Synthesis of artificial humus by hydrothermal carbonization and application to improve soil quality: a review

Global warming, pollution, and industrial agriculture are degrading soils worldwide and threatening food security, thus calling for advanced methods to improve soil quality and fertility, and to sequester carbon. Here, we review the use of artificial humus with focus on the synthesis of artificial humus by hydrothermal carbonization of biomass. We detail the reaction mechanism, parameters controlling the reaction, differences between artificial and natural humus, laboratory and industrial production, biomass components, effects on soil nutrients and microbial biomass, remediation of saline-alkali land, and life cycle assessment. After reaction at 150–230 °C, the conversion rate is high, and the properties of artificial humus are very similar to those of natural humus. The application of this artificial humus improves soil nutrient availability, saline-alkali land, microbial characteristics, and the overall soil health. In saline-alkali soils, the application of artificial humus increases the cation exchange capacity up to 1.78 times, with a short-term pH decrease of 0.26–1.0 and an electrical conductivity decrease of 37–60 µS·cm⁻¹. Nevertheless, large-scale application of this technology still faces challenges such as high equipment investment costs, fluctuations in product characteristics between batches, and the lack of unified quality assessment standards.