Response of soil C and N transformations to condensed tannins and different organic N-condensed tannin complexes

One of the factors that may influence N cycling in boreal forest soils is the ability of tannins to precipitate proteins and certain other organic N compounds, which may lead to creation of recalcitrant complexes. The objective of this study was to determine how additions of condensed tannins (CT) and CT-organic N compounds (OrgN; bovine serum albumin-BSA, chitin, D-ribulose 1,5-diphosphate carboxylase-Rubisco) affect carbon and nitrogen transformations in birch soil. Condensed tannins were extracted from Norway spruce (Picea abies (L) Karst.) needles. Soil samples were taken from the organic layer at two study sites, Kivalo (N-poor soil) and Kerimaki (N-rich soil), and incubated with the above-mentioned compounds in laboratory conditions for one month. For purposes of comparison, uncomplexed OrgN was added to soil samples. Carbon dioxide evolution (C mineralization), net N mineralization and N and C in the microbial biomass were then measured. Addition of CT decreased the amount of C in the microbial biomass, and rates of C mineralization (only in N-poor soil) and net nitrification, suggesting a toxic and/or protein-precipitating effect. Additions of uncomplexed OrgN increased both CO2 production and net mineralization of N. In the N-poor Kivalo soil, additions of uncomplexed OrgN caused a greater increase than did addition of OrgN-CT complexes; in N-rich Kerimaki soil, such pattern was clear only for Rubisco. In N-poor Kivalo soil, additions of chitin and BSA increased the amount of N in microbial biomass, but again the increase was greater for uncomplexed OrgN than for the complex. Changes in amount of C in the microbial biomass (both soils) and in N in the microbial biomass in N-rich Kerimaki soil were less clear. Our study indicates that OrgN-CT complexes undergo some digestion in soil. Depending on the soil, complexes with chitin may act similarly to protein-tannin complexes. (C) 2012 Elsevier B.V. All rights reserved.