Sorption of tannin and related phenolic compounds and effects on soluble-N in soil

Some tannins, plant-derived polyphenolic compounds, can rapidly affix to soil and affect the solubility of labile soil-N but a more complete understanding of the nature and persistence of tannin-soil interactions is needed. Forest and pasture soils from two depths were treated for 1 h with cool (23 °C) water (Control) or solutions that added 10 mg g−1 soil tannic acid (TA), an imprecisely defined mixture of galloyl esters, gallic acid (GA), a phenol, or β-1,2,3,4,6-penta-O-galloyl-d-glucose (PGG), a hydrolyzable tannin. Soluble-C and N, in treatment supernatants, was measured to uncover evidence for sorption of treatments or effects on extraction of soil-N. Significant amounts of soluble-C, added with treatments, were not recovered in supernatants indicating sorption of nearly 90% of the PGG-C, about 75% of the TA-C but less than 25% of the GA-C in surface soil. Disappearance of soluble-C from treatment supernatants was accompanied by a corresponding reduction of total phenolic content. Treatments added a negligible amount of N to soil; but while PGG and TA reduced soluble-N, in extracts from surface soil, GA had little effect. Soluble-N in extracts was composed mainly of organic-N. Effects of tannins persisted in surface soil through 12 washings with hot water (80 °C), suggesting the formation of stable complexes with soil. The proportion of initial soil-C and N remaining after all extractions was higher in samples treated with PGG or TA than either the Control or GA treatment. We conclude PGG readily sorbs to soil and reduces the solubility of soil organic-N unlike GA, its simple monomeric constituent. These differences could be especially important near the surface where quantities of soil organic matter and biological activity are comparatively large and most easily affected by management.