Effects of aluminum, iron, oxygen and nitrate additions on phosphorus release from the sediment of a Danish softwater lake

The effects of oxygen, aluminum, iron and nitrate additions on phosphate release from the sediment were evaluated in the softwater Lake Vedsted, Denmark, by a 34-day experiment with undisturbed sediment cores. Six treatments were applied: (1) Control - O-2 (0 - 20% saturation), (2) O-2 (100% saturation) (3) Al3+ - O-2, (4) Fe3+ + O-2, ( 5) Fe3+ - O-2, and (6) NO3- - O-2. Al-2( SO4)(3)* 18 H2O and FeCl3* 4H(2)O were added in amounts that theoretically should immobilize the exchangeable P-pool in the top 5 cm of the sediment, while sodium nitrate concentrations were increased to 5 mg N l(-1). The four treatments with metals or NO3- reduced the P efflux from the sediment significantly as compared to the suboxic control treatment. Mean accumulated P-release rates for suboxic treatments with Al3+, Fe3+, and NO3- were: - 0.27 mmol m(-2) (st. dev = 0.02 mmol m(-2), N = 5), 0.58 mmol m(-2) (st. dev = 0.30 mmol m(-2), N = 5) and 1.40 mmol m(-2) (st. dev = 0.14 mmol m(-2), N = 5), respectively. The oxic treatment with Fe3+ had a P efflux of 0.36 mmol m(-2) ( st. dev = 0.08 mmol m(-2), N = 5). The two highest P-release rates were observed in the control treatment and the treatment with O-2 (14.50 mmol m(-2) (st. dev = 3.90 mmol m(-2), N = 5) and 2.31 mmol m(-2) (st. dev = 0.80 mmol m(-2), N = 5), respectively). In order to identify changes in the P and Fe binding sites in the sediment as caused by the treatments, a sequential P extraction procedure was applied on the sediment before and after the efflux experiment. Addition of O-2, Fe3+ and NO3- to the sediment increased the amounts of oxidized Fe3+ and P. BD. Al3+ addition resulted in a lower fraction of P. BD but a correspondingly higher fraction of Al-bound P. Addition of Al3+ decreased the Fe-efflux from the suboxic sediment as well as the amount of oxidized Fe3+ in the sediment. This questions the use of Al compounds that contain sulfate because of the possible formation of FeS, which will restrict upward migration of Fe2+ and the formation of new Fe-oxides in the surface sediment. Instead, we suggest the use of AlCl3 for lake restoration purposes.