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 contributes largely to element cycling and transport of natural and anthropogenic compounds. The way and extent to which these processes are affected depends on the amount and the chemical composition of soluble organic matter. Because the amount of soluble organic matter varies seasonally with changes in the microbial activity in soil, it seems reasonable to assume that there may be also seasonal changes in the chemical composition of dissolved organic matter. We examined dissolved organic matter in the seepage waters of organic forest floor layers over a 27-month period (1997-1999) in two forest ecosystems, 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 hydrophobic dissolved organic matter fractions, lignin-derived phenols (CuO oxidation), 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) spectroscopy. For both investigated forest sites we found that the dissolved organic carbon concentrations in forest floor leachates were largest during summer. They peaked after rain storms following short dry periods (106-145 mg dissolved 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 spectroscopy, summer and autumn samples had larger abundances of aromatic and aliphatic structures as well as larger proportions of carboxyl groups whereas the winter and spring samples were dominated by resonances indicating carbohydrates. Wet-chemical analyses confirmed these results. Winter and spring samples were rich in neutral carbohydrates and amino sugars. The summer and autumn samples contained more lignin-derived phenols which were also stronger oxidised than those in the winter and spring samples. Seasonal changes of delta C-13 values were found to reflect the changes in the chemical composition of dissolved organic matter. Most negative values occurred when isotopically light lignin-derived compounds were abundant and less negative values when carbohydrates predominated. The different vegetation, age of the stands, and underlying mineral soils resulted in different concentrations of dissolved organic carbon and in differences in the distribution between hydrophobic and hydrophilic organic carbon. Despite of this, the results suggest that the trends in temporal variations in the composition of dissolved organic matter in forest floor seepage water were remarkably similar for both sites. Dissolved organic matter in winter and spring seems to be mainly controlled by leaching of fresh disrupted biomass debris with a large contribution of bacterial and fungal-derived carbohydrates and amino sugars. Dissolved organic matter leached from the forest floor in summer and autumn is controlled by the decomposition processes in the forest floor resulting in the production of strongly oxidised, water-soluble aromatic and aliphatic compounds. The chemical composition of dissolved organic matter in forest floor seepage water in winter and spring indicates larger mobility, larger biodegradability, and less interaction with metals and organic pollutants than that released during summer and autumn. Thus, the impact of dissolved organic matter on transport processes may vary throughout the year due to changes in its composition.