Drivers of spatial and temporal variability of dissolved organic matter across the terrestrial−aquatic continuum

Dissolved organic matter (DOM) is an important component in carbon and nutrient cycles in terrestrial and aquatic ecosystems. For three decades, concentrations of dissolved organic carbon (DOC) have been increasing in European and North American surface water bodies. The increase has been mainly attributed to export of DOC from terrestrial ecosystems. Depending on the hydrological regime in a catchment (stormflow vs. baseflow conditions), the flow pathways through different soil horizons are varying and in result, the drivers determining the amount and chemical composition DOM vary as well. By studying soil water and surface water at the catchment scale, we aim at identifying the main sources and environmental conditions driving the ongoing trend of increasing DOM in aquatic ecosystems.

To understand the spatial and temporal variations of the export of DOM from soils to surface waters the catchment of the drinking water reservoir Sosa in the Ore Mountains (Germany) is instrumented and monitored along the terrestrial−aquatic continuum for 1 ½ years. We installed plate lysimeters and suction cups to collect soil water at three soil depths, including topsoil organic and subsoil mineral horizons at four different sites (peatland, degraded peatland, cambisol and podzol) representing the potential terrestrial DOM sources within the catchment. In addition, two tributaries of the reservoir were equipped with fluorescence-based probes to continuously monitor DOC. Water samples were taken fortnightly and event-based during heavy rain and snowmelt. All soil and stream water samples were analyzed for DOC, dissolved organic nitrogen (DON), as well as inorganic cations and anions. To identify possible DOM sources, the DOM composition of all samples was additionally analyzed by fluorescence spectroscopy (Excitation-Emission-Matrices – EEMs).

We found the different soils contributed differently to the aquatic DOM, depending on seasons and hydrological conditions. The highest DOC concentrations in the organic layer and upper mineral horizon of the podzol did not correspond with high average DOC concentrations in the stream. However, the stream affected by the peatland had much higher DOC concentrations. All organic topsoil horizons had low DOC concentrations in winter and high concentrations in summer, but only streams fed by peat soils followed this pattern. During stormflow events (snowmelt and strong rainfall), both monitored streams showed DOC concentrations 5 to 6 times higher than the average, illustrating the large potential of all soils (i.e. peatlands, cambisols, podzols) for DOM export. The DOC:DON ratios clearly reflect the differences in DOM composition of the different soils, with high proportions of plant-derived DOM in soil water and the corresponding streams. In conclusion, our research indicates that organic soils, such as peatlands, contribute most to stream DOM under baseflow conditions, while under high-flow conditions, as during snowmelt or rainstorms, mineral soils become additional strong DOM sources. Ongoing analyses of the DOM composition will provide further insights into specific DOM sources and the related spatial and temporal variations of DOM export from soils to surface waters.