Enhanced release of sedimentary phosphorous following nitrate decline in a nitrate-rich reservoir and its management implications

摘要

Nitrate in aquatic systems is controlled to protect human health, whereas it is also used as an inhibitor of sedimentary phosphorus (P) release to prevent freshwater eutrophication. This study was conducted to investigate the effects of nitrate decline on P release from sediments in Tianzhuang Reservoir (Yiyuan County, Shandong, China), which has a high nitrate concentration in the water column and high ferric-bound P to total P (Fe-P/TP) and total iron to total P (T-Fe/TP) ratios in the surficial sediments. We investigated the nitrate and soluble reactive phosphorus (SRP) profiles in sediment pore-waters, estimated their fluxes on the sediment side of the sediment-water interface, and calculated the net production ratio of nitrogen (N) to phosphorus in the surface sediments. We also conducted P release experiments in the laboratory. SRP was significantly negatively correlated with nitrate in porewaters of the surficial 7.5-cm sediments. The diffusional SRP flux in Tianzhuang Reservoir was only 0.191 mg/(m(2).d). The net production ratio of N: P was 2,372: 1 in the surface sediments. Laboratory experiments showed that the concentration of SRP in nitrate-free water was clearly higher than that in nitrate-rich water under the effects of the sedimentary P release. These findings indicated that nitrate was the key factor controlling the sedimentary P release in the reservoir, and that nitrate decline would enhance the release of P from redox-sensitive sediments. Therefore, we recommend that measures for controlling P should be implemented simultaneously when treatments for nitrate pollution are employed in aquatic systems with redox-sensitive, iron-rich sediments.