Elevated nutrient inputs challenge the health and functioning of aquatic ecosystems. To improve riverine water quality management, it is necessary to understand the underlying biogeochemical and physical processes, anthropogenic drivers and their interactions at catchment scale. We hypothesize that the spatial heterogeneity of nutrient sources dominantly controls the variability of in‐stream concentration dynamics among catchments. We investigated controls of mean nitrate (NO₃⁻), phosphate (PO₄³⁻), and total organic carbon (TOC) concentrations and concentration‐discharge (C‐Q) relationships in 787 German catchments of a newly assembled data base, covering a wide range of physiographic and anthropogenic settings. We linked water quality metrics to catchment characteristics using partial least squares regressions and random forests. We found archetypalC‐Qpatterns with enrichment dominating NO₃⁻and TOC, and dilution dominating PO₄³⁻export. Both the mean NO₃⁻concentrations and their variance among sites increased with agricultural land use. We argue that subsurface denitrification can buffer high nitrogen inputs and cause a decline in concentration with depth, resulting in chemodynamic, strongly positiveC‐Qpatterns. Mean PO₄³⁻concentrations were related to point sources, though the low predictive power suggests effects of unaccounted in‐stream processes. In contrast, high diffuse agricultural inputs explained observed positive PO₄³⁻C‐Qpatterns. TOC levels were positively linked to the abundance of riparian wetlands, while hydrological descriptors were important for explaining TOC dynamics. Our study shows a strong modulation of anthropogenic inputs by natural controls for NO₃⁻and PO₄³⁻concentrations and dynamics, while for TOC only natural controls dominate observed patterns across Germany.

The protection of water resources and development of mitigation strategies require large-scale information on water pollution such as nitrate. Machine learning techniques like random forest (RF) have proven their worth for estimating groundwater quality based on spatial environmental predictors. We investigate the potential of RF and quantile random forest (QRF) to estimate redox conditions and nitrate concentration in groundwater (1 km × 1 km resolution) using the European Water Framework Directive groundwater monitoring network as well as spatial environmental information available throughout Germany. The RF model for nitrate achieves a good predictive performance with an R² of 0.52. Dominant predictors are the redox conditions in the groundwater body, hydrogeological units and the percentage of arable land. An uncertainty assessment using QRF shows rather large uncertainties with a mean prediction interval (MPI) of 53.0 mg l⁻¹. This study represents the first nation-wide data-driven assessment of the spatial distribution of groundwater nitrate concentrations for Germany.

Emissions of reactive nitrogen (N_r) give rise to a wide range of environmental problems. Nitrogen budgets for various systems and on different scales are an established tool to quantify the sources and fate of N_r. The national nitrogen budget (NNB) for Germany calculates the nitrogen flows for eight pools: Atmosphere, Energy and Fuels, Material and Products in Industry, Humans and Settlements, Agriculture, Forest and Semi-natural Vegetation, Waste, and Hydrosphere, as well as for the transboundary N-flows. In Germany, in total 6,275 kt N_r a⁻¹ has been introduced into the nitrogen cycle annually (mean 2010 to 2014), of which 43% stem from ammonia synthesis. Domestic extraction and import of nitrogenous fossil fuels (lignite, coal, crude oil) releases another 2,335 kt N_r a⁻¹. Import of food, feed and materials contributes 745 kt N_r a⁻¹, while biological N fixation converts 308 kt N_r a⁻¹ into organically bound nitrogen. In terms of N_r sinks, the combustion and denoxing of fuels and the refining of crude oil converts 2,594 kt N_r a⁻¹ to N₂. In waters, soils, and wastewater treatment plants, denitrification leads to the release of 1,107 kt N_r a⁻¹ as N₂. Via the atmosphere and hydrosphere, Germany exports 755 kt N_r a⁻¹ to neighbouring countries and into coastal waters. On balance, Germany releases 1,627 kt N_r a⁻¹ annually to the environment. However, the NNB as a whole and the individual pool balances involve substantial uncertainties, which have to be considered when interpreting the results.

Nitrate pollution in groundwater and its mitigation strategies is currently a topic of controversial debate in Germany, and the demand for harmonised approaches for the implementation of regulations is increasing. Important factors that need to be considered when planning mitigation measures are the nitrogen inputs into water bodies and the natural nitrate reduction capacity. The present study introduces a nationwide, harmonised and simplified approach for estimating nitrate reduction as an integral quantity across the unsaturated zone and the groundwater body. The nitrate reduction rates vary from 0% to 100%, and are on average 57%, with high values in the north of Germany and low values in the south. Hydrogeological characteristics are associated with the estimated nitrate reduction rates, whereby the influence of aquifer type and redox conditions are particularly relevant. The nitrate reduction rates are substantially higher in porous aquifers and under anaerobic conditions than in fractured, consolidated aquifers and under aerobic conditions. This contribution presents a harmonised conceptual approach to derive the nitrate reduction rate at a 1 km × 1 km resolution. This information can be used when planning and designing mitigation measures to meet the groundwater nitrate limits.

The anthropogenic nitrogen cycle is characterized by a high complexity. Different reactive nitrogen species (NH₃, NH₄⁺, NO, NO₂, NO₃⁻, and N₂O) are set free by a large variety of anthropogenic activities and cause numerous negative impacts on the environment. The complex nature of the nitrogen cycle hampers public awareness of the nitrogen problem. To overcome this issue and to enhance the sensitivity for policy action, we developed a new, impact-based integrated national target for nitrogen (INTN) for Germany. It is based on six impact indicators, for which we derived the maximum amount of nitrogen losses allowed in each environmental sector to reach related state indicators on a spatial average for Germany. The resulting target sets a limit of nitrogen emissions in Germany of 1053 Gg N yr⁻¹. It could serve as a similar means on the national level as the planetary boundary for reactive nitrogen or the 1.5 °C target of the climate community on the global level. Taking related uncertainties into account, the resulting integrated nitrogen target of 1053 Gg N yr⁻¹ suggests a comprehensible INTN of 1000 Gg N yr⁻¹ for Germany. Compared to the current situation, the overall annual loss of reactive nitrogen in Germany would have to be reduced by approximately one-third.