Can we measure condensed tannins from tannin-protein complexes? - A case study with acid-butanol assay in boreal forest soil organic layer

Tannins may influence nitrogen cycle in forest soil. To understand the role of tannins in the decomposition process, we need to know their amount in the soil. Measurements of soil tannins meet great difficulties as tannins may undergo substantial retention. The main aim of this study was to assess the influence of complexation with proteins on condensed tannins (CT) retention. We measured the amount of extractable CT in the humus layer after additions of i) CT, ii) CT followed by addition of bovine serum albumin, BSA (uncomplexed before addition) or iii) CT-BSA complexes. Samples were taken from the humus layer of two sites; Norway spruce soil, which had high levels of organic matter and CT, and silver birch soil, which had lower levels of organic matter and CT. For tannin additions, we used CT extracted and purified from Norway spruce needles. To estimate extractable CT, we employed acid-butanol assay which is commonly used in condensed tannin chemistry. The complexation of CT with BSA remarkably decreased CT recovery from the humus layer in comparison to CT added alone or without prior complexation. The recovery of added CT also depended on the forest soil, and was higher in spruce soil. As complexation with proteins strongly decreases the recovery of CT it should be taken into account in studies concerning tannins in humus layer. According to our studies, only some of the CT from the CT-BSA complex in humus layer can be detected in the assay.