Effects of microbial communities associated with biochar and compost teas on plant growth and insect herbivory

Background/Question/Methods

Biochar and compost tea are soil amendments that are used in sustainable agriculture to promote plant growth, induce disease resistance, and reduce insect herbivory.  Microorganisms are thought to be the primary mediators of these effects, however, little is known about the microbial communities associated with compost teas and biochar.  Smaller-sized bacteria tend to have slower growth rates than larger-sized bacteria and often have host-dependent lifestyles; therefore, smaller-sized bacteria in compost teas may have different effects on plant growth and insect herbivory than larger, fast-growing bacteria also present in the teas. We tested this hypothesis by fractionating microbial communities in vermicompost tea using filtration (< 0.40 μm) and enrichment culture. The microbial communities associated with whole compost teas, tea filtrates (filter size < 0.40 µm), and a microbial enrichment were characterized using Community Level Physiological Profiling (CLPP) and heterotrophic plate counts. Biochar was then inoculated with whole teas, tea filtrates, or microbial enrichments, mixed with garden soil, and planted with eggplant (Solanum melongena). After 50 days of indoor growth, the plants were placed in the field for one week and plant growth (total leaf area, leaf biomass and root biomass) and herbivory by the eggplant flea beetle (Epitrix fuscula) were measured. 

Results/Conclusions

Eggplants grown in soil amended with biochar + filtered tea showed a significant (< 0.05) decrease in herbivory by flea beetles compared to plants amended with biochar + microbial enrichment.  In addition, plants grown with biochar + tea filtrates and biochar + microbial enrichments also had a significant increase (p< 0.05) in root and leaf biomass compared to biochar + whole tea and/or biochar control.  These results suggest that: 1) there are biological, and perhaps chemical, components in vermicompost tea that can impact plant growth and susceptibility to insect herbivory when used in combination with biochar, and 2) these effects are differentially mediated by the types of microorganisms and chemicals that are present in whole tea, tea filtrate and microbial enrichment as well as the interactions that occur between the biotic and abiotic components of the teas.