Soil-microbial communities respond less than plant communities to synthetic- or bio-herbicides applied to address the exotic grass-fire cycle in rangelands

Authors: LAZARUS, BRYNNE - Us Geological Survey (USGS); Mueller, Rebecca; GERMINO, MATTHEW - Us Geological Survey (USGS)

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2025
Publication Date: 8/20/2025
Citation: Lazarus, B.E., Mueller, R.C., Germino, M.J. 2025. Soil-microbial communities respond less than plant communities to synthetic- or bio-herbicides applied to address the exotic grass-fire cycle in rangelands. Science of the Total Environment. 991. Article 179831. https://doi.org/10.1016/j.scitotenv.2025.179831.
DOI: https://doi.org/10.1016/j.scitotenv.2025.179831

Interpretive Summary: Chemical herbicides are widely used to control invasive plants, but can have unintended and detrimental effects to ecosystems. Identifying alternatives to chemical herbicides, such as bioherbicides, could have both ecological and economic benefits. In addition, the responses of the microbial community, which play a large role in ecosystem functioning, have been largely overlooked in studies of herbicide effects. Here, we examined the effect of a widely used chemical herbicide, imazapic, along with a bacterial bioherbicide ACK55, and their combined effects, on invasive annual weeds at two sites near Boise Idaho. We found site-specific effects, such that the bioherbicide was effective at one site, and the chemical at the other. Plant and microbial responses were largely linked, suggesting that plant-soil feedbacks are not disrupted by herbicide disturbances. Despite the transient effects of ACK55 on the plant community, we found no evidence that the added bacteria survived in the soils, suggesting that other organisms more adapted to the site might have stronger effects on invasive plants. We also found minimal shifts in the microbial community with imazapic, indicating that chemical application does not lead to a loss of microbial diversity.

Technical Abstract: Rationale: Land managers often use the pre-emergent chemical herbicide imazapic and have shown interest in using the bioherbicide (Pseudomonas fluorescens strain ACK55) to help break the annual grass-fire cycle in semiarid sagebrush steppe ecosystems, but little work has been done on how these treatments might affect the soil microbial community (soil microbiome). Questions remain regarding the potential for imazapic to negatively alter soil microbes or shift microbial community composition in a way that feeds back on plant growth. In addition, whether ACK55 survives after application, and if it does, how its addition might shift the resident microbial community, has not been widely studied. Measuring the survival of ACK55 has been hindered by lack of sequencing data for the ACK55 isolate, which has limited our ability to determine if applied bioherbicides have established post-application using current high-throughput sequencing methods. Methods: In this study we treated two distinct cheatgrass-invaded sagebrush steppe plant communities with imazapic (Plateau®) and ACK55 (Battalion Pro®) individually and in combination. We quantified plant communities for three years after treatment and soil microbial communities in the first year after treatment using targeted sequencing of the 16S and LSU rRNA genes for bacteria and fungi, respectively. We also cultured the commercialized strain of ACK55 (BattalionPro) and sequenced the full-length 16S rRNA gene for more robust taxonomic and phylogenetic classification. We asked 1) whether and how the treatments affected plant and soil microbial communities and 2) whether the bioherbicide we applied could be detected in the soil in fall one year after application of the bioherbicide. Results: The largest differences in microbial community composition were found between the two plant communities, and microbial community composition was strongly correlated with plant community composition, even with larger shifts in the plant community relative to the microbial community. Imazapic treatments caused detectable but relatively small shifts in bacterial and archaeal but not fungal communities, and only when herbicide application also resulted in large shifts in composition of one of the plant communities. Phylogenetic analyses revealed that the curated source germplasm for ACK55 bioherbicide is more likely to be P. salmonii rather than P. fluorescens, and there was no evidence that the ACK55 bacterium was present in soil one year after application. ACK55 had only small and transient effects on plant cover in one of the plant communities, which were accompanied by a small reduction in species richness of soil fungi. Significance: Longer term observations of microbial communities are needed for soils treated with imazapic, but we found no evidence for loss of microbial diversity with imazapic applications. We also found no evidence that ACK55 established within field soils, suggesting additional work is needed to uncover the drivers of limited establishment in soils.