Preprints
https://doi.org/10.5194/hess-2024-109
https://doi.org/10.5194/hess-2024-109
31 May 2024
 | 31 May 2024
Status: this preprint is currently under review for the journal HESS.

Nitrate and Water Isotopes as Tools to Resolve Nitrate Transit Times in a Mixed Land Use Catchment

Christina Franziska Radtke, Xiaoqiang Yang, Christin Müller, Jarno Rouhiainen, Ralf Merz, Stefanie R. Lutz, Paolo Benettin, Hong Wei, and Kay Knöller

Abstract. To understand the transport and fate of nitrate in catchments and its potential hazardous impact on ecosystems, knowledge about transit times (TT) and age of nitrate is needed. To add to that knowledge, we analyzed a 5-year low-frequency dataset followed by a 3-year high-frequency data set of water and nitrate isotopic signatures from a 11.5 km2 headwater catchment with mixed land use within the Northern lowlands of the Harz mountains in Germany. For the first time, a combination of water and nitrate isotope data was used to investigate nitrate age and transport and their relation to water transit times. To do so, the numerical model tran-SAS based on Storage Age Selection (SAS) functions was extended using biogeochemical equations describing nitrate turnover processes to model nitrification and denitrification dynamics along with the age composition of discharge fluxes. The analysis revealed a temporally varying offset between nitrate and water median transit times, with a larger offset at the beginning of wet periods due to higher proportions of young nitrate that is released more quickly with increasing discharge compared to water with larger transit times. Our findings of the varying offset between water and nitrate transit times underline the importance of analyses of solute transport and transformation in the light of projected more frequent hydrological extremes (droughts and floods) under future climate conditions.

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Christina Franziska Radtke, Xiaoqiang Yang, Christin Müller, Jarno Rouhiainen, Ralf Merz, Stefanie R. Lutz, Paolo Benettin, Hong Wei, and Kay Knöller

Status: open (until 26 Jul 2024)

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Christina Franziska Radtke, Xiaoqiang Yang, Christin Müller, Jarno Rouhiainen, Ralf Merz, Stefanie R. Lutz, Paolo Benettin, Hong Wei, and Kay Knöller
Christina Franziska Radtke, Xiaoqiang Yang, Christin Müller, Jarno Rouhiainen, Ralf Merz, Stefanie R. Lutz, Paolo Benettin, Hong Wei, and Kay Knöller

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Short summary
Most studies assume no difference between transit times of water and nitrate, because nitrate is transported by water. With an 8-year high-frequency dataset of isotopic signatures of both, water and nitrate, and a transit time model, we show the temporal varying difference of nitrate and water transit times. This finding is highly relevant for applied future research related to nutrient dynamics in landscapes under anthropogenic forcing and for managing impacts of nitrate on aquatic ecosystems.