Effects of nitrate on phosphorus release from lake sediments

Phosphorus (P) release from sediment is a key process affecting the effectiveness of eutrophication mit-igation. 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 stim-ulating the growth of phytoplankton and increase the decomposition rates and oxygen consumption at the sediment water interface. To test the effect of different NO3 & minus; concentrations, we conducted a three-month experiment in 15 cement tanks (1 m(3) ), 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 NO3-1; was maintained at high levels: NO3-(1);>= 5 +/- 7 mg L & minus;1 (range of median values), there was no effect of NO3-1 on net P release from the sediment, likely because the positive effects of NO3 & minus; (increasing oxidation) was counteracted by a promotion of phytoplankton growth. ii) after NO3-1 addition was terminated NO3-1; dropped sharply to a low level (NO3- 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 NO3-1; on P release de-creased, 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 & minus; exceeding 5-7 mg L-1. (C) 2021 Published by Elsevier Ltd.