Lignin-based metal chelate fertilizers: preparation, properties and applications

Lignin, as a renewable natural polymer, is recognized as a promising metal chelating agent due to its reactive functional groups capable of chelating metal ions. However, the relatively limited number of reactive functional groups in lignin restricts its chelating capacity to a certain extent and hence limits its application as a metal chelating fertilizer. This review focuses on various lignin modification methods, including sulfonation, sulfomethylation, amination, oxidation, carboxymethylation, demethylation, and phenolization, which can increase the number of functional groups available for metal chelation in lignin. Furthermore, the methods for preparing lignin-based metal chelate fertilizers utilizing modified lignin with necessary classifications and summaries were systematically introduced. The application effects of lignin-based metal chelate fertilizers in plant cultivation are discussed, with particular emphasis on their role in enhancing plant nutrient absorption, improving soil fertility, alleviating plant stress, and reducing the uptake of heavy metals by crops. Overall, the aim of this review is to provide theoretical support and practical guidance for the further application of lignin in the agricultural field as an indispensable component in metal chelate fertilizers.