Biochar and Ash Fertilization Alter the Chemical Properties of Basket Willow (Salix viminalis L.) and Giant Miscanthus (Miscanthus x giganteus)

This article presents the results of a two-year study investigating the effects of the fertilization of soil, with biochar and ash from plant biomass, on selected properties of the pyrolysis products obtained from basket willow (Salix viminalis L.) and giant miscanthus (Miscanthus x giganteus). The study was designed to determine whether soil enrichment through the use of organic fertilizers (ash added at the rate of 1.5 t ha−1, biochar added at the rate of 11.5 t ha−1 and a combination of them) in the cultivation of energy crops would affect the quality of pyrolysates obtained from these plants. The research goal was to use biochar and biomass ash to produce high-quality pyrolysates with fertilizing potential. The aboveground parts of the plants were subjected to the pyrolysis process, which was carried out in constant conditions, i.e., a temperature of 500 °C and duration of 10 min. The pyrolysates obtained were examined for their pH value, the content of absorbable forms of phosphorus (P2O5), potassium (K2O) and magnesium (Mg), as well as total carbon and the total content of selected macro- and micro-elements. The results of the current study show the beneficial effects of these soil fertilizers, reflected by the high quality and enhanced mineral contents of the biochars obtained. The highest total increase in the contents of absorbable forms of P, K and Mg was found in the pyrolysis products from basket willow fertilized with ash alone, amounting to 21.6% in relation to the pyrolysates from the control sample. As for the pyrolysates from the biomass of giant miscanthus, the greatest total increase in the contents of the elements, amounting to 44.4%, was identified when biochar and ash were used in combination. Soil amendments such as biochar and ash used for growing bioenergy crops can alter the aboveground plant quality. The subsequent pyrolysates created from these plants may be enriched and can be an alternative to mineral fertilizers. Natural amendment, such as high-quality pyrolysates, can be used in the cultivation of many plants. Additionally, conversion of plant biomass into pyrolysates is important for the environment, affecting the balance of carbon in the atmosphere through its capture and storage in a stable form outside the atmosphere, e.g., in soil.