Research papersQuantifying changes and trends of NO3 concentrations and concentration-discharge relationships in a complex, heavily managed, drought-sensitive river system
Graphical abstract
Conceptual diagram of the effects of catchment legacy nitrogen and drought conditions on stream nitrate-nitrogen concentrations, export pattern (slope b) and export regime (CVc/CVq). As the legacy nitrogen decreases and drought intensifies, stream nitrate-nitrogen concentrations will decrease, and the relationship with streamflow tends toward enrichment pattern and chemodynamic.
Introduction
Over the past century, dramatic increases in global agricultural land conversion and urbanization, along with widespread fertilizer use and population growth has led to the deterioration of stream water quality worldwide (Basu et al., 2022, Liu et al., 2020a, Schilling et al., 2016). A particular problem is nitrate nitrogen (NO3) pollution due to its high mobility and impact on water eutrophication (Liu et al., 2022a, Wu et al., 2022). Given that streamflow is the main transport pathway of NO3, clarifying the stream NO3 concentration-discharge (C-Q) relationships can improve understanding of the biogeochemical processes governing NO3 and thus help to prevent and control stream NO3 pollution in catchments (Musolff et al., 2017, Thompson et al., 2011).
In response to increasing stream NO3 pollution, stringent NO3 management measures to reduce input loads have been adopted in a wide range of catchments (Van Meter et al., 2018, Wu et al., 2021, Yang et al., 2022). However, the response of stream NO3 concentrations and C-Q relationships to such management measures can have long time lags due to long water residence times. In addition, mainly due to geographic variations in climate and landscape characteristics, inconsistent results have been reported in terms of dominant controlling factors (Ebeling et al., 2021, Guo et al., 2022, Lintern et al., 2021, Liu et al., 2022a, Liu et al., 2022b, Zhi and Li, 2020). Moreover, the response of stream NO3 concentrations and C-Q relationships to climate change is still largely unknown. These factors limit an improved understanding of the driving mechanisms of stream NO3 in complex landscape catchments as well as projection of likely future trends.
Climate is a major driver of N biogeochemical processes, especially over longer-time scales, and both climate and consequent biogeochemical processes have undergone profound changes in many areas over the past half century (Guo et al., 2022, Liu et al., 2023, Yang et al., 2022). Currently, climate warming is intensifying and increasing droughts will potentially reduce streamflow and stream depths over extensive areas (Davidson and Janssens, 2006, Wu et al., 2022). In shallower streams, the relative contact area of the water volume with the sediment surface, and therefore exposure to denitrification at the benthic interface is enhanced. Thus, the majority of N removal usually occurs upstream with shallower stream depths (Alexander et al., 2000, Liu et al., 2022a). For example, studies in the Mississippi River catchment have shown that a rapid decline in the average first-order rate of N loss with increasing stream depth - from 0.45 days−1 to 0.005 days−1 (Alexander et al., 2000). Reduced streamflow can even result in streams becoming intermittent, with disconnected flow pathways affecting the hydrological transport of NO3 (Wu et al., 2021). These changes will affect stream NO3 concentrations and C-Q relationships, and there is an urgent need to consider climate change in predictions of stream NO3 concentrations and C-Q relationships as well as the effects of current and future management measures.
Catchments have diverse landscape characteristics and anthropogenic pressures that influence NO3 export-retention and hydrological transport processes (Goyette et al., 2019, Liu et al., 2021, Liu et al., 2020a, Wu et al., 2022). Stream NO3 concentrations and C-Q relationships differ at different spatial scales due to variations in landscape composition and configuration (Ebeling et al., 2021, Liu et al., 2020a, Zhi and Li, 2020). Given that many landscape ecosystem functions (e.g. release and retention of NO3 and water) also change at seasonal and interannual scales (Guo et al., 2022, Liu et al., 2022a), stream NO3 concentrations and C-Q relationships vary temporally. Previously, the percentage of agricultural land was considered to be the main driver of increased stream NO3 concentrations due to high N fertilizer inputs, which led to an enrichment and usually chemostatic behavior of stream NO3 concentrations (Basu et al., 2010, Thompson et al., 2011). Recently, the impact of agriculture on stream NO3 concentrations is gradually decreasing due to the reduction of N inputs (Basu et al., 2022, Krupa et al., 2011, Van Meter et al., 2018). Also, the enrichment or dilution patterns of nutrients depending on streamflow variations in agricultural catchments have been inconsistently reported (Miller et al., 2017, Zhi and Li, 2020), and little is known about the effects of export regimes (C-Q relationships; i.e. chemostatic vs. chemodynamic). Meanwhile, understanding of long-term changing stream NO3 concentrations and C-Q relationships is still limited for larger, heavily managed catchments with complex mosaics of landscape characteristics and human interventions but is particularly important for informing management of stream NO3 in the future.
