Tannin surges increase metal oxide loss and potentially destabilize soil aggregates

The unclear mechanisms of interaction between plant-derived tannins and soil metal oxides (Al, Fe, Mn), especially in tannin-rich ecosystems like eucalyptus plantations, hinder accurate predictions of long-term soil health under continuous tannin exposure. Therefore, controlled soil column leaching experiments were conducted using different tannin types and concentrations. Metal oxide speciation (free, amorphous, and complexed) was quantified by a modified parallel extraction method, while aggregate stability was assessed through mean weight diameter (MWD) and geometric mean diameter (GMD) measurements. Results demonstrate that hydrolyzable tannins (e.g., gallic acid) exhibit superior metal mobilization capacity compared to condensed tannins (e.g., epigallocatechin gallate), establishing tannin chemistry as the primary determinant of oxides reactivity. A sequential dissolution-recomplexation pathway was identified, resulting in a 4 %-26 % decrease in amorphous oxides coupled with a 6 %-149 % increase in complexed oxides. The strong positive correlation between aggregate stability and complexed manganese content confirms its crucial cementing role. These findings resolve the apparent paradox of short-term stabilization versus long-term degradation in tannin-rich ecosystems by revealing that while initial complexation enhances aggregation, persistent tannin input ultimately depletes amorphous oxides, compromising soil structure. This mechanistic understanding provides a scientific basis for assessing metal-leaching risks and for developing sustainable management strategies in affected ecosystems.