PS 34-236 - Soil chemistry and plant assemblage responses to roadside restoration methods

Abstract: Vegetation restoration often entails actions targeting a single desirable outcome. However, we may have multiple restoration goals, such as reducing weed invasion and fire risk, and enhancing pollinator habitat. Theoretically, multiple goals can be achieved by establishing native vegetation that provides biotic resistance to invasion via limiting similarity and niche complementarity, but the efficacy of methods used to establish such plant assemblages are largely unclear. In particular, there is increasing interest in using soil amendments to enhance restoration, especially soil microbe amendments. Here, we hypothesized that soil amendments can be combined in ways that result in plant assemblages with low invasive plant abundances, low fire fuel loads, and greater pollinator resources. We measured responses of soil chemical properties and plant assemblages to three soil amendments: a mix of bacterial and fungal strains, a high-micronutrient non-synthetic fertilizer, and a wool-based fertilizer designed to increase soil water holding capacity and nutrient availability. We expected that the microbial addition would act synergistically with fertilizer amendments to enhance resource availability for plants, thereby benefitting native plant establishment. To test this, we established two factorial experiments, crossing spring applications of microbes with each fertilizer amendment, established at three roadside sites in southeastern Idaho that were cleared and seeded with native grasses and forbs. We found that within one month of applying the amendments the micronutrient fertilizer enhanced soil copper content, nitrate-N content and electrical conductivity, but iron, potassium, manganese, zinc, and phosphorus content were unaffected, as well as organic matter content and pH. Soil properties were unaffected by the other amendments. Plant aboveground dry biomass and water content in both summer and early fall were unaffected by the soil amendments, yet species cover in the fall indicated compositional differences. Counter to expectations, we found antagonistic interaction effects of the microbial amendment and both fertilizers on native plant cover. Native cover was enhanced by the microbial amendment alone, but not in combination with either fertilizer, and this was driven by native forbs. Introduced plant cover did not respond strongly to the treatments, though introduced forbs had lower cover and introduced grasses had higher cover in the presence of wool-based fertilizer. Altogether, these results suggest that soil microbe amendments may aid establishment of pollinator habitat, but not when combined with fertilizer amendments. While the amendments had little impact on invasion resistance and fire fuel loads, additional monitoring is needed to understand treatment impacts through time.