Discharge Driven Nitrogen Dynamics in a Mesoscale River Basin As Constrained by Stable Isotope Patterns

Area of Science:

• Environmental Science
• Hydrology
• Geochemistry

Background:

• Understanding nitrogen (N) dynamics in river catchments is crucial for managing water quality and ecosystem health.
• Anthropogenic activities, especially agriculture, significantly impact nitrogen loads and isotopic signatures in aquatic systems.
• River catchments exhibit complex hydrological and biogeochemical processes that influence nitrogen fate and transport.

Purpose of the Study:

• To evaluate large-scale nitrogen dynamics and trends in a river catchment with a pronounced anthropogenic gradient.
• To investigate the influence of different nitrogen sources, environmental conditions, and denitrification on nitrate isotopic signatures.
• To understand the relationship between discharge and nitrate isotopic signatures in major river tributaries.

Main Methods:

• Conducted a two-year monitoring program across the 3200 km(2) Bode River catchment, including 133 water sampling points.
• Utilized nitrate dual-isotope signatures (δ¹⁵N and δ¹⁸O) to trace nitrogen sources and transformations.
• Interpreted hydrochemical and isotope data based on discharge, using observed or simulated data from the mesoscale hydrological model (mHM).

Main Results:

• Nitrate isotopic signatures across the catchment are primarily influenced by nitrogen sources, environmental conditions during nitrate formation, and denitrification.
• A distinct relationship between discharge and nitrate isotopic signatures was observed in major tributaries.
• Higher discharge periods showed a greater wash-out of isotopically lighter nitrate from soil nitrification, while in-stream denitrification was more pronounced during low flow periods.

Conclusions:

• Nitrate isotope analysis provides valuable insights into complex nitrogen cycling in river systems influenced by anthropogenic pressures.
• Hydrological variability plays a significant role in modulating nitrate isotopic signatures through differential transport and transformation processes.
• The findings highlight the importance of considering both source contributions and in-stream processes for effective nitrogen management in river catchments.