Dissolved organic nitrogen in urban streams: Biodegradability and molecular composition studies

Area of Science:

• Environmental Chemistry
• Microbiology
• Organic Geochemistry

Background:

• Dissolved organic nitrogen (DON) biodegradability and molecular makeup in aquatic systems remain poorly understood.
• Urbanization impacts water quality, but its specific effects on DON's molecular composition and reactivity are unclear.

Purpose of the Study:

• To investigate the biodegradability and molecular composition of DON in streams influenced by varying urban land use.
• To identify the types of DON compounds present and their susceptibility to microbial degradation.

Main Methods:

• Coupling 5-day bioassay experiments with high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS).
• Analyzing DON in streams draining 28% and 83% urban land use.
• Weekly sampling over 21 weeks to assess DON concentrations.

Main Results:

• Urban streams showed no significant correlation between urban land use and DON concentration.
• >80% of DON consisted of lignin-like compounds, with only 5-7% being labile.
• Labile DON comprised lipid- and protein-like structures; microbial degradation shifted DON towards more oxygenated and less saturated formulas.

Conclusions:

• Urban streams contain complex DON mixtures, primarily recalcitrant lignin-like compounds.
• Microbial degradation caused minor changes to the DON carbon skeleton, with limited net removal of organic-bound nitrogen.
• The biodegradability of DON is highly dependent on its molecular structure, with labile fractions dominated by protein- and lipid-like compounds.