Phosphorus release in relation to composition and isotopic exchangeability of sediment phosphorus

Phosphorus release was studied in simple batch experiments with varying pH and redox conditions. The results were related to changes of the fractional distribution and isotopic exchangeability of sediment phosphorus from six shallow, eutrophic lakes with different types of sediments. Fractional distribution of exchangeable phosphorus was a better indicator of the most significant fractions related to the phosphorus release from different types of sediments than fractional composition of phosphorus itself, since there were large fractions with relatively low exchangeabilities and vice versa (for example HCl-RP in the Swedish fakes and BD-P in Lake Balaton, respectively). Exchangeability of total sediment phosphorus could be related to the adsorption capacity of the sediments. pH had a strong effect on phosphorus release both in calcareous and non-calcareous sediments, while redox-dependency was more definite in the iron-rich ones. Induced changes in concentrations of the Al- and Ca-bound phosphorus by nitrate and acetate treatments could be explained by their pH-modifying effects. Acetate treatments usually decreased the Fe-P contents in an extent, which could only be attributed to the reduction of iron, followed by a liberation of phosphorus. A part of this phosphorus, however, was resorbed on other phases depending on the pa-conditions and the adsorption capacity of the sediments. Acetate and nitrate additions had a significant effect on the organic forms of phosphorus, as a consequence of the increased bacterial activity and mineralization, respectively. pH-enhanced phosphorus release may more often be the most important factor for the regulation of the internal phosphorus loading in shallow, eutrophic lakes with oxic bottom sediments than it has been recognized before. Resorption or release of the inorganic phosphorus produced in the course of mineralization is also regulated by the pH-conditions and adsorption capacity of the oxic sediments.