FORAGE MANAGEMENT SYSTEMS FOR SMALL-SCALE RUMINANT PRODUCTION IN THE APPALACHIAN REGION
Title: Kinetics and binding capacity of six soils for structurally defined hydrolyzable and condensed tannins and related phenols
Submitted to: Journal of Soils and Sediments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 18, 2011
Publication Date: January 11, 2012
Citation: Schmidt, M.A., Halvorson, J.J., Gonzalez, J.M., Hagerman, A.E. 2012. Kinetics and binding capacity of six soils for structurally defined hydrolyzable and condensed tannins and related phenols. Journal of Soils and Sediments. 12:366-375.
Interpretive Summary: In this study we investigate chemical interactions between tannins and soils. Tannins are produced by plants and are present in plant tissues such as leaves, bark, and roots. Tannins enter the soil during rain storms when water washes through leaves, or during autumn when leaves start to decompose. Once in the soil tannins alter soil processes that are important to the agricultural industry or to forest ecosystem management. Understanding the chemical interaction between tannins and soil may help manage the economic and environment impacts of agricultural processes. This study used six different model compounds that represent the two major classes of tannins and six different soil types to determine how fast tannins bind to soil, how much tannin binds, and how tannin and soil properties affect binding. All of the tannin compounds bound to soil quickly, with the slowest compound binding to soil within 6 hours of being applied. The maximum amount of tannin that binds to soil is dependent on both the size and water-solubility of the compound. Maximum binding was observed with the least water-soluble compounds, so that for some compounds almost all the material applied to the soil was retained. The ability of soils to bind tannin could be predicted from four soil properties (sand, silt, clay, and pH). We also developed an extraction method to remove tannins from soils for analysis. This method uses several different solvents to extract the different types of tannins that may be present in soil. This method is a novel extraction procedure that is capable of removing more tannin from soils then previous extraction methods. This work is useful for soil scientists assessing potential impacts tannins have on agricultural and ecosystem processes.
We investigated tannin-soil interactions by assessing the kinetics of sorption and sorption capacities, and their relationship to the chemical properties of six polyphenolic compounds and the textures of six soils. We developed a new extraction procedure for recovering tannins from soil samples by successive extraction with solvents of decreasing polarity. Sorption of polyphenolic compounds: methyl gallate, catechin, oenothein B, pentagalloyl glucose, epigallocatechin gallate, and procyanidin dimer was determined using six soils with textures ranging from 7% silt-89% sand to 52% silt-22% sand. Sorption kinetics and capacity of the soils were determined using room temperature mixing with high performance liquid chromatography (HPLC) to determine polyphenol concentration. Tannins were extracted from soils loaded with known amounts of the model compounds using solvents with a range of polarities, and establishing recovery by HPLC. Multivariable regression was used to establish relationships between polyphenol properties and sorption, and between soil texture and sorption. Sorption of the polyphenols followed the Langmuir isotherm with unique binding capacity and kinetics of sorption for each compound. Tannin sorption was correlated to molecular weight and polarity. For an Ultisol pasture soil, up to 18.7 mg g-1 soil of the large, hydrophobic compound epigallocatechin gallate was bound compared to only 5.11 mg g-1 soil of the smaller, more polar compound methyl gallate. Kinetics of sorption also varied with sorption reaching equilibrium between 75 to 350 min. Silt and sand composition affected polyphenol sorption in a unique fashion for each polyphenol. Using sequential extraction with solvents ranging from polar (water) to non-polar (hexane) we extracted up to 42% of the material that had been loaded on the soil as a mixture of five polyphenols. This work demonstrates that the likely fate of tannins and related polyphenols from plant sources is rapid sorption, with little likelihood of release of unmodified tannins from the soil by leaching. Tight sorption impedes recovery and analysis of soil tannins, but better methods for extraction may improve our knowledge of tannins in the soil.