Biochar aged or combined with humic substances: fabrication and implications for sustainable agriculture and environment-a review

Purpose: Humic substances (HSs) and biochar (BC) are carbon-based soil amendments. These amendments improve soil health and fertility, enhance nutrient pools and carbon content, remove soil pollutants, and enhance plant performance. As a result, they contribute to agro-environmental sustainability and the development of a circular bioeconomy. However, there is a lack of research on the effects of HSs-aged BC or the co-application of BC and HSs on the agro-environmental system. Therefore, further studies are needed to understand the impacts of these amendments on the agro-environmental system. Methods: This study utilizes a novel technique based on BC aging with HSs to investigate the BC-aging process, factors influencing it, as well as the impact of BC and HSs on soil physicochemical properties, nutrient pools, microbial communities, immobilization of metal ions in the soil, and plant performance. We gathered original research articles, meta-analysis papers, book chapters, conference proceedings, and technical notes from high-quality peer-reviewed journals and reputable websites. Results and discussion: The extensive literature evaluation revealed that the potential benefits of BC are closely related to variations in the physicochemical composition of the BC and soil because microorganisms do not prefer fresh BC for colonization. In some studies, BC showed a detrimental impact on the soil microbiome. Therefore, the influence of BC on the soil microbiome, nutrient pool, pollutant removal, and plant growth strongly depends on the residence time of BC in the soil and its prior aging with HSs. Aging BC with HSs is more effective than using fresh BC as it enhances nutrient pools, accessibility to plants, pollutant amelioration capacity, microbial activities, and consequently, plant performance due to the presence of surface functional groups and the adsorbed nutrient-rich organic molecules. Conclusions: The soil fertility traits and plant performance were impacted by aging or a combination of BC with HSs. However, detailed characterizations and continuous experiments are required to gain in-depth insights into the interaction mechanisms between the aging of BC with HSs via the liquid soaking technique and soil fertility traits.