Responses of coastal sediment phosphorus release to elevated urea loading

Increased urea is one of the common nitrogen forms polluting coastal waters and affecting nutrient dynamics. To investigate the effects of urea on sediment phosphorus (P) release, we carried out a 2-month mesocosm experiment with six targeted loadings of urea (0-0.6 mg N L^-1 d^-1). Results showed that: i) urea was rapidly transformed into ammonium and then nitrate (NO₃^-). ii) When nitrogen occurred as urea or ammonium, minor P release was observed. iii) After urea were mostly converted to NO₃^-, P release became clearer. iv) NO₃^- had a dual effect by promoting P release through decreasing sediment pH and increasing alkaline phosphatase activity or by inhibiting P release through improving sediment oxidation. v) The overall effects of urea on P release depended on the ultimate NO₃^- concentrations, being prominent when NO₃^- ≥ 11 mg N L^-1. Our findings are of relevance when determining nitrogen reduction targets needed for combating eutrophication.