Do late-successional tannin-rich plant communities occurring on highly acidic soils increase the DON/DIN ratio

Previous studies suggested that late-successional tannin-rich plant communities increase the amount of dissolved organic N (DON) relative to dissolved inorganic N (DIN) in decomposing litter. We devised an experiment to test this hypothesis by adding varying proportions of black spruce (Picea mariana) and tannin-rich Kalmia angustifolia litter to forest floor samples collected on six black spruce cutovers. An increasing proportion of Kalmia litter increased condensed tannin and total phenolic concentrations over the course of a 46-week incubation period. Mineral N concentrations did not vary among treatments in spite of much higher total N concentrations in Kalmia litter. This was more likely due to the formation of protein-tannin complexes rather than microbial immobilization of N, as indicated by the decline in available C with increasing Kalmia litter on two of the five sampling dates. There was a significant positive linear trend between the proportion of Kalmia litter and the DON/DIN ratio on one sampling date (week 13) only. Results suggest that the DON/DIN ratio is controlled by confounding factors (e.g., tannins bonding with non-extractable humus particles) and has limited value for describing ecological succession.