Plant fibers for renewable growing media: acidification or inoculation with biocontrol fungi to reduce the N drawdown in extruded Miscanthus straw

Peat based growing media are not sustainable and not easily colonized by biocontrol organisms. Plant fibers may allow for partially replacing peat in growing media and they may have the potential to carry biocontrol organisms. However, they may also carry plant pathogens and they may vary in their degree of biodegradability and their interaction with N, and thus they may affect the quality of growing media.
Several defiberization techniques were tested for flax shives, miscanthus and reed straw for their ability 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.
Flax shives obtained from various sources were colonized by viable microsclerotia of the plant pathogen Verticillium dahliae. However, when sufficiently high temperatures were reached during defiberization, this technique killed the plant pathogen on the shives, allowing the use of defiberized flax shives in growing media.
Untreated flax shives, miscanthus or reed straw were characterized by high N immobilization. Using these plant fibers as peat replacement in growing media may thus reduce N availability for plants. According to the incubation trials results, extruding, retruding and applying disc refining significantly reduced the N immobilization for miscanthus straw without affecting the pH of the fiber, while steam explosion significantly reduced the pH and resulted in higher N immobilisation. Extruding and retruding the flas shives increased the N fixation.
Defiberized pure miscanthus or reed straw and flax shives were easily colonized by fungal biocontrol strains, e.g. Trichoderma spp. . The effect of inoculation with fungi or acidification on N fixation was tested for extruded miscanthus fibers. Inoculation with shiitake (Lentinula edodes) or Agrocybe aegerita, i.e., 2 fungi without biocontrol properties was effective in reducing the N immobilization only in a specific pH range and at high colonization degree and/or for fungi with fast colonization, but never increased the N fixation when successfully inoculated. Inoculation of extruded Miscanthus fibers with different strains of biocontrol fungi did not increase nor decrease the N immobilization.
Reducing the pH of the fibers between pH 2.5 and 4.7 was effective in reducing the microbial activity and the N fixation. Finetuning the conditions is necessary when applying acidification or inoculation for reducing the N fixation in plant fibers.
N immobilization in growing media is related to biological activity. We compared 3 methods to determine the microbial biomass on plant fibers in growing media incubated with mineral N fertilizers, i.e., fumigation-extraction, plfa and Q-PCR, and used water-extractable C to measure the easily available C in the plant fibers.
The extrusion of miscanthus straw with or without inoculation of a biocontrol organism reduced the N immobilization when compared with the pure fiber. In case of flax shives, defiberization of plant fibers kills plant pathogens that may be present on the fibers when sufficiently high temperatures are reached during defiberization. Inoculation of plant fibers with biocontrol fungi may be beneficial for increasing the disease resistance in growing media, and we found that inoculation of extruded Miscanthus fibers with biocontrol fungi did not increase nor decrease the N immobilization.