How Important is Denitrification in Riparian Zones? Combining End‐Member Mixing and Isotope Modeling to Quantify Nitrate Removal from Riparian Groundwater

Abstract

Riparian zones are important buffer zones for streams as they are hotspots of nitrate transformation and removal in agricultural catchments. However, mixing of water from different sources and various transformation processes can complicate the quantification of nitrate turnover in riparian zones. In this study, we analyzed nitrate concentration and isotope data in riparian groundwater along a 2-km stream section in central Germany. We developed a mathematical model combining end-member mixing and isotope modeling to account for mixing of river water and groundwater and quantify nitrate transformation in riparian groundwater. This enabled us to explicitly determine the extent of denitrification (as process leading to permanent nitrate removal from riparian groundwater) and transient nitrate removal by additional processes associated with negligible isotope fractionation (e.g., plant uptake and microbial assimilation) and to perform an extensive uncertainty analysis. Based on the nitrogen isotope data of nitrate, the simulations suggest a mean removal of up to 27% by additional processes and only about 12% by denitrification. Nitrate removal from riparian groundwater by additional processes exceeded denitrification particularly in winter and at larger distance from the river, underlining the role of the river as organic carbon source. This highlights that nitrate consumption by additional processes predominates at the field site, implying that a substantial fraction of agricultural nitrogen input is not permanently removed but rather retained in the riparian zone. Overall, our model represents a useful tool to better compare nitrogen retention to permanent nitrogen removal in riparian zones at various temporal and spatial scales. Plain Language SummaryNitrogen is an important nutrient for agricultural crops. However, excessive nitrogen input into surface water in the form of nitrate can lead to algae blooms and lack of oxygen. The riparian zones of rivers are important buffer zones where groundwater is connected to soils, which are rich in soil organisms and organic matter pools fueling reaction processes. Hence, plants and bacteria can remove nitrate from riparian groundwater before it reaches the river. Bacterial consumption of nitrate (denitrification) leads to complete removal of nitrogen via release of nitrogen gas into the atmosphere. In contrast, other biogeochemical processes such as nitrate uptake by plants merely result in nitrogen retention within riparian zones. To quantify the role of denitrification relative to other processes, we developed a novel model combining concentration and isotope data of nitrate and applied it to a groundwater study site in Central Germany. We found that nitrate removal from riparian groundwater by additional processes largely exceeded denitrification. Hence, a major fraction of nitrogen inputs was retained in the riparian zone and may eventually end up in the river. Such information is highly relevant for many river ecosystems at risk of eutrophication because of high nitr...

Disentangling the Impact of Catchment Heterogeneity on Nitrate Export Dynamics From Event to Long‐Term Time Scales

“…Recent studies by Lutz et al. (2020) and Trauth et al. (2018) reported a removal by riparian denitrification of up to 12% of nitrate from groundwater entering the Selke River along a 2‐km section downstream of Meisdorf.…”

“…Nevertheless, there is evidence for significant denitrification in the riparian zones, especially during LFSs. Recent studies by Lutz et al (2020) and Trauth et al (2018) reported a removal by riparian denitrification of up to 12 % of nitrate from groundwater entering the Selke River along a 2 km section downstream of Meisdorf. Additionally, a stable isotope study by Mueller et al (2015) in the Bode catchment (which includes the Selke catchment) found evidence for significant denitrification in the stream beds during LFSs whereas denitrification in the groundwater was not evident, findings which are in line with those of Hannappel et al (2018).…”

Optimal Transport for Assessing Nitrate Source‐Pathway Connectivity

“…Further evidence for limited denitrification is provided by the lack of a significant negative relationship between ln[NO 3 − ] and δ 15 N NO3 or δ 18 O NO3 (not plotted), which is expected when NO 3 − is reduced as δ 15 N NO3 is enriched (Kendall et al, 2007). Thus, our results indicate that while biogeochemical processing (e.g., denitrification) may play a role, there is likely a large imprint of physical source‐mixing that influences NO 3 − composition (Lutz et al, 2019).…”

“…Systems experiencing considerable denitrification typically have a slope between 0.43 and 1.0 (Granger & Wankel, 2016;Hu et al, 2019 (Kendall et al, 2007). Thus, our results indicate that while biogeochemical processing (e.g., denitrification) may play a role, there is likely a large imprint of physical source-mixing that influences NO 3 − composition (Lutz et al, 2019).…”

“…Loadograph separation only requires nitrate concentration and discharge, which are commonly becoming ubiquitous, particularly with the widespread use of aquatic nitrate sensors that provide continuous estimates of nitrate concentration (Burns et al, 2019). Further, with regard to nitrate sourcing, stable isotopes are commonly employed (Lutz et al, 2019; H. Zhang, Kang, et al, 2020; Wang et al, 2020; Z. Zhang, Chen, Cheng, Li, et al, 2020; Z. Zhang, Chen, Cheng, & Soulsby, 2020) but sources have also been unmixed with elemental concentration data alone (Katz et al, 2011; Yue et al, 2017). For example, the relationship between nitrate and chloride concentration provides a simple way to estimate whether nitrate is derived from wastewater or agriculture origin (Chen et al, 2009).…”