Publication Details

Category Text Publication
Reference Category Journals
DOI 10.5194/hess-23-3503-2019
Licence creative commons licence
Title (Primary) Trajectories of nitrate input and output in three nested catchments along a land use gradient
Author Ehrhardt, S.; Kumar, R. ORCID logo ; Fleckenstein, J.H.; Attinger, S.; Musolff, A.
Source Titel Hydrology and Earth System Sciences
Year 2019
Department CHS; HDG
Volume 23
Issue 9
Page From 3503
Page To 3524
Language englisch
Data and Software links

Increased anthropogenic inputs of nitrogen (N) to the biosphere during the last few decades have resulted in increased groundwater and surface water concentrations of N (primarily as nitrate), posing a global problem. Although measures have been implemented to reduce N inputs, they have not always led to decreasing riverine nitrate concentrations and loads. This limited response to the measures can either be caused by the accumulation of organic N in the soils (biogeochemical legacy) – or by long travel times (TTs) of inorganic N to the streams (hydrological legacy). Here, we compare atmospheric and agricultural N inputs with long-term observations (1970–2016) of riverine nitrate concentrations and loads in a central German mesoscale catchment with three nested subcatchments of increasing agricultural land use. Based on a data-driven approach, we assess jointly the N budget and the effective TTs of N through the soil and groundwater compartments. In combination with long-term trajectories of the CQ relationships, we evaluate the potential for and the characteristics of an N legacy.

We show that in the 40-year-long observation period, the catchment (270 km2) with 60 % agricultural area received an N input of 53 437 t, while it exported 6592 t, indicating an overall retention of 88 %. Removal of N by denitrification could not sufficiently explain this imbalance. Log-normal travel time distributions (TTDs) that link the N input history to the riverine export differed seasonally, with modes spanning 7–22 years and the mean TTs being systematically shorter during the high-flow season as compared to low-flow conditions. Systematic shifts in the CQ relationships were noticed over time that could be attributed to strong changes in N inputs resulting from agricultural intensification before 1989, the break-down of East German agriculture after 1989 and the seasonal differences in TTs. A chemostatic export regime of nitrate was only found after several years of stabilized N inputs. The changes in CQ relationships suggest a dominance of the hydrological N legacy over the biogeochemical N fixation in the soils, as we expected to observe a stronger and even increasing dampening of the riverine N concentrations after sustained high N inputs. Our analyses reveal an imbalance between N input and output, long time-lags and a lack of significant denitrification in the catchment. All these suggest that catchment management needs to address both a longer-term reduction of N inputs and shorter-term mitigation of today's high N loads. The latter may be covered by interventions triggering denitrification, such as hedgerows around agricultural fields, riparian buffers zones or constructed wetlands. Further joint analyses of N budgets and TTs covering a higher variety of catchments will provide a deeper insight into N trajectories and their controlling parameters.

Persistent UFZ Identifier
Ehrhardt, S., Kumar, R., Fleckenstein, J.H., Attinger, S., Musolff, A. (2019):
Trajectories of nitrate input and output in three nested catchments along a land use gradient
Hydrol. Earth Syst. Sci. 23 (9), 3503 - 3524 10.5194/hess-23-3503-2019