Submitted to: Soil Biology and Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/25/2007
Publication Date: 1/1/2008
Citation: Halvorson, J.J., Gonzalez, J.M. 2008. Tannic acid reduces recovery of water-soluble carbon and nitrogen from soil and affects the composition of Bradford-reactive soil protein. Soil Biology and Biochemistry. 40(1):186-197. Interpretive Summary: Tannins, produced by plants, may interact with many compounds in soil but specific information is needed to determine their effects on nutrient cycling and soil organic matter. We added tannins to soil samples to determine if they would affect the recovery of water soluble soil-C and -N or influence the composition of BRSP, related to the important soil protein, glomalin. Tannins reduced the amount of soluble C and N and affected the amount and composition of BRSP extracted from soil. We conclude tannins can reduce the solubility of soil carbon and nitrogen and the validity of the BRSP assay. This information is important to researchers and land managers needing an understanding of soil organic matter accumulation and needs further study as a potential means for controlling nutrient losses from soil.
Technical Abstract: Tannins are plant-derived polyphenolic compounds that precipitate proteins, bind to metals and complex with other compounds and may be particularly important in soil ecosystems. Solutions of tannic acid, or other phenolic compounds, were added to soil samples to determine if they would affect recovery of soluble soil-C (WSC) or -N (WSN) or influence the extraction and composition of Bradford Reactive Soil Protein (BRSP), associated with glomalin. Tannic acid-C added with water was not completely recovered from samples, and the amount of total net WSC and WSN recovered was reduced, suggesting formation of insoluble complexes and precipitation. By comparison, non-tannin phenolics like gallic acid, or its ester, methyl gallate, substances related to tannic acid, had little effect on extraction of WSC or WSN while a simple gallotannin, derived from tannic acid, inhibited extraction most. The amount of BRSP and carbon and nitrogen concentrations in BRSP increased when samples were treated with tannic acid before extraction, a procedure that includes autoclaving. Increases were greatest in the 10-20 cm depth compared to 0-5 cm. Accompanying these were declines in the ratio of light absorbance at 465 and 665 nm (E4/E6 ratio) of BRSP extracts suggesting formation of larger or heavier molecules. In contrast, C and N composition in lyophilized BRSP were unaffected in 0-5 cm samples and decreased in 10-20 cm samples when tannic acid was added to the BRSP extract solution after the extraction process. We conclude a) tannins can reduce the solubility of labile soil carbon and nitrogen, at least temporarily and b) given unpredictability of response associated with phenolic substances, the Bradford assay should not be relied on to quantify pools or composition of soil proteins like glomalin.