One factor used to determine wetland mitigation success is the establishment of native wetland plant species. Although mycorrhizal associations are known to be present in 70 to 95% of all plant species and have been linked to seedling establishment as well as nutrient and carbon flux within plant communities, the presence or absence of mycorrhizal fungi are not assessed or addressed during wetland mitigation work. Three experiments were devised to examine the effects of mycorrhizal fungi on the germination and growth of native wetland plant species in soils and field sites from natural, restored, and created wetlands.
Greenhouse experiment I was a small scale greenhouse experiment in which soil from Siebenthaler Fen (SF), a high quality wetland, was used to inoculate plants with mycorrhizal fungi to determine its effect on germination and growth. Inoculation significantly affected shoot height of 30% of plant species and fresh weight of 100% of plant species. Field soils significantly affected both fresh and dry weight of 70% of plant species independent of inoculation.
Greenhouse experiment II was a large-scale greenhouse experiment in which a produced soil inoculum was used to infect plants with mycorrhizal fungi to determine its effect on the growth of four native wetland plants. Inoculation significantly affected shoot height, dry weight, and arbuscular colonization of Mimulus ringens L. The overall affect of field soils were few and highly varied.
In the field experiment, native wetland plants were inoculated with MycoGrow Soluble in the greenhouse prior to transplantation into natural, restored, and created wetlands to determine the effect of soil inoculation containing mycorrhizal fungi on plant growth and establishment. Soil inoculation significantly decreased shoot dry weight of M. ringens but did not impact the shoot height, leaf count, or shoot count of any species. Field site location significantly affected shoot height of M. ringens, C. vulpinoidea, and C. stipata.
In this study, the effects of soil inoculation containing mycorrhizal fungi varied greatly between plant species and field sites and soils throughout all three experiments. When inoculated, the growth of some plants increased, some were not affected, and others decreased. Though not true for all species, M. ringens displayed clear correlations between inoculation and growth in the greenhouse. The plant shoot height, dry weight, and arbuscular colonization of M. ringens were all significantly affected by inoculation. For M. ringens, inoculation increased arbuscular colonization while reducing plant shoot height and dry weight. In the field, shoot dry weight of M. ringens was once again significantly decreased by inoculation; however, a direct correlation to arbuscular colonization was not found. However, the shoot dry weight of all four species combined reflected the level of site disturbance originally used to choose these field sites, independent of mycorrhizal treatment. While certain plant species may in fact benefit from the presence of mycorrhizal fungi in the soil, the degree to which the plants are impacted by mycorrhizae is strongly dependent on the condition of the site and soil in which they are grown. Therefore, it appears to be more beneficial to select a mitigation site with minimally disturbed, hydric soils that will support the growth of native wetland plants than it is to attempt the re-introduction of mycorrhizal fungi to disturbed areas through soil inoculation.