Influence of natural organic matter on the bioavailability and preservation of organic phosphorus in lake sediments

Abstract

Information about the bioavailability and sequestration of organic phosphorus (Po) in sediments is fundamental to understanding biogeochemical cycling of phosphorus (P) in eutrophic lakes. However, the processes governing preservation of Po in sediments are still poorly understood. Sequential extraction of Po by H2O (H2O-Po) and NaOH–EDTA (NaOH–EDTA Po), in combination with enzymatic hydrolysis/31P NMR, was applied to estimate the bioavailability of Po in sediments of Lake Tai (Ch: Taihu), China. Of H2O-Po and NaOH–EDTA Po, 45.5–89.4% and 30.4–71.3% respectively were hydrolyzed by phosphatase, and therefore considered to be biologically available. Of NaOH–EDTA Po, 28.7–69.6% could not be hydrolyzed by phosphatase; this portion was characterized by 31P NMR as monoester P and/or diester P. Simulation experiments of hydrolysis of model Po compounds in the presence of humic acids (HA), which were used as a model for natural organic matter (NOM), and metals, including Al, Ca, and Fe, have demonstrated that enzymatic hydrolysis of labile monoester P was weakly reduced by HA or metal ions. Condensed phosphate (e.g., pyrophosphate) and phytate-like P (e.g., inositol phosphates) were resistant to enzymatic hydrolysis in the presence of HA and/or metal ions, which indicated that they may be possibly preserved in sediments. These observations suggest that NOM in sediments can be a significant factor determining the bioavailability and preservation of Po in sediments. The presence of metals would enhance the effect of NOM on preservation of Po in sediments. Formation of Po–metal–HA or Po–metal complexes might be mechanisms responsible for these processes.