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 d). Results showed that: i) urea was rapidly transformed into ammonium and then nitrate (NO). ii) When nitrogen occurred as urea or ammonium, minor P release was observed. iii) After urea were mostly converted to NO, P release became clearer. iv) NO 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 NO concentrations, being prominent when NO ≥ 11 mg N L. Our findings are of relevance when determining nitrogen reduction targets needed for combating eutrophication.
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