Export of dissolved organic carbon and nitrogen from Gleysol dominated catchments - The significance of water flow paths

In this study, we estimated whether changes in hydrological pathways during storms could explain the large temporal variations of dissolved organic carbon (DOC) and nitrogen (DON) in the runoff of three catchments: a forest and a grassland sub-catchment of 1600m2delineated by trenches, and a headwater catchment of 0.7 km2. The average annual DOC export from the sub-catchments was 185 kg DOC ha-1y-1for the forest, 108 kg DOC ha-1y-1for the grassland and 84 kg Doc ha-1y-1for the headwater catchment. DON was the major form of the dissolved N in soil and stream water. DON export from all catchments was approximately 6 kg N ha-1y-1, which corresponded to 60% of the total N export and to 50% of the ambient wet N deposition. DOC and DON concentrations in weekly samples of stream water were positively correlated with discharge. During individual storms, concentrations and properties of DOC and DON changed drastically. In all catchments, DOC concentrations increased by 6 to 7 mg DOC 1-1compared to base flow, with the largest relative increment in the headwater catchment (+350%). Concentrations of DON, hydrolysable amino acids, and phenolics showed comparable increases, whereas the proportion of carbohydrates in DOC decreased at peak flow. Prediction of DOC and DON concentrations by an end-member mixing analysis (EMMA) on the base of inorganic water chemistry showed that changes in water flow paths largely explained these temporal variability. According to the EMMA, the contribution of throughfall to the runoff peaked in the initial phase of the storm, while water from the subsoil dominated during base flow only. EMMA indicated that the contribution of the DOC and DON-rich topsoil was highest in the later stages of the storm, which explained the highest DOC and DON concentrations as the hydrograph receded. Discrepancies between observed and predicted concentrations were largest for the reactive DOC compounds such as carbohydrates and phenolics. They occurred at base flow and in the initial phase of storms. This suggests that other mechanisms such as in-stream processes or a time-variant release of DOC also played an important role.