Nitrogen and dissolved organic carbon (DOC) losses from an artificially drained grassland on organic soils

Abstract. Nitrate–nitrogen (NO₃–N) as well as dissolved organic carbon (DOC) and nitrogen (DON) concentrations and losses were studied for three and two years, respectively, in a small catchment dominated by a degraded peatland used as intensive grassland. Concentrations in the shallow groundwater were spatially and temporally very variable, with NO₃–N being the most dynamic component (7.3 ± 12.5 mg L⁻¹) and ranging from 0 to 79.4 mg L⁻¹. Average NO₃–N concentrations of 10.3 ± 5.4 mg L⁻¹ (0 to 25.5 mg L⁻¹) in the ditch draining the catchment and annual NO₃–N losses of 19, 35 and 26 kg ha⁻¹ confirmed drained peatlands as an important source of diffuse N pollution. The highest NO₃–N losses occurred during the wettest year. Resulting from concentration of 2.4 ± 0.8 mg L⁻¹ (0.7 to 6.2 mg L⁻¹), DON added a further 4.5 to 6.4 kg ha⁻¹ to the N losses and thus formed a relevant (15%) component of the total N losses. Ditch DOC concentrations of 24.9 ± 5.9 mg L⁻¹ (13.1 to 47.7 mg L⁻¹) resulted in DOC losses of 66 kg ha⁻¹ in the wet year of 2006/2007 and 39 kg ha⁻¹ in the dry year of 2007/2008. Ditch DOC concentration were lower than the groundwater DOC concentration of 50.6 ± 15.2 mg L⁻¹ (14.9 to 88.5 mg L⁻¹). Both DOC and N concentrations were governed by hydrological conditions, but NO₃–N reacted much faster and clearer on rising discharge rates than DOC, which tended to be higher under drier conditions. In the third year of the study, the superposition of a very wet summer and land use changes from grassland to arable land in a part of the catchment suggests that, under re-wetting conditions with a high groundwater table in summer, NO₃–N would diminish quickly, while DOC would remain on a similar level. Further intensification of the land use, on the other hand, would increase N losses to receiving water bodies.