Urea contributions to dissolved ‘organic’ nitrogen losses from intensive, fertilised agriculture

• Runoff of urea nitrogen was a significant, but cryptic, nitrogen loss pathway.
• Runoff soon after fertiliser application resulted in highest urea load losses.
• Urea analysis should be added to agronomic and water quality studies.

While nearly 60% of global nitrogen fertilizer use is in the form of urea, and urea is increasingly implicated in aquatic eutrophication, little is known of the scale or temporal dynamics of urea export from intensive agriculture. Annual paddock urea nitrogen exports in surfacewater runoff were quantified across multiple years from several sugarcane farms across Australia’s Great Barrier Reef catchment area. Study results suggest that runoff of undegraded urea can represent a significant proportion of the total ‘dissolved organic nitrogen’ pool leaving paddocks, and an important form of nitrogenous export from fertilised agricultural land uses. Situations where substantial rainfall or irrigation-driven surfacewater runoff occur within 1–2 weeks of fertiliser application particularly provide scope for major, and in some cases dominant, losses of undegraded urea from paddocks. Fertiliser derived urea can be a significant, bioavailable and anthropogenic form of dissolved ‘organic’ nitrogen export that warrants further attention in many field and catchment scale research applications.