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 ₃ ^- ) 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 ₃ ^- concentrations, we conducted a three-month experiment in 15 cement tanks (1 m ³ ), with five targeted concentrations of NO ₃ ^- : control, 2 mg L ^-1 , 5 mg L ^-1 , 10 mg L ^-1 , and 15 mg L ^-1 . The results showed that: i) when NO ₃ ^- was maintained at high levels: NO ₃ ^- ≥5-7 mg L ^-1 (range of median values), there was no effect of NO ₃ ^- on net P release from the sediment, likely because the positive effects of NO ₃ ^- (increasing oxidation) was counteracted by a promotion of phytoplankton growth. ii) after NO ₃ ^- addition was terminated NO ₃ ^- dropped sharply to a low level (NO ₃ ^- ≤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 NO ₃ ^- on P release decreased, while the promotion effects at high NO ₃ ^- concentrations continued. The results thus supported our hypotheses of a dual effect on sediment P release and suggest dose-dependent effect of NO ₃ ^- loading on stimulating P release from the sediment, being clear at high NO ₃ ^- exceeding 5-7 mg L ^-1 .
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