Abstract. 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 C–Q 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
C–Q 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 C–Q
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.
Low-flow frequency curves for selected long-term stream gaging stations in eastern United States
