Seasonal variations in the chemical composition of dissolved organic matter in organic forest floor layer leachates of old-growth Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) stands in northeastern Bavaria, Germany

Organic matter dissolved in the percolation water of forest soils contribut es largely to element cycling and transport of natural and anthropogenic co mpounds. The way and extent to which these processes are affected depends o n the amount and the chemical composition of soluble organic matter. Becaus e the amount of soluble organic matter varies seasonally with changes in th e microbial activity in soil, it seems reasonable to assume that there may be also seasonal changes in the chemical composition of dissolved organic m atter. We examined dissolved organic matter in the seepage waters of organi c forest floor layers over a 27-month period (1997-1999) in two forest ecos ystems, a 160-year-old Scots pine (Pinus sylvestris L.) stand and a 90-year -old European beech (Fagus sylvatica L.) forest. The forest floor leachates were analysed for bulk dissolved organic C, C in hydrophilic and hydrophob ic dissolved organic matter fractions, lignin-derived phenols (CuO oxidatio n), hydrolysable neutral carbohydrates and uronic acids, hydrolysable amino sugars, and stable carbon isotope composition. In addition, we studied the samples by use of liquid-state C-13-nuclear magnetic resonance (NMR) spect roscopy. For both investigated forest sites we found that the dissolved organic carb on concentrations in forest floor leachates were largest during summer. The y peaked after rain storms following short dry periods (106-145 mg dissolve d organic C l(-1)). The proportions of C in the hydrophilic fractions were largest in winter and spring whereas in summer and autumn more C was found in the hydrophobic fraction. According to liquid-state C-13-NMR spectroscop y, summer and autumn samples had larger abundances of aromatic and aliphati c structures as well as larger proportions of carboxyl groups whereas the w inter and spring samples were dominated by resonances indicating carbohydra tes. Wet-chemical analyses confirmed these results. Winter and spring sampl es were rich in neutral carbohydrates and amino sugars. The summer and autu mn samples contained more lignin-derived phenols which were also stronger o xidised than those in the winter and spring samples. Seasonal changes of de lta C-13 values were found to reflect the changes in the chemical compositi on of dissolved organic matter. Most negative values occurred when isotopic ally light lignin-derived compounds were abundant and less negative values when carbohydrates predominated. The different vegetation, age of the stands, and underlying mineral soils r esulted in different concentrations of dissolved organic carbon and in diff erences in the distribution between hydrophobic and hydrophilic organic car bon. Despite of this, the results suggest that the trends in temporal varia tions in the composition of dissolved organic matter in forest floor seepag e water were remarkably similar for both sites. Dissolved organic matter in winter and spring seems to be mainly controlled by leaching of fresh disru pted biomass debris with a large contribution of bacterial and fungal-deriv ed carbohydrates and amino sugars. Dissolved organic matter leached from th e forest floor in summer and autumn is controlled by the decomposition proc esses in the forest floor resulting in the production of strongly oxidised, water-soluble aromatic and aliphatic compounds. The chemical composition o f dissolved organic matter in forest floor seepage water in winter and spri ng indicates larger mobility, larger biodegradability, and less interaction with metals and organic pollutants than that released during summer and au tumn. Thus, the impact of dissolved organic matter on transport processes m ay vary throughout the year due to changes in its composition.