•Nitrate has a dual effect on sediment phosphorus release in shallow lakes•Nitrate can reduce sediment phosphorus release through oxidation•Nitrate can promote phosphorus release by stimulating phytoplankton growth•Alkaline phosphatase secreted by phytoplankton explains the phosphorus release•The effects of nitrate loading on sediment phosphorus release are dose-dependent Phosphorus (P) release from sediment is a key process affecting the effectiveness of eutrophication mitigation. We hypothesized that high nitrate (NO3−) input may have dual effect on sediment P release: reduce the sediment P release by improving the oxidation of sediment or promote P release by stimulating the growth of phytoplankton and increase the decomposition rates and oxygen consumption at the sediment water interface. To test the effect of different NO3− concentrations, we conducted a three-month experiment in 15 cement tanks (1 m3), with five targeted concentrations of NO3−: control, 2 mg L−1, 5 mg L−1, 10 mg L−1, and 15 mg L−1. The results showed that: i) when NO3− was maintained at high levels: NO3−≥5–7 mg L−1 (range of median values), there was no effect of NO3− on net P release from the sediment, likely because the positive effects of NO3− (increasing oxidation) was counteracted by a promotion of phytoplankton growth. ii) after NO3− addition was terminated NO3− dropped sharply to a low level (NO3−≤0.4 mg L−1), followed by a minor P release in the low N treatments but a significant P release in the high N treatments, which likely reflect that the inhibition effect of NO3− on P release decreased, while the promotion effects at high NO3− concentrations continued. The results thus supported our hypotheses of a dual effect on sediment P release and suggest dose-dependent effect of NO3− loading on stimulating P release from the sediment, being clear at high NO3− exceeding 5–7 mg L−1. Display omitted