A review on the production of nutrient-enriched biochar: Insights from the evolution of nitrogen, phosphorus, and potassium

Biochar, a carbonaceous solid produced by thermochemical conversion of biomass, is commonly used for soil improvement. In comparison with the biochars derived from wood chips and grass, the biochar produced from nutrient-enriched feedstocks (e.g., sewage sludge, animal manure, microalgae, and wetland plants) contained higher contents of nutrients and showed greater potential for soil application. To achieve the efficient utilization of nutrients in biochar, the behavior of nitrogen (N), phosphorus (P), and potassium (K) during pyrolysis of nutrient-enriched feedstock should be clarified. This study provides a systematic review of the migration and transformation of N, P, and K during the production of biochar derived from nutrient-enriched biomass, with special emphasis on the effects of nutrient-enriched biochar on soil available nutrients. Perspectives and challenges for agricultural applications of biochar are discussed as well. The migration and transformation of N, P, and K are affected by the pyrolysis temperature and the properties of the feedstock during the pyrolysis process. The application of nutrient-enriched biochar could provide additional nutrients, change the physicochemical properties of soil, and modify microbial community to elevate the content of soil available nutrients. Therefore, the production of nutrient-enriched biochar from N/P/K-enriched biomass wastes for soil applications is a promising scheme for sustainable agriculture.