Chemical Changes During 6 Years of Decomposition of 11 Litters in Some Canadian Forest Sites. Part 1. Elemental Composition, Tannins, Phenolics, and Proximate Fractions

Slowing or even cessation of litter decomposition with time is well-known, but there is insufficient understanding of the chemical changes that contribute to increasing recalcitrance. Samples from the Canadian Intersite Decomposition Experiment (CIDET) were used to determine 6-year chemical changes for all 11 litters from a site with rapid initial decomposition (Morgan Arboretum, MAR) and for three litters at three colder sites. Six-year mass remaining was 17-37% at MAR, with higher values at the colder sites. Atomic C/N ratios declined and phenolics and condensed tannins generally decreased to minimal values. However, for the three species compared across four sites, phenolics and tannins showed small increases for species with the lowest initial values and also tended to increase with increasing mass loss. For the foliar litters at MAR, there was an average increase in proportion of acid-unhydrolyzable residue (AUR) and decreases in proportions of acid-hydrolyzable (ACID) and extractable fractions, with final AUR/(ACID + AUR) ratios within 0.55-0.66. Principal component analysis showed that foliar litters (and to a lesser extent wood) became more alike after 6 years, decomposition being associated with increase of Fe, Al, N, and AUR concentrations and decrease of K, Mg, tannins, phenolics, and non-polar and water-soluble fractions. However, litters were also affected by site soil chemistry, with some high 6-year accumulations of Ca, Mg, Fe, Al, Mn, and Mg at two sites. Increasing recalcitrance likely arises from increasing dominance of complex, less-soluble organic structures, collectively represented by AUR, together with increases in heavy elements such as Al and Fe, which also specifically bind and stabilize organic matter.