Toward Understanding of Long‐Term Nitrogen Transport and Retention Dynamics Across German Catchments

Abstract

Long‐term nitrogen (N) transport and retention dynamics across catchments are not well understood. Using a process‐based model for 89 German catchments, results across study catchments reveal that most N surplus (during 1950–2014) was removed by denitrification (mean ± standard deviation: 58 ± 15%) while the remaining fraction was mostly stored in the soil (14% ± 11%). The mean groundwater transit times in these catchments varied from 3.2 to 20.3 years. These results indicate that past N inputs could continue to affect surface and groundwater quality in the coming years. We identified four catchment groups with distinct archetypal N transport and retention dynamics, which are linked to the catchments' climate, topographic, and geological conditions. Overall, our results shed light on long‐term N dynamics in German catchments and how they are linked to catchment characteristics, emphasizing the role of long‐term N accumulation and transport for water quality management and evaluation programs.

The value of instream stable water isotope and nitrate concentration data for calibrating a travel time‐based water quality model

“…A better constraint on the parameter α corresponded to a narrower 95PPU of the TT 50 time series in Experiment 3 (Figure 5c) compared to Experiment 1 (Figure 5a). This is because α is one of the determining factors influencing water age selection preference (Nguyen, Kumar, et al, 2022; Nguyen, Sarrazin, et al, 2022), thus affecting both the TTD and TT 50 . We also found a correlation between α and β in Experiment 1 ( r = 0.41), which decreased in Experiment 2 ( r = 0.34) and 3 ( r = 0.38).…”

“…The subsoil hydrochemical conditions of the Upper Selke allow for more effective denitrification than the Lower Selke. Despite showing a generally smaller TT50 than the Lower Selke (Nguyen, Sarrazin, et al, 2022; Winter et al, 2021), the Upper Selke is characterized by a significant TT50 variability (Figure 5), which allows denitrification under low‐flow conditions (see Da50 number in Figure 8). In contrast, the Lower Selke, has a more stable TT50 time series and, although having a generally longer TT50 than the Upper Selke, the transport time scale is still shorter than the very long reaction time scale due to very low denitrification rates.…”

“…In contrast, the Lower Selke, has a more stable TT50 time series and, although having a generally longer TT50 than the Upper Selke, the transport time scale is still shorter than the very long reaction time scale due to very low denitrification rates. As a consequence, denitrification is limited (Nguyen, Sarrazin, et al, 2022; Winter et al, 2021). Additionally, Hannappel et al (2018) showed a clear sign of ongoing denitrification in the Upper Selke based on groundwater samples.…”

“…These three experiments aim to explore simulated TTDs, calibrated parameters and their correlations, when using different calibration targets. In all experiments, we did not re‐simulate the hydrological fluxes (i.e., Q and I ), and the NO 3 − leaching flux (i.e., NO 3 − in I ) with the mHM model, but instead we took the best simulation from a prior study by Nguyen, Kumar, et al (2022), Nguyen, Sarrazin, et al (2022) for the same area. We selected the simulation of Q , I and NO 3 − in I with the highest efficiency for observed Q and instream NO 3 − concentrations, ensuring the most accurate representation of hydrological and water quality processes for our study.…”

“…Several initiatives have attempted to regulate agricultural fertilizer inputs such as the Nitrate Directive (EU Commission, 1991) and the Water Framework Directive (EU Commission, 2020). Despite these efforts, high nitrate concentrations still impact water quality (Bodirsky et al, 2014; Bouraoui & Grizzetti, 2014; Van Meter et al, 2023) due to legacy effects, that is, accumulation of N in the soil and long transport times of nitrate through soil and groundwater (Lutz et al, 2022; Nguyen, Sarrazin et al, 2022; Van Meter & Basu, 2017). This situation can create time delays between implemented measures and actual improvements (Kopáček et al, 2013; Puckett et al, 2011; Van Meter et al, 2016), and calls for further actions to enhance water quality status (Solomon et al, 2015) and protect drinking water supplies (Carrard et al, 2019).…”