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 (NO) 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 NO concentrations, we conducted a three-month experiment in 15 cement tanks (1 m), with five targeted concentrations of NO: control, 2 mg L, 5 mg L, 10 mg L, and 15 mg L. The results showed that: i) when NO was maintained at high levels: NO≥5-7 mg L (range of median values), there was no effect of NO on net P release from the sediment, likely because the positive effects of NO (increasing oxidation) was counteracted by a promotion of phytoplankton growth. ii) after NO addition was terminated NO dropped sharply to a low level (NO≤0.4 mg L), 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 NO on P release decreased, while the promotion effects at high NO concentrations continued. The results thus supported our hypotheses of a dual effect on sediment P release and suggest dose-dependent effect of NO loading on stimulating P release from the sediment, being clear at high NO exceeding 5-7 mg L.