Soil inoculation increases biomass of non-native grasses at a mine restoration site

Background/Question/Methods

Surface mines can be challenging to restore, as topsoil is often lost or severely disturbed. This disturbance, along with reclamation practices such as the stockpiling of topsoil, often leads to loss of mycorrhizal propagules and other soil biota. Although mycorrhizal communities may regenerate without intervention in other systems, the intensity and scale of mining may impede the recovery of this critical symbiosis. Invasion by non-native plants is another concern in mine restoration, and non-native plants that are less dependent on symbionts may have an advantage over natives in the absence of soil mutualists. In order to ask whether soil inoculation increases native plant biomass or density in mine restoration, three inoculation treatments were compared in eight experimental blocks at a former mercury mine in northern California. Stockpiled topsoil was placed over mine spoils at depths ranging from 2cm to 50cm, and was seeded with native grasses, while non-native species recruited from adjacent grassland. Plots were treated with commercial arbuscular mycorrhizal (AM) inoculum, soil inoculum collected from rhizospheres of native grasses, or were not treated with additional inoculum beyond the background in the stockpiled topsoil (control). In April 2011, aboveground biomass of native and non-native grasses was collected, dried, and weighed.

Results/Conclusions

Contrary to predictions, non-native grass biomass was more responsive to inoculation than native grass biomass. In control plots, biomass of non-native and native grass was similar. In rhizosphere soil treated plots, biomass of non-native grasses was approximately 2.5 times higher in than in controls, while native grass biomass was unaffected. Commercial AM inoculum had no significant effect on either native or non-native grass biomass. To examine AM colonization rates and effects on tissue phosphorus content, Vulpia microstachys was selected for analysis because it established at much higher rates than other natives (Vulpia density averaged 30 individuals/m², whereas other native grass species averaged 2-11 individuals/m²). Inoculation treatment did not significantly affect AM colonization or shoot phosphorus content of Vulpia. Topsoil depth was included in all statistical analyses as a covariate. Deeper topsoil resulted in higher biomass, but lower density, of native grasses. Shoot P increased with topsoil depth, while AM colonization was not significantly affected by depth. There was no interaction between depth and inoculation treatment. Results indicate that while soil inoculation may increase plant biomass in mined areas, in some cases it could disproportionately increase growth of less desirable species; thus caution should be used when implementing an inoculation treatment.