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:
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.