Litter decomposition and humus characteristics in Canadian and German spruce ecosystems: information from tannin analysis and 13C CPMAS NMR

Influences of litter and site characteristics were investigated during the decomposition of black spruce (Picea mariana (Mill.) B.S.P.) and Norway spruce (Picea abies (L.) Karst.) needle litter in litterbags in two black spruce sites in Canada (6 and 12 months) and two Norway spruce sites in Germany (6 and 10 months). Mass losses were greater for black spruce litter (mean 25.2%) than for Norway spruce (20.8%), despite lower quality of black spruce litter in terms of lower N (10.1 versus 17.1 mg g−1), higher C-to-N ratio (49.0 versus 30.3) and higher content of alkyl C (surface waxes and cutin), indicated by CPMAS 13C NMR spectroscopy. However, Norway spruce litter was higher in condensed tannins than black spruce (37.8 and 25.3 mg g−1, respectively). Tannins were lost rapidly from both species, especially in the first 6 months, with losses in 10–12 months of 75–89% of the fraction extractable in acetone/water and 40–70% of the residual fraction. Losses were greater in the German sites (mean 75.2%, 10 months, versus 68.4%, 12 months), which had earthworms present and higher temperature, precipitation and catalase activity, the latter being positively correlated with tannin loss. There was a much larger contrast in the organic layers; with the Canadian sites having lower C-to-N ratios and higher N concentrations (C-to-N, 20.3 and 29.7; N, 26.0 and 13.8 mg g−1 for Canadian and German sites, respectively). The 13C NMR spectra showed that they were poorly decomposed and unusually high in condensed tannins (consistent with chemical analysis of 28.7 and 37.6 mg g−1, Canada; and 3.5 and 5.0 mg g−1, Germany), with depletion of lignin structures. Differences in other inputs (bark, wood, roots, understorey vegetation) and in site properties (climate, decomposer community, earthworm activity) may be responsible for the considerable differences in humus properties, which would not be expected from differences in the chemical composition and short-term decomposition of needle litter. The tannin accumulation, lignin depletion and N sequestration in the black spruce sites may be related to accumulation of unavailable N and associated forest management problems in these ecosystems.