Plant fibers for renewable growing media: potential of defibration, acidification or inoculation with biocontrol fungi to reduce the N drawdown and plant pathogens.

Plant fibers allow for the partial replacement of peat in growing media and have potential to carry and sustain biocontrol organisms. However, they may also carry plant pathogens and they may vary in their degree of biodegradability and their interaction with N, and in this way thus affect the quality of growing media.

We tested 3 types of plant fibers (flax, miscanthus and reed) for replacement of peat in growing media. In order to (1) kill plant pathogens present on the plant fibers, (2) reduce the N fixation risk of the fibers and (3) provide fibers serving as carrier of biocontrol fungi, various defibration techniques were tested, including extrusion, retruding, disc refining and steam explosion.

Flax shives obtained from various sources were naturally colonized by viable microsclerotia of the plant pathogen Verticillium dahliae. When sufficiently high temperatures were reached during defibration, this technique killed the plant pathogen on the shives.

Untreated flax shives, miscanthus or reed straw were characterized by high N immobilization and thus reduce N availability for plants if used as growing medium. We found a clear positive correlation between the risk for N immobilization versus pH, the hemicellulose content and water-extractable C concentration of the fiber.

Unlike peat, defibrated pure miscanthus, reed straw and flax shives were easily colonized by fungal biocontrol strains. Inoculation of extruded miscanthus fibers with biocontrol fungi did not increase nor decrease the N immobilization. Reducing the pH of the fibers was effective in reducing the microbial activity and the N fixation.

Defibrated plant fibers have potential for peat replacement and as carrier for biocontrol fungi. Peat replacement is important in relation to the circular economy, climate change mitigation (prevention of carbon loss and greenhouse gas emissions) and the conservation of fragile ecosystems. Successful application of biocontrol fungi may reduce the need for using chemical crop protection agents in horticulture.