19 Jan 2022
19 Jan 2022
Status: this preprint is currently under review for the journal HESS.

How do inorganic nitrogen processing pathways change quantitatively at daily, seasonal and multi-annual scales in a large agricultural stream?

Jingshui Huang1,2, Dietrich Borchardt2, and Michael Rode2 Jingshui Huang et al.
  • 1Chair of Hydrology and River Basin Management, Technical University of Munich, Arcisstrasse 21, 80333 Munich, Germany
  • 2Department of Aquatic Ecosystem Analysis, Helmholtz Centre for Environmental Research - UFZ, Brueckstrasse 3a, 39114 Magdeburg, Germany

Abstract. Instream nitrogen (N) processing consists of complex interacting and highly time-varying pathways. To understand the role of a large agricultural stream river reaches in processing N thoroughly, it is urgently needed to continuously quantify high temporal resolution N processing pathways, reflecting seasonal shifts and multi-annual overarching effects. To this end, the hydrodynamic and river water quality model WASP 7.5.2 was applied in the 27.4-km reach of the 6th order agricultural stream Lower Bode (central Germany) for 5 years (2014–2018). Paired high-frequency data (15-min interval) of discharge (Q), nitrate (NO3), dissolved oxygen (DO), and Chlorophyll-a at upstream and downstream stations were used as model boundaries and for model constraints. The WASP model simulated 15-minute intervals of Q, NO3 and DO with Nash-Sutcliffe-Efficiency values higher than 0.9 for calibration and validation, enabling the calculation of gross and net dissolved inorganic N (DIN) uptake and pathway rates on a daily, seasonal and multi-annual scale. Results showed daily DIN net uptake rate ranged from −17.4 mg N m−2 d−1 to 553.9 mg N m−2 d−1. The highest daily net uptake could reach almost 30 % of total input loading, which occurred at extreme low flow in summer 2018. The growing season (spring and summer) accounted for 91 % of the average net annual DIN uptake in the measured period. In spring, both the DIN gross and net uptake were dominated by the phytoplankton uptake pathway. In summer, benthic algae assimilation dominated the gross DIN uptake. Conversely, the reach became a DIN net source with negative daily net uptake values in autumn and winter, mainly because the release from benthic algae surpassed uptake processes. Over the five years, average DIN gross and net uptake rates were 124.1 and 56.8 mg N m−2 d−1, which accounted for only 2.7 % and 1.2 % of the total loadings the study reach in the Lower Bode, respectively. 5-year average gross DIN uptake decreased from assimilation by benthic algae through assimilation by phytoplankton to denitrification. Our study highlights the value of combining river water quality modelling with high-frequency data in obtaining reliable instream DIN-budget, which facilitates our ability to manage N in aquatic systems. This study provides a methodology that can be applied to any large stream to quantify N processing pathway dynamics and complete our understanding of N cycling.

Jingshui Huang et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2021-615', Camille Minaudo, 25 Feb 2022
    • AC2: 'Reply on RC1', Jingshui Huang, 21 Apr 2022
  • RC2: 'Comment on hess-2021-615', Anonymous Referee #2, 24 Mar 2022
    • AC1: 'Reply on RC2', Jingshui Huang, 21 Apr 2022

Jingshui Huang et al.

Jingshui Huang et al.


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Short summary
In this study, we set up a water quality model using a 5-year paired high-frequency water quality dataset from a large agricultural stream. The simulations were compared with the 15-min interval measurements and showed very good fits. Based on these, we quantified the N uptake pathway rates and efficiencies at daily, seasonal, and yearly scales. This study offers an overarching understanding of N processing in large agricultural streams across different temporal scales.