Indirect effects of soil amendments on plant traits and the microbiome in post-wildfire forest recovery

Authors: Barnard, David; Mahood, Adam; MACDONALD, JACOB - Colorado State University; AMUNDSON, KAELA - Colorado State University; FEGEL, TIM - Us Forest Service (FS); Gleason, Sean; Guimond, Madeline; KAISER, SOPHIE - Harvard College; SPARKS, KYA - Colorado State University; WILKINS, MIKE - Colorado State University; RHOADES, CHARLES - Us Forest Service (FS)

Submitted to: Forest Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/25/2025
Publication Date: 7/5/2025
Citation: Barnard, D.M., Mahood, A.L., Macdonald, J., Amundson, K., Fegel, T., Gleason, S.M., Guimond, M.M., Kaiser, S., Sparks, K., Wilkins, M., Rhoades, C. 2025. Indirect effects of soil amendments on plant traits and the microbiome in post-wildfire forest recovery. Forest Ecology and Management. 594. Article e122953. https://doi.org/10.1016/j.foreco.2025.122953.
DOI: https://doi.org/10.1016/j.foreco.2025.122953

Interpretive Summary: Land managers are using organic soil amendments to reduce erosion and stimulate vegetation recovery after wildfires, but little is known about how these treatments affect how plants operate within the context of vegetation community and ecosystem functioning. We found that organic soil amendments had minimal direct effects on plant community composition, but did have detectable indirect effects on plant functional traits through a cascade of interactions via the effects of soil amendments on soil chemistry and microbial composition. Land managers can use this information in their use of organic soil amendments, knowing that it will not dramatically alter plant communities, and may help certain plants deal with moisture limitation.

Technical Abstract: As wildfires intensify across the western United States, understanding long-term ecosystem recovery is increasingly critical. Post-fire soil amendments, such as woody mulch or biochar, are commonly used to stabilize soils and promote vegetation recovery, yet their effects on plant functional strategies and plant–microbe interactions remain poorly understood. Thirteen years after a high-severity wildfire in Colorado, we evaluated the impacts of these treatments on soil properties, microbial communities, and plant functional traits, including plant height, specific leaf area (SLA), and leaf dry matter content (LDMC). While statistical means comparisons showed limited treatment effects on vegetation composition or plant traits, random forest and exploratory mediation models revealed complex, non-linear, and indirect relationships among soil chemistry, microbial community composition, and plant responses. Notably, microbial community shifts and soil nutrient changes, especially in nitrogen form, emerged as key drivers of plant trait variation. Vaccinium scoparium responded more strongly to NH4' and ectomycorrhizal fungi, while Oreochrysum parryi was associated with NO3- and bacterial/archaeal diversity. Biochar increased soil carbon and microbial diversity, whereas mulch enhanced soil moisture and reduced NO3- availability, leading to contrasting effects on microbial and plant responses. These findings demonstrate that post-fire soil amendments can have meaningful yet indirect impacts on plant resource use strategies and microbial dynamics, which may not be captured using conventional analyses. Our results highlight the value of trait-based approaches and integrative modeling for guiding restoration strategies aimed at enhancing ecosystem function and resilience in fire-affected landscapes.