  We investigated the interactions involving the native mycorrhizal fungi, non-mycorrhizal soil microbes, soil, and two plant species (a C₃ and C₄ grass) along the contamination gradient.

The native C₃ study species Deschampsia flexuosa, is dominant along the gradient and established naturally; the C₄ Sorghastrum nutans, is native to Pennsylvania but not to the site and was introduced during restoration.  Because C₄ grasses are obligate mycotrophs, we expected S. nutans to have a different effect on and response to the soil symbiont community than the C₃ grass.  We carried out a full factorial greenhouse experiment using field-collected seeds of D. flexuosa and S. nutans, soil, AMF spores, and non-mycorrhizal microbes from both high and low contaminated ends of the gradient.  After 11 weeks of growth in the greenhouses, we harvested above and belowground plant biomass, and quantified AMF root colonization and AMF sporulation.

Results/Conclusions

Our results indicate that context-dependent function is an important factor driving specific ecological interactions between plants and soil microbes. We found that soil origin significantly affected plant growth.  Plants from both species grew much larger in soil from low contaminated (LC) origin than high contaminated (HC) origin.  Furthermore, we found that the efficacy of AMF in promoting plant growth depended on AMF origin.  Specifically, AMF from LC improved survivorship of S. nutans in HC soil compared to AMF from HC.

We also found that the origin of non-mycorrhizal soil microbes affects the benefit provided to plants and likely interacts with AMF in affecting AMF function.  Non-mycorrhizal soil microbes from HC origin decreased mean plant size in D. flexuosa while microbes from LC origin increased mean plant size compared to plants with no non-mycorrhizal soil microbes added.  Our results may be useful for improving our basic ecological understanding of plant-soil interactions and ecotypic variation/context-dependent function.  There are also potential applications for restoration of heavy metal polluted sites.