Fractionation and bioavailability of phosphorus in sediments of Huaihe River, China

The results of sequential extraction of organic phosphorus (P) from sediment profiles of a riverine wetland located in the Huaihe Watershed, China, indicated that organic P is an important P fraction. Total microbial phosphorus (Biomass-Po), humic-associated phosphorus (HA-Po) and residual phosphorus (Res-Po) increased distinctly from the bottom to the surface layer of the sediments, and they accounted for 8.0% to 17.4%, 3.3% to 34.5%, and 29.9% to 63.2% of total organic phosphorus (TOP), respectively. In contrast, fulvic-associated phosphorus (FA-Po), accounting for 17.3% to 32.6% of TOP, showed no clear depth-related trend and exhibited characteristics of accumulation. Relative to FA-Po, HA-Po was more easily mineralized to other P species, such as dissolved reactive phosphorus (DRP), which can be ingested directly by aquatic organisms. Furthermore, alkaline phosphatase activity (APA) increased from 4.58 to 44.03 mu mol g(-1) h(-1) with decreasing depth in the sediment profile. Significant positive correlations observed between APA and all P species, except FA-Po, suggested that APA facilitated the transformation of HA-Po and Biomass-Po to inorganic P in the sediment samples. Finally, strongly significant positive correlations between iron (Fe)/aluminum (Al) and HA-Po and with Biomass-Po suggested that organic P associated with Fe/Al cations, relative to calcium (Ca)/manganese (Mn), were easier to be hydrolyzed and mineralized under anaerobic conditions.