Tannins Influence Soil Chemical Processes.

Tannins, plant secondary compounds, can affect soil and water quality by interacting with inorganic and organic compounds.  However, the fate of tannins and their effect on soil metal cycling dynamics and soil chemical processes is poorly understood.  We examined the effects of commercial available tannins and related compounds on the solubility of plant nutrients and toxicants in soils.  Soils collected from across the US and Canada, were treated, with tannins or phenolic compounds and the supernatant was analyzed for metals.  There was a tannin or phenolic effect on the type metal extracted.  Highest amounts of extracted Ca, Mg, K, and Mn occurred when soils were treated with gallic acid, whereas highest amount of extracted Al and Fe were obtained with methyl gallate.  These results suggest tannins and related phenolics interact with metals by (a) ligand exchange and (b) redox reactions.  Ligand exchange might be particularly important for plant nutrient dynamics when fertilizers and amendments are applied to supply Ca, Mg, and K.  Conversely, redox reactions affect transition metals such as Mn which is reduced and solubilized from Mn(IV) to Mn(II) and tannins or related phenolics are oxidized; the oxidized products of tannins might lead to the formation of humic-like substances through the formation of polymers with other soil organic compounds.  Soil management may be improved through an understanding of factors that affect the quality and quantity of plant secondary compounds like tannins entering soil.  These may not only to increase plant productivity through improved nutrient availability but also to enhance formation and stabilization of soil organic matter to increase nutrient cycling and ultimately soil and water quality.  In addition, the extraction of Al and Fe, together with the solubilization of Mn, might be an environmental concern, since these metals can be toxic to plants and pollute water.