Transformation of internal sedimentary phosphorus fractions by point injection of CaO2

The transformation of sedimentary phosphorus (P) fractions by point injection of CaO2 was investigated in this study. The results showed that the addition of CaO2 increased the dissolved oxygen (DO) in the overlying water as well as its pH. Simultaneously, the concentrations of total organic carbon (TOC), ammonia nitrogen (NH4+-N), total phosphorus (TP) and dissolved inorganic P (DIP) in the overlying water were sharply reduced by the point injection of CaO2, indicating that this accelerated the mineralisation rate of organic matter. It also favoured the formation and maintenance of the micro-oxidation environment in the deep sediments. NH4Cl-P, Fe/Al-P and Ca-P increased significantly, while the Res-P decreased sharply, especially close to the injection point of CaO2, accelerating the transformation of P. These results can all be attributed to the oxidation of organic P and the adsorption of dissolved P from the overlying water. Additionally, the NaHCO3-extractable phosphorus (Olsen-P) and algal available phosphorus (AAP) were reduced, and their percentages in Ca-P and Fe/Al-P also decreased, especially close to the injection point of CaO2. The bioavailable P decreased from 25.6% (the initial) to 17.2% (the mean for the different distances) after the point injection of CaO2, when the sum of the Olsen-P and AAP was used to calculate the bioavailable P. This indicates that point injection of CaO2 favours not only the transformation of P species but also inhibition of the bioavailable P. This is attributable to acceleration of the transformation from bioavailable to inert P.