Qualitative and quantitative changes in water-extractable organic compounds in the organic horizon of boreal coniferous forests

In order to investigate the changes in soil organic matter along a natural decomposition gradient, we determined the concentrations and stocks of water-extractable carbon (WEC), water-extractable nitrogen (WEN), soluble phenolics and sugars in the litter (L), fermentation (F), and humus (H) layers on two site types (mesic Norway spruce and sub-xeric Scots pine forests) and in two climatic zones (north and south boreal). The WEC, WEN, phenolic and sugar concentrations decreased considerably from the L to the H layer, but the reduction was stronger in the north than in the south. Despite the highest concentrations in the L layer, the stocks of water-extractable organic matter (WEOM) were the highest in the H layer. Our investigation demonstrates for the first time that the organic stocks in the H layer outweigh the higher concentrations of water-extractable compounds in the L layer, thus making the H layer the most important stock for water-extractable compounds.