Sorption of Tannin-C by Soils Affects Soil Cation Exchange Capacity.

Some tannins, produced by plants, are able to sorb to soil, and thus influence soil organic matter and nutrient cycling. However, studies are needed that compare sorption of tannins to other related phenolic compounds, evaluate their effects across a broad range of soils, and determine if sorption of tannin-C is associated with soil cation exchange capacity (CEC). Soils collected from across the US and Canada, were treated with model tannins or tannin-related phenolics. Tannin treatments included the mixture of gallotannins known as tannic acid (TA), a specific gallotannin (pentagalloylglucose, PGG), and a condensed tannin from sorghum grain (SOR). Tannin-related phenolics included a flavonoid (catechin, CAT), a phenolic acid (gallic acid, GA), and its ester (methyl gallate, MG). We measured the proportion of compound-C that sorbed by difference and determined CEC by the cobalt-hexamine method before and after treatment. Patterns of sorption varied with treatment, but were consistent across various geographic locations, and for a wide array of soil amendments. Highest sorption of treatment-C was observed for tannins, PGG (60-70%), TA (40-50%) and SOR (25-35%), compared to the phenolic compounds (< 20%). Soil CEC was modified by treatment, but changes were not just a simple function of the amount of treatment-C sorbed. Cation exchange capacity increased with PGG but decreased with GA. These studies indicate that tannins affect both soil C and ion exchange capacity in ways that vary with tannin chemistry and with soil geochemistry. However, more work remains to integrate chemical and biological mechanisms into a functional understanding of soil-tannin interactions that can be used to develop management strategies.