Nutrient recovery in pyrolysis biochars for agricultural applications–Biochemical compositions and nutrient extraction techniques

Biochar has a very wide range of uses in industrial processes, wastewater treatment, agriculture and soil remediation. Scaling up biochar deposits in soils has been proposed for carbon sequestration in the context of climate mitigation strategies. Although certain contaminants in feedstock can be eliminated through pyrolysis, others persist, and the process itself produces additional hazardous substances. Considerable uncertainty exists regarding interactions with soil microorganisms and longer-term dynamics. Large-scale deposits of biochar in soils may pose risks, and geostorage has been suggested as a safer alternative. Both geostorage and biochar deposits in deep soil layers would restrict availability to plants and soil ecosystems of valuable nutrients, such as phosphorus. Against this background, a systematic search was undertaken in the Web of Science Core Collection of publications up to February 2024 to provide an overview of existing studies on methods to retrieve nutrients from biochar. The search produced a total of 17 relevant publications. Findings point to the need for additional studies and work to develop nutrient extraction methods that are both efficient and environmentally safe. None of the reviewed studies tested the extracted phosphorus as fertilizer, and simultaneous extraction of plant nutrients from biochar might be a more advantageous approach.