Contribution of exogenous humic substances to phosphorus availability in soil-plant ecosystem: A review

Phosphorus (P) is one of the largest nutrients limiting crop productivity. Meanwhile, P deficiency is a common phenomenon in agricultural soils around the world. Humic substances, as macromolecular polymer, accelerate and strengthen process which transforms P into bio-available forms via a range of chemical reactions and biological interactions. There is now an urgent need to comprehend the work carried out on the interaction among humic substances, soil and plant to better understand their role in the transformation and promotion of soil bioavailable P for plant growth. Herein, we discuss the factors and mechanisms of humic substances influencing P cycling in soil-plant systems, which focus on their contribution to soil P mobilization and plant P acquisition. This review covers how humic substances influence the mobilization and transformation of P in soils, including release of P from residues, and competitive adsorption of P and humic acid or fulvic acid to metallic minerals, as well as exchange with P adsorbed by humic substances. It then discusses a range of contributions to plant available P acquisition such as the release of organic acids from roots caused by humic substances, and promoting the solubilize and/or hydrolyze phosphate by plant and their associated microbes. Notably, we also discuss the challenges of artificial humic substances influencing P cycling in soil-plant systems, which may alleviate the global deficit of soil P resources. Overall, humic substances have become promising for sustainable agriculture over time and have great potential to meet specific soil-plant systems.