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 m−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.