Functions of Calcium-bound Phosphorus in Relation to Characteristics of Phosphorus Releasing Bacteria in Sediment of a Chinese Shallow Lake (Lake Wabu)

Sediment phosphorus (P) release accelerates lake eutrophication, while retention capacity and release potential of different P fractions, calcium-bound P (CaCO₃∼P) in particular, still remains unclear. Fractionation and sorption behaviors of phosphorus were studied in sediment of a Chinese shallow lake (Lake Wabu) and two inflowing rivers from December 2011 to December 2012. Abundance of P releasing bacteria was analyzed, and their main species were isolated using a culture-dependent method and identified by their 16S rDNA sequences. CaCO₃∼P release abilities of these bacteria were also tested. In sediments of both the lake and rivers studied, the rank order of the different P extracts was CaCO₃∼P > iron-bound P > acid-soluble organic P > hot NaOH-extractable organic P. At the same time, CaCO₃∼P content and equilibrium P concentration (EPC₀) values in river sediments were significantly higher than those in the lake. Additionally, EPC₀ changes non-monotonically with increasing CaCO₃∼P content, forming a V-shaped curve, with the lowest EPC₀ at an intermediate CaCO₃∼P content (around 180 mg kg⁻¹). Below this threshold, CaCO₃∼P was a component strengthening P retention; moreover, CaCO₃∼P became an active species responsible for P release. Noticeably, between the two parts divided by this threshold, the differences in abundance of inorganic phosphorus solubilizing bacteria (IPB) and organic phosphorus mineralizing bacteria (OPB) were insignificant and the dominant IPB species clustered together. By contrast, OPB was distinguished from each other, whose dominant species isolated from the part with higher CaCO₃∼P content, namely Novosphingobium sp., exhibited a stronger ability to solubilize CaCO₃∼P. Shortly, with lower content, CaCO₃∼P tends to stabilize P in sediment; while with higher content or under eutrophic condition, it shifted into P source, with some OPB species becoming the main factors to drive its release.