Fig. 1

Mechanisms of the effects of organic matter on soil P chemistry. 1. Competitive sorption can occur in three scenarios: (1) OM blocking surface charge on clay or oxide mineral leads to phosphate desorption; (2) OM adsorbing on negatively charged surface of clay or oxide mineral leads to negative phosphate repulsion; (3) OM complexing with Fe adsorbed on clay or oxide mineral surface leads to the formation of Fe–OM which can be released, leaving a positively charged surface available for phosphate sorption. 2. Competitive metal complexation occurs via ternary complexation between SOM and phosphate through cations like Ca2+, depending on SOM structures. In the absence of binary complexation with either Ca2+ or Fe3+ (red circle), the negative charge on SOM is unavailable to adsorb CaPO4−. However, when the negative charge on SOM is accessible, CaPO4− can form a ternary complex (blue circle). 3. Mineralization of Organic P: microbes induce enzymatic activities to break down organic P compounds into inorganic phosphate. This process involves the degradation of mono-, di-, and phosphotriesters by specific enzymes which release inorganic phosphate for plant uptake