Response of Andropogon gerardii to arbuscular mycorrhizal fungi in the context of phytoremediation
Phytoremediation, or the use of plants to restore polluted soils, holds significant promise for the treatment of heavy metal contamination. Phytoremediation provides a cleaner and cheaper alternative to destructive traditional methods. Incorporating obligate symbionts like arbuscular mycorrhizal fungi (AMF) is vital to the effectiveness of phytoremediation. AMF are crucial in promoting growth and survival that, for some plants, might otherwise be impossible. However, there is scant information about tolerance of AMF species to metalliferous soils, and this has set back phytoremediation’s application in restoration. We performed a greenhouse experiment to examine the efficacy of AMF in soils polluted with Zn, Cd, and Pb from nearly a century of zinc smelting. We measured performance of two Andropogon gerardii ecotypes: seeds collected from recent plantings on site and from a natural grassland on a serpentine outcrop, which is high in Ni, Mg, Cr, and Fe. We used whole soil and root inoculum (10ml) to apply AMF species: Rhizophagus clarus, Claroideoglomus etunicatum, a 1:1 mixture of the two, or sterile inoculum. Plant ecotype and AMF factors were fully-factorial with two levels of soil metal contamination, thereby producing 16 distinct treatment combinations. After 10 weeks, we assessed plant height, biomass, and mycorrhizal root colonization.
Plant height, biomass, and AMF colonization varied with all three experimental factors: plant ecotype, level of soil metal contamination, and AMF treatment. The serpentine plant ecotype experienced higher levels of colonization than the on-site ecotype, but lower levels of biomass growth. As expected, plants grown in lower levels of soil contamination had higher colonization on average and greater biomass accumulation. The importance of AMF to A. gerardii in these soil environments is evinced by plants without AMF having 600-700% less biomass than those with AMF. In addition, Rhizophagus clarus produced the most mycorrhizal colonization, nearly double colonization when both AMF species were applied. Plants inoculated with R. clarus and those with both AMF species experienced similar growth, significantly outperforming plants inoculated with C. etunicatum alone; this difference in plant growth among AMF treatments was particularly evident with the on-site plant ecotype and in low soil contamination. Thus, AMF diversity is not necessarily an absolute benefit. Provided that AMF are present, A. gerardii thrives in high stress environments, signifying it as an operative phytoremediator. The results of this study will provide a greater understanding of the importance of AMF to phytoremediation and hopefully allow for increased implementation of phytoremediation.