Concentrations of secondary compounds in Scots pine needles at different stages of decomposition

A litterbag experiment was conducted to study chemical changes in decomposition of Scots pine needle litter in central Finland. Concentration of secondary metabolites (terpenes, resin acids and total phenolics) and main nutrients (N, P, K, Ca and Mg) were analysed four times over a 19-month period. After 19 months decomposition, the mass loss by pine needle litter was about 22%. Initial concentrations of monoterpenes and total phenolics were 36 and 27% lower in needle litter than in green needles, respectively, while more (44%) resin acids were found in needle litter than in green needles. Concentrations of monoterpenes were 6%, resin acids 35% and total phenolics 17% of the initial concentration after 19 months decomposition. In green needles and senescent brown needles most common monoterpenes were α-pinene and 3-carene. During the decomposition monoterpenes, sabinene, myrcene, limonene+β-phellandrene, terpinolene and bornylacetat, were lost to a greater extent than α-pinene, camphene and tricyclene. At the end of decomposition experiment several oxygen-containing hydrocarbons were detected in terpene samples. The most commonly identified compounds were verbenone and verbenol. Resin acid composition also changed substantially during decomposition, neoabietic acid decomposed faster than other resin acids. Dehydroabietic acid was the main resin acid in needle litter after 19 months decomposition. The concentrations of N, P and Ca significantly increased during decomposition, while concentrations of Mg and K decreased. The results suggest that degradation of secondary organic compounds in needle litter is a slow process and these compounds might have effects on decomposer organisms for several years after needle abscision.