Biochar and Hydrochar from Organic Side-Streams Induce Species-Specific Responses in Plants

Biochar (BC) and hydrochar (HC) have gained attention as multipurpose materials for soil improvement, plant growth promotion, carbon removal and waste reduction – key components of the circular economy. Heterogeneous side-stream and waste biomasses together with thermochemical processes produce variable char products whose effects on soil properties and plants may be divergent and dependent on the environmental context. We produced BC and HC from demolition wood, tomato leaves and biogas digestate and characterized chemical and physical properties of feedstock materials and char products. Growth chamber experiments were established to evaluate the phytotoxicity of the char products and their effects on seedling emergence, biomass and nutrient concentrations in three crop indicator species (barley, radish and rape) grown in a boreal soil. We found that both slow pyrolysis and hydrothermal carbonization (HTC) produce mostly good-quality char products within the limits of the EU Fertilising Products Regulation (FPR) 2019/1009. The effects of BC and HC applications on soil properties reflected the characteristics of the char products and application rates. In general, all char treatments increased soil pH, carbon content and organic matter. Effects of char products on emergence and growth depended on the plant species, sowing time, char treatment and application rate, with many interactive effects of treatments. Immediate phytotoxicity was shown by the highest application rate (50 t ha−1) of tomato BC which reduced emergence in all species. Overall, char application had explicit positive effects on biomass in barley only. Our results suggest that pyrolysis and HTC are valuable methods in processing different side-stream and waste materials into char products that are potential soil amendments. Harmful effects of char application are rare with high-quality products, but the observed patterns of short-term plant responses emphasize the need for environment- and species-specific testing of soil amendments.