Abstract
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.
Highlights
In terrestrial, freshwater and marine ecosystems nitrogen (N) species are essential and often limiting nutrients (Webster et al, 2003; Elser et al, 2007)
Based on the above discussion we find for our study catchment, the denitrification in the soils, including the riparian zone, may partly explain the retention of NO3–N, but there is unlikely to be a single explanation for the observed imbalance between input and output
Based on our data-driven analyses, we propose the following conceptual model (Fig. 7) for N export from the Holtemme catchment, which is able to plausibly connect and synthesize the available data and findings on travel times (TTs), concentration trajectories and C–Q relationships and allows for a discussion on the type of N legacy
Summary
Freshwater and marine ecosystems nitrogen (N) species are essential and often limiting nutrients (Webster et al, 2003; Elser et al, 2007). Changes in strength of their different sources like atmospheric deposition, wastewater inputs and agricultural activities caused major changes in the N cycle (Webster et al, 2003). Two major innovations from the industrial age accelerated anthropogenic inputs of reactive N species into the environment: artificial. The different input sources of N show diverging rates of change over time and space. While the atmospheric emissions of N oxides and ammonia have strongly declined in Europe since the 1980s (EEA, 2014), the agricultural N input through fertilizers declined but is still at a high level (Federal Ministry for the Environment and Federal Ministry of Food, 2012). In the cultural landscape of western countries, most of the N emissions in surface and groundwater bodies stem from diffuse agricultural sources (Bouraoui and Grizzetti, 2011; Dupas et al, 2013)
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