Nitrogen and dissolved organic carbon (DOC) losses from an artificially drained grassland on organic soils
Nitrate–nitrogen (NO 3–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 NO 3–N being the most dynamic component (7.3 ± 12.5 mg L −1) and ranging from 0 to 79.4 mg L −1. Average NO 3–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 NO 3–N losses of 19, 35 and 26 kg ha −1 confirmed drained peatlands as an important source of diffuse N pollution. The highest NO 3–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 NO 3–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, NO 3–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.