Strong indirect herbicide effects on mycorrhizal associations through plant community shifts and secondary invasions.

Million of acres of U.S. wildlands are sprayed with herbicides to control invasive species, but relatively little is known about non-target effects of herbicide use. We combined greenhouse, field, and laboratory experiments involving the invasive forb spotted knapweed (Centaurea stoebe) and native bunchgrasses to assess direct and indirect effects of the forb-specific herbicide picloram on arbuscular mycorrhizal fungi (AMF), which are beneficial soil fungi that colonize most plants. Picloram had no effect on bunchgrass viability and their associated AMF in the greenhouse, but killed spotted knapweed and reduced AMF colonization of a subsequent host grown. Results were similar in the field where AMF abundance in bunchgrass-dominated plots was unaffected by herbicides one year after spraying based on 16:1ω5 phospholipid fatty acid (PLFA) and neutral lipid fatty acid (NLFA) concentrations. In spotted-knapweed-dominated plots, however, picloram application shifted dominance from spotted knapweed, a good AMF host, to bulbous bluegrass (Poa bulbosa), a poor AMF host. This coincided with a 63% reduction in soil 16:1ω5 NLFA concentrations but no reduction of 16:1ω5 PLFA. Because 16:1ω5 NLFA quantifies AMF storage lipids and 16:1ω5 PLFA occurs in AMF membrane lipids, we speculate that the herbicide-mediated reduction in host quality reduced fungal carbon storage, but not necessarily fungal abundance after one year in the field. Overall, in greenhouse and field experiments, AMF were only affected when picloram altered host quantity and quality. This apparent lack of direct effect was supported by our in-vitro trial where picloram applied to AMF mycelia did not reduce fungal biomass and viability. We show that the herbicide picloram can have profound, indirect effects on AMF within one year. Depending on herbicide-mediated shifts in host quality, rapid interventions may be necessary post herbicide applications to prevent loss of AMF abundance. Future research should assess consequences of these potential shifts for the restoration of native plants that differ in mycorrhizal dependency.

Do invasive plants structure microbial communities to accelerate decomposition in intermountain grasslands?

Invasive plants are often associated with greater productivity and soil nutrient availabilities, but whether invasive plants with dissimilar traits change decomposer communities and decomposition rates in consistent ways is little known. We compared decomposition rates and the fungal and bacterial communities associated with the litter of three problematic invaders in intermountain grasslands; cheatgrass (Bromus tectorum), spotted knapweed (Centaurea stoebe) and leafy spurge (Euphorbia esula), as well as the native bluebunch wheatgrass (Pseudoroegneria spicata). Shoot and root litter from each plant was placed in cheatgrass, spotted knapweed, and leafy spurge invasions as well as remnant native communities in a fully reciprocal design for 6 months to see whether decomposer communities were species-specific, and whether litter decomposed fastest when placed in a community composed of its own species (referred to hereafter as home-field advantage-HFA). Overall, litter from the two invasive forbs, spotted knapweed and leafy spurge, decomposed faster than the native and invasive grasses, regardless of the plant community of incubation. Thus, we found no evidence of HFA. T-RFLP profiles indicated that both fungal and bacterial communities differed between roots and shoots and among plant species, and that fungal communities also differed among plant community types. Synthesis. These results show that litter from three common invaders to intermountain grasslands decomposes at different rates and cultures microbial communities that are species-specific, widespread, and persistent through the dramatic shifts in plant communities associated with invasions.

Restoring ecological properties of acidic soils contaminated with elemental sulfur.

Elemental sulfur (S0) accumulates in the environment from anthropogenic sources as a byproduct from oil and gas refining and from trap and skeet shooting targets. Bacteria can oxidize S0 to H2SO4, which acidifies soil. We explored whether combinations of soil amendments can be used to remediate acidic soils contaminated with S0 by restoring soil chemistry, plant growth, and bacterial communities in a greenhouse. Results were compared to a contamination gradient in a field that had been limed with CaMg(CO3)2 two years prior. Amendments in the greenhouse included CaCO3 by itself, and in combination with fertilizer, compost, biochar, and chitin. Amended soils were incubated for one week and half of all containers were planted with Poa nevadensis. We sequenced bacterial DNA from a subset of amended soils and along the field gradient. CaCO3 additions in the greenhouse initially raised the pH of contaminated soil to values found in uncontaminated soils. However, pH decreased over time, which was likely caused by the oxidation of S0 to H2SO4. This was also apparent in the field, where CaCO3 additions raised pH to 4 but not to the desired value of 5 or higher. Plants in the greenhouse failed to grow in the unamended contaminated soil, but CaCO3 alone reduced concentrations of toxic cations and resulted in more plant growth than in the uncontaminated soil. CaCO3 also partially restored the bacterial communities in the greenhouse and in the field by increasing richness and diversity to near values found in uncontaminated soil, suggesting that bacteria can be resilient to prolonged acidic conditions. Organic amendments did not provide a significant benefit to restoration. This study demonstrates that acid neutralization alone can restore abiotic and biotic components and productivity of soils contaminated with S0, but multiple CaCO3 applications may be required to avoid future acidification.