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

Stefanie R. Lutz,Kay Knöller,Andreas Musolff,Boris M. Van Breukelen,Jan H. Fleckenstein,Nico Trauth

doi:10.1029/2019wr025528

Stefanie R. Lutz, Kay Knöller + Show 4 more

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https://doi.org/10.1029/2019wr025528

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Abstract

AbstractRiparian 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 28% by additional processes and only about 9% 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.

Highlights

Despite efforts to reduce nutrient inputs, the contamination of freshwater resources with nitrate (NO3−) poses a continuing problem in many European countries (European Union, 2010)
This enabled us to explicitly determine the extent of denitrification and transient nitrate removal by additional processes associated with negligible isotope fractionation and to perform an extensive uncertainty analysis
We summarize under the term additional processes all uptake and transformation processes other than denitrification that lead to NO3− removal from riparian groundwater and subsequent N retention in the riparian zone

Summary

Introduction

Despite efforts to reduce nutrient inputs, the contamination of freshwater resources with nitrate (NO3−) poses a continuing problem in many European countries (European Union, 2010). NO3− pollution of its freshwater resources (European Commission, 2016). The excessive N can be transformed to NO3− and leach to groundwater or enter rivers via direct runoff. In this context, riparian zones can act as buffers against NO3− pollution, as they are hydrologically and biogeochemically active zones where uptake and transformation of nutrients occur (Anderson et al, 2014; Dhondt et al, 2003; Hill, 1996; Mayer et al, 2007; Osborne & Kovacic, 1993; Vidon & Hill, 2004; Vought et al, 1994)

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