Chemical Changes During 6 Years of Decomposition of 11 Litters in Some Canadian Forest Sites. Part 2. 13C Abundance, Solid-State 13C NMR Spectroscopy and the Meaning of Lignin

There is still a poor understanding of how changes in the organic composition of litter contribute to slowing or even cessation of decomposition. Using C-13 nuclear magnetic resonance (NMR) spectroscopy of samples from the Canadian Intersite Decomposition Experiment (CIDET), we asked whether increasing lignin per se could account for the well-known increase in acid-unhydrolyzable residue (AUR), and secondly, using three litters from four sites with different mean annual temperatures, whether changes in organic composition would follow similar trajectories with C mass loss. At 6 years, there was 16-39% C remaining for 10 foliar litters and wood blocks at a site with rapid initial decomposition, and higher amounts remaining for three species at three colder sites. C-13 NMR spectra obtained with rapid cross-polarization (CP) mainly showed increasing similarity among the foliar litters, although wood showed little change in composition. Foliage generally showed loss of O- and di-O-alkyl C, mainly from carbohydrate, and increase in alkyl, aromatic, phenolic and carboxyl C. However, O-alkyl C loss was limited, especially for litters with slow initial decomposition, and many litters showed relatively small changes in intensity distribution. Quantitative C-13 (BD) spectra showed similar trends, but even smaller changes in C composition, and 6-year CP difference spectra showed that C was lost across the whole range of structures. Changes in delta C-13 were small and variable, but could be correlated to some extent with loss of carbohydrates versus tannins. Lignin was not selectively preserved, and the increase of resistant structures derived from lignin, tannins, and cutin collectively accounts for increasing AUR. Compositional changes of NMR C fractions across sites with different temperatures were small and inconsistent, likely due to the influence of other site factors; however, changes in their contents did largely follow consistent trajectories with %C remaining.