Evidence for the role and fate of water-insoluble condensed tannins in the short-term reduction of carbon loss during litter decay

Abstract

Warmer temperatures associated with climate change have the potential to accelerate litter decay and subsequently release large amounts of carbon stored in soils. Condensed tannins are widespread secondary metabolites, which accumulate to high concentrations in many woody plants and play key roles in forest soil nutrient cycles. Future elevated atmospheric CO2 concentrations are predicted to reduce nitrogen content and increase tannin concentrations in plant tissues, thus reducing litter quality for microbial communities and slowing decomposition rates. How the distinct condensed tannin fractions (water-soluble, acetone:MeOH-soluble and solvent-insoluble) impact soil processes, has not been investigated. We tested the impact of condensed tannin and nitrogen concentrations on decay rates of poplar and Douglas-fir litter at sites spanning temperature and moisture gradients in coastal rainshadow forests in British Columbia, Canada. The three condensed tannin fractions were quantified using recent improvements on the butanol-HCl assay. Decay was assessed based on carbon remaining, while changes in litter chemistry were primarily observed using two methods for proximate chemical analyses. After 0.6 and 1 year of decay, more carbon remained in poplar litter with high, compared to low, condensed tannin concentrations. By contrast, more carbon remained in Douglas fir litter than poplar litter during this period, despite lower condensed tannin concentrations. Rapid early decay was especially attributed to loss of soluble compounds, including water-soluble condensed tannins. Water-insoluble condensed tannin fractions, which were transformed to acid-unhydrolyzable residues over time, were associated with reduced carbon loss in high condensed tannin litter.