Effects of tannins on soil carbon, cation exchange capacity, and metal solubility

Background/Question/Methods

Tannins sorb to soil, affect the solubility of soil nitrogen, and soil chemical processes that may be important for the formation of soil organic matter and nutrient cycling. However, studies are needed, comparing different classes of tannins and related compounds, to determine if soils have a maximum storage capacity for phenolics; if tannins have a role in the solubility/mobility of metals; and if tannins influence important soil properties like cation exchange capacity. To address these questions, forest and pasture soils, collected at 0-5 and 10-20 cm depth, were repeatedly treated with water (control) or solutions containing tannins or related compounds. After each application, the amount of compound that sorbed to the soil was determined as were metals in solution.

Results/Conclusions

With each successive application, less treatment carbon (C) was bound to soil, until a maximum storage capacity was reached. The amount of bound-C was several times higher for tannins than for smaller phenolic compounds. Surface soil had the highest storage capacity for tannins. Sequential washes of tannin “loaded” soil with cool and hot water resulted in the release of some treatment-C indicating it was weakly held by the soil but the majority of tannin-C remained on the soil. Direct measurements of total soil C at the end of the experiment confirmed that meaningful amounts of each compound remained on soil. Cation exchange capacity increased in samples treated with a gallotannin but decreased in samples treated with its simple monomeric constituent. The amount of Ca and Mn detected in solution varied among the different phenolic treatments. Higher Ca content found in solution may result from the acidity of the solutions of phenolic compounds. Higher Mn in solution may result from the redox reaction of Mn (IV) oxides with the phenolic compounds, producing the soluble Mn²⁺and quinones. Quinones are very reactive compounds that can self-polymerize and/or copolymerize with other biomolecules to form humic-like substances. These studies contribute to an assessment of the impacts of tannins and other phenolic compounds on soil organic matter formation, nutrient cycling, and toxicity risk of some metals and improve the management of soils.