Evidence for organic phosphorus activation and transformation at the sediment-water interface during plant debris decomposition

The processes and mechanisms through which phosphorus (P) is released from sediment and organic P is transformed, induced by the decomposition of plant (duckweed (Lemma minor L)) debris, were studied experimentally. In the simulation experiments, the dissolved oxygen concentration, pH, and oxidation-reduction potential at the water-sediment interface first decreased rapidly. The lowest oxidation-reduction potential reached was 225.4 mV, and the solution became weakly acidic (pH 5.14) and anoxic (dissolved oxygen concentration 0.17 mg. L-1). The dissolved oxygen concentration, pH, and oxidation-reduction potential then became stable. The soluble reactive P, total dissolved P, and total P concentrations in the overlying water all increased rapidly because of the particulate P and dissolved organic P released as the plant debris decomposed. P-31 NMR analysis of the solution showed that orthophosphate monoesters were the main organic P compounds in the sediment. The orthophosphate monoester and orthophosphate diester concentrations were higher during the first 7 d of the experiment (at 71.2 and 153 mg.kg(-1), respectively) than later (60.8 and 14.6 mg.kg(-1), respectively). The decomposition of the duckweed could have mineralized the orthophosphate monoesters and orthophosphate diesters to give orthophosphate. The results indicated that the decomposition of aquatic plant debris is a key factor in the release of P from sediment even when external P is excluded. It is therefore necessary to remove plant debris from freshwater ecosystems to control the release of P from plant debris and sediment. (C) 2017 Elsevier B.V. All rights reserved.