Wood fibers are available as a domestic, renewable raw material for producing peat-reduced substrates. However, their proportion in blends has so far usually been limited to a maximum of 30 % (v/v). One of the main reasons for this is the insufficient stability of wood fibers against microbial decomposition. The associated nitrogen (N) immobilization can impair the N supply in substrate-based plant production. In the present study, an attempt was made to counter this problem by thermal treatment of wood fibers. The aim of the procedure is to reduce the availability of easily decomposable cell wall polymers and to cross-link their degradation products with lignin. A product resulting from the development work showed a stable N balance in an incubation test under laboratory conditions. In a vegetation trial with cherry laurel (Prunus laurocerasus cv. 'Rotundifolia'), the effect of thermal modification of wood fibers on plant growth and nitrogen dynamics was investigated. Thermally treated and untreated wood fibers were mixed with white peat in proportions of 10, 30, and 50% (v/v). In the variants with 50% (v/v) thermally modified wood fibers, a further treatment with the addition of wetting agents were included. A pure white peat substrate served as a control.

After one year of cultivation, the fresh mass yield of cherry laurel, grown in a substrate with untreated wood fibers at a mixing ratio of 30% (v/v), was significantly reduced by about one-quarter compared to the peat control. In substrates with thermally modified wood fibers, growth reductions of a similar magnitude only occurred at a mixing ratio of 50% (v/v). When the thermally modified wood fibers, comprising 50% (v/v) of the substrate mixture, were additionally treated with a wetting agent, the yield reductions of the cultivated shrubs could be limited to about 10%

Trees protect themselves against microbial decay by forming lignin, which encases the easily decomposable cellulose and hemicellulose within the tree’s fibrils. However, when wood chips are fiberized to produce wood fibers for growing media, this matrix is destroyed, allowing microbes easy access to the decomposable cell wall compounds. Due to the high C/N ratio of these compounds, substantial nitrogen immobilization is common in wood-fiber based substrates. To counteract this undesirable phenomenon, the influence of adding lignin to wood fibers was investigated. Lignin can be extracted from “black liquor”, a lignin-rich byproduct of the pulp industries (e. g. paper production). In this study, different types of lignin and procedures to add the lignin on wood fibers have been compared. The general procedure was based on dissolving lignin in a base. The lignin addition to wood fibers was performed in an extruder allowing for intensive mixing of both components. Afterwards an acid was used to precipitate the lignin on the inner and outer surfaces of the wood fibers. When using a lignin extracted via the LignoBoost® process, at least 50 % (m/m) of the added lignin was strongly fixed on the wood fibers even after intensive washing. This indicated that the applied lignin entered the inner surfaces of the wood fibers. When using nitric acid to fix the lignin on the wood fibers, it was found that both microbial respiration and nitrogen immobilization were significantly reduced in incubation studies. Furthermore, the germination of Chinese cabbage was not affected by mixing 49 % (v/v) of a lignin-treated wood fiber in a peat/compost substrate, indicating plant tolerability. However, the addition of lignin also introduces potassium and nitrogen to the growing medium, which should be considered when determining suitable fertilizer dressings.

Wood fibres are among the most important peat substitutes in potting soils and growing media. However, the influence of the defibration process on the wood fibre quality - especially the N immobilization - and energy demand, and thus the carbon footprint, is unknown. Furthermore, nowadays wood fibers are exclusively made form coniferous species. However, due to climate change the proportion of conifers in our forests decreases, making the use of other wood assortments, e. g. beech, inevitable. By a systematic variation of the process parameters a total of 32 diversified fibre samples were produced from fresh spruce and beech using a thermomechanical and unpressurized laboratory refiner respectively. The electrical energy demand and throughput were measured during defibration and a size distribution analysis was carried out to assess the fibre morphology. The N immobilization was determined in a short-term plant response test with Chinese cabbage, from which a N balance was calculated. The investigations show, that irrespective of the processing parameters N immobilization of wood fibres produced by unpressurized refiner process was lower compared to the thermochemical refiner. However, the effect of wood species on N immobilization was considerably larger than of processing. N immobilization of spruce wood fibers was about half as high as of beech wood fibers. From a process engineering point of view, both the thermomechanical and the unpressurized refiner process allow an extensive influence on the fiber morphology and energy demand by adjusting the process parameters. Wood fibres produced in a thermomechanical refiner tend to have a lower shive content and a higher degree of defibration, but due to the less complex and cost-intensive plant technology and simpler process control the unpressurized refiner process is overall considered to be the superior processing technique.

Steam-exploded wood fibers (SEWF), a byproduct of animal feed production derived from a patented process that extracts hemicellulose from wood biomass, offer significant potential as a sustainable alternative to peat in horticulture. This process leaves behind fibrous material rich in cellulose and lignin, which could be less prone to microbial decomposition, making it a promising candidate for stable growing media.

In this study, phytotoxicity and physicochemical properties of SEWF were evaluated, including carbon content, C/N ratio, CEC, and nutrient availability. Results showed that SEWF exhibits higher carbon content and C/N ratios, with pH levels comparable to peat. However, the CEC of SEWF was significantly lower, likely due to its distinct organic composition and reduced acid-functional groups, affecting its buffering capacity. The application of 2 kg/m³ of dolomite lime or 2% biorest effectively raised the pH of SEWF to levels suitable for horticultural use, while biochar had minimal impact. Although SEWF contained lower levels of total and soluble nutrients compared to peat, the higher ratio of soluble to total nutrients suggests relatively better nutrient availability for plant uptake. The dry matter content in SEWF at the end of the plant growth ranged from 30-40%, which is higher compared to peat's 15%, indicating lower water retention and potentially explaining the reduced NO₃-N concentration. Nevertheless, physical tests are needed to verify water retention properties of SEWF. Germination tests revealed that while all seeds emerged with radicles, further development was inhibited, likely due to phytotoxic compounds such as phenols, furfural, and hydroxymethylfurfural generated during steam explosion. These hydrophilic compounds may be mitigated through washing, with ongoing research focused on evaluating germination index and development rate in washed SEWF.

 This study highlights the potential of SEWF to serve as a sustainable alternative to peat, with further processing needed to reduce phytotoxicity and improve water retention.