Nutrient losses from artificially drained catchments in North-Eastern Germany at different scales

Abstract

Tile drainage, which is common agricultural practice to improve moisture and aeration conditions of the soil, shortens the residence time of water in the soil and therefore contributes to the still elevated diffuse pollution levels of surface water bodies. To evaluate the environmental impacts on surface water bodies, it is a matter of particular interest how the tile drain discharge and its solute signal translate to the higher scales i.e. ditches, streams and rivers. At the experimental field site ‘Dummerstorf’ in the federal state Mecklenburg-Vorpommern (Germany), a hierarchical water quantity and quality measurement program comprising automatic sampling stations at a tile drain outlet, a ditch and a brook was therefore conducted in a small (16 km 2) rural lowland catchment (2003–2005). The flow rates and the solute concentrations showed parallel patterns on all scales. In the case of nitrate–nitrogen, concentrations invariably increased at increasing flow rates while sulphate–sulphur, magnesium, and, to a lesser extent, potassium concentrations tended to decrease at increasing flow rates. These patterns indicate, firstly, the importance of artificial drainage on the larger catchment's hydrology and hydrochemistry and, secondly, a heavy nitrate–nitrogen enrichment of the soil. Nevertheless, a scale effect due to mixing, land use diversity and in-stream processes causing lower NO 3–N as well as higher SO 4–S and K concentrations at higher scales was observed. At all scales, nitrate–nitrogen concentrations frequently exceeded the drinking water guideline of 11.3 mg l −1 and often fell within the worst water quality class (>20 mg l −1) in Germany. High loss rates always occurred at high flow rates, and in the case of nitrate, an increase of the flow rate evoked a disproportionately high increase of the solute load. Generally, the loss rates of nitrogen were low to average with 3.4–21.8 kg ha −1, but relatively high taking into account the comparably low flow rates measured. Loss rates of the other solutes were low (SO 4–S: 23.2–57.9 kg ha −1, K: 1.9–6.2 kg ha −1, Mg: 5.7–22.0 kg ha −1). However, as only years with a below average amount of precipitation could be considered, higher loss rates of all solutes are to be expected in wetter years.