Organic carbon in streams—effects of agricultural land use practices | Organischer Kohlenstoff in Bächen – Auswirkungen ackerbaulicher Bewirtschaftungspraktiken

Agriculture is the dominant land use form in Lower Austria, covering more than 45% of the total area. Agriculture delivers significant amounts of dissolved organic carbon (DOC) to our streams, changing basic metabolic processes at the water-sediment interface and affecting the ecological state and the health of the aquatic systems. The aim of this study was to investigate the effects of agricultural practices on the amount and the quality of dissolved organic carbon discharged into streams and to clarify the consequences of these inputs onto the cycling of aquatic carbon and the oxygen consumption within the aquatic systems.In summer 2017 and 2018, we took water samples from streams with different land use, wastewater effluents, and the Danube and analyzed them for concentrations of dissolved organic carbon and nutrients. In laboratory experiments, we studied the effects of leachates from different organic sources, such as forest soil, cropland soil, leaves, and manure on the microbial respiration in stream sediments. In addition, we performed percolation experiments to investigate the effects of fertilization on the concentration of dissolved organic matter leached from different soil types.The concentrations of both dissolved organic carbon and nutrients increased with increasing agricultural and urban land use. In addition, we observed an enrichment with labile organic components, indicating increased pollution by microbes and elevated microbial turnover rates. Labile organic matter sources, such as e.g. manure leachate, significantly increased the microbial oxygen consumption in the sediments. However, the potential of stimulating microbial respiration depends on the amount and quality of the organic carbon sources as well as on the structure and the pollution of the sediments. In our percolation experiments, the application of nitrate-based fertilizers, whether mineral or organic, significantly decreased the amount of DOC in soil pore water. We assume that this reduction was caused by an increased mineralization of the organic matter in the soil, stimulated by the imported nitrate. Our study highlights the significance of a sufficiently long soil passage to protect streams in agricultural regions from excessive inputs of labile organic carbon. This factor gains in importance in the face of the predicted increase of long dry periods and heavy rain events in the future, which will increase the input of soil material into surface waters.