Sophie Ehrhardt1, Pia
Ebeling1, Rémi Dupas2, Rohini
Kumar3, Jan H. Fleckenstein1,4, and
Andreas Musolff1
1Department of Hydrogeology, Helmholtz-Centre for
Environmental Research, 04318 Leipzig, Germany.
2UMR SAS, INRAE, Institut Agro, 35000 Rennes, France.
3Department of Computational Hydro Systems,
Helmholtz-Centre for Environmental Research, 04318 Leipzig, Germany.
4Bayreuth Center of Ecology and Environmental
Research, University of Bayreuth, 95440 Bayreuth, Germany.
Corresponding author: Sophie Ehrhardt (sophie.ehrhardt@ufz.de)
Key Points:
- Time lags of nitrogen transport in Western European catchments were
five years on average and mainly explained by hydroclimatic
variability
- Almost three-quarters of the diffuse N input was retained in the
catchment, mainly controlled by subsurface parameters and specific
discharge
- Biogeochemical legacy likely exceeded hydrologic legacy in most of the
238 analyzed catchments
Abstract
Excess nitrogen (N) from anthropogenic sources deteriorates freshwater
resources. Actions taken to reduce N inputs to the biosphere often show
no or only delayed effects in receiving surface waters hinting at large
legacy N stores built up in the catchments’ soils and groundwater. Here,
we quantify transport and retention of N in 238 Western European
catchments by analyzing a unique data set of long-term N input and
output time series. We find that half of the catchments exhibited peak
transport times larger than five years with longer times being evident
in catchments with high potential evapotranspiration and low
precipitation seasonality. On average the catchments retained 72% of
the N from diffuse sources with retention efficiency being specifically
high in catchments with low discharge and thick, unconsolidated
aquifers. The estimated transport time scales do not explain the
observed N retention, suggesting a dominant role of biogeochemical
legacy in the catchments’ soils rather than a legacy store in the
groundwater. Future water quality management should account for the
accumulated biogeochemical N legacy to avoid long-term leaching and
water quality deteriorations for decades to come.
Plain language summary
Despite different regulations that limit anthropogenic nitrate input to
the biosphere, there is in many cases no or only delayed improvement in
groundwater or surface water contamination. One reason for this mismatch
are legacies either by accumulated nitrate in the soil or nitrate with
slow transport pathways in the groundwater to the river. We assessed
long-term data covering nitrate in- and output for Western-European
catchments to quantify (1) the needed transport time until reappearance
in the river and (2) the quantity of reappeared nitrate.
The transport time through the catchment had its peak at 5 years and was
mainly controlled by hydrological parameters as high seasonality in
precipitation favored faster transports. Furthermore 72% of the nitrate
was retained in the catchment, mainly controlled by subsurface
characteristics as thick and unconsolidated material favored retention
either by holding nitrate in the soil or by supporting a bacterial
process that released nitrate to the atmosphere. We hypothesized that
most of the retained nitrate is accumulated in the soil. This huge pool
has on the one hand the potential of being recycled and on the other
hand the danger of leaching slowly, which would constitute a future
long-lasting contamination source for groundwater and surface waters.
1. Introduction
Nitrogen (N) can be a limiting nutrient in terrestrial, freshwater and
marine ecosystems (Webster et al., 2003). However, the N cycling in
these ecosystems is modified and disturbed by humans through inputs from
atmospheric deposition, agricultural fertilizers and waste water. High N
inputs especially in economically developed countries have led to
increased riverine dissolved inorganic nitrogen (DIN) fluxes, causing
ecological degradation in aquatic systems and posing a threat to
drinking water safety (Dupas et al., 2016; Sebilo et al., 2013;
Wassenaar, 1995). Diffuse agricultural sources (mineral fertilizer and
manure) constitute most of the N emissions into waters in European
countries (Bouraoui and Grizzetti, 2011; Dupas et al., 2013).
