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

摘要

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.