Rainstorms peak governs nitrate export pathways and patterns: Insights from high-frequency sampling and stable...

Area of Science:

• Environmental Science
• Hydrology
• Water Quality

Background:

• Agricultural lands are major sources of nitrate runoff into aquatic systems.
• Rainstorm events exacerbate nitrate pollution, threatening water quality and ecosystems.
• The influence of intra-event rainstorm structure on nitrate transport is poorly understood.

Purpose of the Study:

• To investigate how the structure of rainstorm events influences hydrological responses and nitrate transport.
• To identify and quantify nitrate export pathways under different rainstorm regimes.
• To understand the mechanisms (dilution vs. chemostasis) governing nitrate concentration changes during rainstorms.

Main Methods:

• Conducted high-frequency monitoring and sampling during 20 rainstorm events.
• Classified rainstorm regimes into early-, center-, and late-peak based on intensity and peak timing.
• Utilized stable water isotopes to trace nitrate sources (soil water, groundwater, rainwater).
• Analyzed concentration-discharge relationships to determine export patterns and behaviors.

Main Results:

• Early-peak rainstorms: Rainwater and shallow soil water were primary nitrate sources; dilution and chemostasis had comparable roles.
• Center-peak rainstorms: Deep soil water dominated nitrate export; chemostatic behavior led to the highest nitrate concentrations (4.97 mg L⁻¹).
• Late-peak rainstorms: Multiple soil layers contributed; dilution behavior was dominant, resulting in lower nitrate concentrations (3.00 mg L⁻¹).

Conclusions:

• Intra-event rainstorm structure significantly alters nitrate export pathways and patterns.
• Hydrological pathway activation during rainstorms is more complex than previously thought.
• Findings provide a basis for targeted watershed management to control nitrate pollution.