Effects of nitrate on phosphorus release from lake sediments

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.