Crucial factors determining the response of activated sludge-derived fertilizer particles to crop root exudates

In this study, six types of wastewater activated sludge were collected from South Korea, and the relationship between crop nutrient release kinetics and the compositional, structural and physicochemical properties of both raw activated sludges and mineral-like particles generated through acid/heat digestion followed by alkali precipitation was evaluated. Compared to raw activated sludge, all the particles exhibited up to a 72-fold higher ∆k _H [difference in k _H (Higuchi dissolution constant) values] between HCl- and citric acid-driven pH changes, suggesting that their assembled structures are more vulnerable to disruption by citric acid (i.e., one of the regular root exudates), irrespective of sludge type. Redundancy analysis indicates that the root responsiveness is positively influenced by metallic contents (e.g., Ca) and negatively influenced by hydrophobic aliphatics. Partial least squares-structural equation modeling also reveals that enriched inorganics and phenolic features in particles directly affects their dissolution behaviors. Spearman correlation also suggests that radical scavenging ability, fluorescence wavelengths and some metallic cations (i.e., Ca and Fe) are crucial for root exudate-driven dissolution. Given that radical scavenging ability, fluorescence wavelengths, and phenolic content are associated with phenolic polymer structures, incorporating the phenolic structures and its coordination with metal cations appears to be critical for inducing root responsiveness.