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. [Display omitted] •Urea entering coastal waters was rapidly transformed into NH4+ and then NO3−.•Effects of urea on P release depended on the ultimate concentration of NO3−.•Nitrate has a dual effect on coastal sediment P release.•Increased alkaline phosphatase and sediment acidification explain the P release.•Increased oxide-layer thickness of the sediment explains the P retention.