Phenolic compounds and terpenes in soil organic horizon layers under silver birch, Norway spruce and Scots pine

The aim of this study was to monitor the concentration of some plant secondary metabolites, such as low- and high-molecular-weight phenolics, condensed tannins (proanthocyanidins), and sesqui-, di- and triterpenes, in litter (L), fermentation (F) and humified (H) layers of the soil organic horizon in stands dominated by silver birch (Betula pendula Roth.), Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.), and from samples taken from birch leaves and spruce and pine needles. Concentrations of low- and high-molecular-weight phenolics and terpenes from the four most dominant species of ground vegetation taken from the stands were also determined. In general, the L layer showed higher concentrations of both phenolic compounds and terpenes than the F and H layers did. Concentrations of terpenes decreased relatively more with soil depth than did concentrations of total phenolics (=low+high) or condensed tannins. Of the total phenolics, the proportion of low-molecular-weight phenolics increased from the L to the H layer with all tree species. Concentrations of all terpenes were highest under pine and lowest under birch. Concentrations of the studied secondary metabolites in the ground vegetation species were similar under different tree species. Blueberry (Vaccinium myrtillus L.) and lingonberry (Vaccinium vitis-idaea L.) contained considerably higher concentrations of total phenolics than did feather moss (Pleurozium schreberi (Brid.) Mitt.) and wavy hair-grass (Deschampsia flexuosa (L.) Trin.). Concentration of total phenolics in soil correlated positively with soil respiration and microbial biomass C, and terpenes showed positive correlation with soil C-to-N ratio.