Recent Advances and Research Trends in the Eco-friendly Lignin-based Fertilizers: Production Technologies, Process Mechanisms and Performance Appraisal

In circular bioeconomy viewpoint, sustainable development of biorefineries requires to use the lignocellulosic biomass completely to produce high value-added bioproducts. The lignin products market was 750 million US$ in 2018 and is predicted to reach 1460 million US$ by the end of 2025. In industrial processes, the large amounts of industrial lignins as technical lignins or biorefinery lignins are formed as by‐products. Annually, about 150-180 million tons of industrial lignins is generated in worldwide. Most of industrial lignins are directly combusted to obtain heat, which not only is a loss of organic matter but also leads to environmental pollution issues. Therefore, developing the lignin-based fertilizers has become an important research topic for eco-friendly agricultural practices. Interestingly, lignin can be used as slow‐release carriers, coating materials, soil conditioners and chelated micro-fertilizers due to its excellent slow-release properties, chelating and other functionalities. Lignin-based fertilizers have several specific properties including the slow dissolution, adsorptivity, biocompatibility, controlled-release, biodegradability, nonvolatility, long-term stability, anti-leaching, low pollution, high fertilizer efficiency, low price and higher biological activity. Lignin can delay the dissolution of the nutrients (nitrogen, phosphate, etc.) in the modified fertilizer to improve the slow-release performance and also delay hydrolysis of urea by inhibiting the soil urease activity and inhibit the conversion of NH4+-N to NO3--N, thereby increasing the utilization of NH4+-N. Lignin‐based fertilizers prepared by sustainable chemical, coating and micro-chelation modifications. This review, exhaustively scrutinizes and reports the recent research advances in the lignin extraction methods, underlying mechanisms, characterization and applications of the above methods for preparing lignin‐based fertilizers.