To address these research gaps, we investigated the dynamics of stream NO3 concentrations and C-Q relationships in the large, complex and heavily managed catchment of the River Spree and 67 of its sub-catchments in Germany, with different land use, soil, climate and topographic characteristics, and which is increasingly drought prone. The study used both intensive synoptic surveys and long-term observations (2000–2021). We hypothesized that stream NO3 concentrations and C-Q relationships would differ according to landscape spatial heterogeneity and changes in environmental conditions over long-term time scales. We aimed to
Identify spatial–temporal patterns of NO3 concentrations in a large, complex and heavily managed catchment;
Investigate C-Q relationships at different spatial scales and over longer time periods.
Quantify the influence of different environmental controls on NO3 dynamics and understand the impact of recent intensifying drought conditions on stream NO3 concentrations and C-Q relationships.
Section snippets
Study area
The study catchment of the River Spree is located in north-eastern Germany (52°32′10″N, 13°12′31″E) (Fig. 1a), with a total catchment area of 10105 km2. The Spree - ∼400 km in length - provides the main drinking water supply for Berlin, the capital city of Germany with a population of ∼ 4 million. The region has a typical mid-continental climate with significant seasonal variations in meteorological conditions, with a wetter growing season from April to September and a generally drier winter
Spatial-temporal patterns of NO3 concentrations
Stream NO3 concentrations showed seasonal variability, with a decreasing long-term trend and strong spatial heterogeneity (Fig. 2). From the upstream to downstream sections at A1, W1, W2, U1 and U2, mean stream NO3 concentrations were 22.3, 8.65, 0.47, 0.47 and 0.80 mg L-1, respectively, during the 2000–2021 period (Fig. 2a). NO3 first decreased downstream from A1 to W2 (P < 0.0001), but then increased again from W2 further downstream (P < 0.01) (Fig. 2b). The long-term decreasing temporal
Patterns and trends in stream NO3 concentrations and C-Q relationships
We found stream NO3 concentrations exhibited a decreasing temporal trend during the observation period (2000–2022), except for the W2 observation site, which is midstream of the Spreewald Wetland Reserve (Fig. 2). This is primarily explained by the strict reduction of N fertilizer inputs in Germany (∼30%), beginning in 1990 (Kirschke et al., 2021), which has resulted in varying reductions of stream NO3 concentrations. As in other catchments, there is still significant NO3 pollution (>3 mg L-1)
Conclusion
Our work characterized spatial–temporal patterns in stream NO3 concentrations and C-Q relationships in a large, complex and heavily managed catchment, which is drought sensitive. Stream NO3 concentrations and C-Q relationships varied across spatial–temporal scales. In the upstream catchment, where arable farming is dominant, we found a high-risk degree of river eutrophication due to high NO3 concentrations resulting from high N inputs to fertile soils and substantial nitrogen legacy, which is
CRediT authorship contribution statement
Ji Liu: Conceptualization, Methodology, Visualization, Writing – original draft, Writing – review & editing. Doerthe Tetzlaff: Conceptualization, Investigation, Supervision, Writing – original draft, Writing – review & editing. Tobias Goldhammer: Investigation, Writing – review & editing. Songjun Wu: Writing – review & editing. Chris Soulsby: Conceptualization, Supervision, Writing – original draft, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
Ji Liu is supported and funded by Alexander von Humboldt Foundation. Tetzlaff’s contribution was partly funded through the Einstein Research Unit “Climate and Water under Change” from the Einstein Foundation Berlin and Berlin University Alliance. Contributions from Soulsby have been supported by the Leverhulme Trust through the ISO-LAND project (grant no. RPG 2018 375) and the Einstein Foundations MOSAIC project. We thank Thomas Rossoll and technical staff of the Chemical Analytics and
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