Phosphorus fractions and fluxes in the soils of a free surface flow constructed wetland in Hong Kong

This study investigated the distribution of phosphorus (P) fractions in soils, and the magnitude and seasonal variations of P flux across the soil-water interface at the Hong Kong Wetland Park (HKWP), the largest free surface flow constructed wetland in the territory. Iron-bound P was the dominant P form comprising 29-59% of the total soil P, and its concentration was considerably lower than that in a nearby eutrophic marsh. The HKWP soils demonstrated a mean P release of 0.05 +/- 0.01 mg In-2 d(-1), which was very low compared with the range reported for other constructed wetlands. Significantly higher soil P fluxes were obtained under anaerobic conditions that could possibly be attributed to the release of redox-sensitive iron-bound P to the overlying water column. Moreover, soil P flux was significantly higher in the dry season than the wet season, which was likely a result of differences in water column P concentration and hence diffusive flux. These findings suggest that the HKWP soils comprising fishpond bund materials, river sand and decomposed granite would not contribute to significant degradation in water quality. (C) 2014 Elsevier B.V. All rights reserved.