Several regulations at federal, national or international levels have
been implemented e.g. the EU Nitrate Directive (CEC, 1991) or the Clean
Water Act (EPA, 1972) in the US – aiming particularly at reducing N
inputs to the terrestrial system. Despite the reduction in inputs, there
is often no or only little improvement in water quality observed in many
catchments (Meals et al., 2010; Bouraoui and Grizzetti, 2011; Vero et
al., 2017). The inadequacy of implemented measures to improve water
quality can be related to transport and retention in the catchments
responding to changes in the nutrient inputs. The latter is closely
connected to a legacy accumulation of N (e.g. Thomas & Abbott, 2018;
Van Meter & Basu, 2015; Wang & Burke, 2017) - a buildup of large N
stores in the catchment that are not or only slowly exported. This
legacy acts as long-term memory of catchments and has been hypothesized
to buffer stream concentration variability (Basu et al., 2010).
N legacies can be attributed to two major components: the biogeochemical
and the hydrologic N storage. The first one is related to biogeochemical
transformation processes of N in the unsaturated (vadose) zone, often
leading to a large buildup of an organic N pool in the soil matrix and
only slowly converting to mobile nitrate (NO3; Van Meter
& Basu, 2017). Hydrologic legacy describes the pool of dissolved N in
the groundwater and unsaturated zone, subjected to very slow transport
processes (Van Meter & Basu, 2015). This transport is controlled by the
travel time, i.e., the time rainfall needs to travel through a catchment
(Kirchner et al., 2000). The diversity of subsurface flow paths in a
catchment creates a distribution of travel times (Kirchner et al., 2000)
varying from days to decades (e.g. Howden et al., 2011; Jasechko et al.,
2016; McMahon et al., 2006; Sebilo et al., 2013) also integrating
information on timing, amount, storage and mixing of water and thus
solutes (Heidbüchel et al., 2020). Therefore, slow travel times and a
resulting temporary storage of reactive N in the unsaturated zone
(Ascott et al., 2017; Ehrhardt et al., 2019), can create similar time
lags as the biogeochemical legacy of N stored in the soil N pool
(Bingham & Cotrufo, 2016; Bouwman et al., 2013; Sebilo et al., 2013).
Due to the high complexity of hydrological and biogeochemical processes
in catchments, a good understanding of the share of the two different
legacy storages and the fate of N remains challenging.
Data-based joint quantification and characterization of N transport
timescales and retention under different land-use and management
practices can provide an evidence based entry point to better understand
N trajectories for reactive N transport at catchment scale (e.g.
Ehrhardt et al., 2019; Van Meter and Basu, 2015). More specifically,
comparing quantity and temporal patterns of diffuse N input and riverine
N concentrations from catchments allow to estimate N transport time (TT)
scales as well as retention (Dupas et al., 2020; Ehrhardt et al., 2019).
Retention is defined here as the “missing N” that is either stored in
a catchment due to the buildup of legacies or permanently removed by
denitrification. The estimated TT of N integrates time delays by
biogeochemical immobilization and mobilization in the soils and the TT
through the vadose zone and groundwater. So far, only a few studies
investigated retention and TTs simultaneously as availability of
long-term data often limits the number of studied catchments (e.g. Dupas
et al., 2020; Ehrhardt et al., 2019; Howden et al., 2010; Van Meter et
al., 2017; Van Meter et al., 2018) although the identification and
quantification of legacy effects is of critical importance for
predicting future N dynamics and for implementing effective restoration
efforts (Bain et al., 2012). Here we analyze a large-sample database of
238 Western European catchments with different geophysical and
hydro-climatological characteristics and at least 20 years of
observations with regards to observed nitrogen (1) TT scales and (2)
retention. Furthermore, we connect these results to catchment
characteristics to discuss their (3) main controlling factors. These
research objectives are used to improve the understanding of catchment
responses to changes in input and the fate of retained N being
associated with different legacy stores and/or denitrification.
2. Materials and Methods
2.1. Study area
For data on water quantity and quality, we relied on three national data
sets. Water quality data for French catchments are publicly available at
http://naiades.eaufrance.fr/, while water quantity data are
available at http://hydro.eaufrance.fr/. For Germany, Musolff
(2020) provided a database for water quality and water quantity.