Linking the composition of dissolved organic matter and nutrient cycling in streams of temperate forested catchments

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Dissolved organic matter (DOM) is an important component of the carbon (C) and nutrient cycles in terrestrial and aquatic systems. Changing environmental conditions, such as decreasing acidic and nitrogen deposition and changing water regimes with more frequent and intense storm and drought events, strongly influence the dynamics of DOM with unknown effects on important ecological functions of DOM as its role in C and nutrient cycling. The stoichiometry of dissolved organic carbon (DOC), nitrogen (DON) and phosphorous (DOP) plays a crucial role in regulating ecosystem functioning. Soils determine the input of organic matter into the streams and depending on hydrological conditions surface organic horizons or subsoil mineral horizons will control amount and composition of stream DOM. However, changes in DOM composition because of changes on its source have not been linked to the role of DOM for nutrient cycling. Thus, the overall objective of the requested project is to shed light upon the complex interactions at the interface of terrestrial and aquatic ecosystems controlling amount, composition and functionality of DOM in streams. Our overarching hypothesis is that the composition of DOM determines its contribution to nutrient cycling in forested streams, i.e. serving as a C, N or P source for microorganisms. The selected forested catchments with differing bedrock and soil offer a suitable gradient in potential nutrient limitations to study the envisaged effects of DOM composition on its function. DOM composition will be analyzed along the soil-water-continuum (i.e. linking the composition of soil organic matter and DOM) in the selected catchments at different hydrological conditions. Differences in DOM composition will be used to determine the most important DOM sources in the catchment depending on the flow conditions. These studies on DOM composition will be linked to nutrient pulse addition experiments in the streams determining whether DOM serves mainly as a C, N or P source for microorganisms. This strong focus on field observations and experiments will be accompanied by incubation experiments in the laboratory to further elucidate the linkages between the composition of soil organic matter and DOM and the consequences for nutrient cycling along the terrestrial aquatic continuum. Methods of different complexity will be combined to determine the composition of organic matter in soils and water (Fluorescence Excitation-Emission Matrices, Fourier-Transform Infrared Spectrometry, Pyrolysis Gas Chromatography Mass Spectrometry, 31P-Nuclear Magnetic Resonance Spectroscopy). Only the combination of the unique expertise of the Czech and German partners with the well characterized long-term monitoring catchments of the Czech partner and the modern lab equipment of the German partner will ensure the success of the project, i.e. elucidating the role of DOM composition for its ecological function under the pressure of ongoing environmental changes.