Multi-scale quantification of non-point source nitrogen in a coastal river using an integrated multi-method...

Area of Science:

• Environmental Science
• Ecosystem Management
• Water Quality Assessment

Background:

• Coastal ecosystems face significant stress from excessive nitrogen inputs, primarily from human activities via rivers.
• Quantifying non-point source (NPS) nitrogen contributions remains a persistent technical challenge for effective management.

Purpose of the Study:

• To develop and apply an integrated multi-method framework for tracing nitrogen sources at administrative scales.
• To analyze the spatiotemporal dynamics of NPS contributions under varying hydrological conditions in the Xiaoqing River Watershed.

Main Methods:

• Integrated framework combining Pollutant Discharge Coefficient Method (PDCM), isotopic analysis, and Soil and Water Assessment Tool (SWAT) hydrological modeling.
• Downscaling nitrogen contributions to administrative management units for refined pollution source targeting.
• Quantifying spatial distribution and seasonal shifts in nitrogen sources and transport pathways.

Main Results:

• PDCM and SWAT showed moderate positive correlations in nitrogen loads and source contributions, enhancing pollution targeting.
• Isotopic analysis and SWAT simulations revealed distinct seasonal shifts: urban domestic sewage (point sources) dominated dry seasons (~45%), while agricultural NPS prevailed in wet seasons (>65%).
• SWAT model quantified NPS spatial distribution and identified seasonal differences in nitrogen transport to the river.

Conclusions:

• The integrated framework significantly enhances the accuracy and comprehensiveness of NPS source tracing.
• Provides a robust, administrable scientific basis for refined nitrogen management strategies in complex coastal watersheds.
• Highlights the critical need for seasonal and spatially explicit approaches to manage nitrogen pollution effectively.