Estimating bioavailability of soil particulate phosphorus to Microcystis aeruginosa

Bioavailable phosphorus has been reported to include not only phosphorus in dissolved form but also the fraction of particulate phosphorus that is readily usable. Here, a chemical extraction scheme in combination with an algal bioassay was conducted to estimate the potential bioavailability of particulate phosphorus in soil collected from a Chinese cabbage field. Microcystis aeruginosa was cultured in a medium containing the soils sequentially extracted with 1 M NH4Cl, 0.11 M bicarbonate dithionite, 1 M NaOH, and 0.5 M HCl as the sole source of phosphorus. Analyses of chlorophyll-a, suspended solids, particulate organic carbon, and particulate organic nitrogen showed that M. aeruginosa could utilize some of the phosphorus present in each soil extract. These results imply that bioavailable phosphorus includes not only aluminum/iron-bound P and clay-bound P but also carbonate/apatite-bound P and residual P which have been considered not to be available to algae.