Determining bioavailable phosphorus in some paddy soils by algal assay method

The loss of phosphorus (P) from agricultural lands and its entry into water resources has caused the algal bloom and eutrophication phenomena, and has marked the control of P entrance to aquatic ecosystems as an important challenge for the management of these resources. Algal assay is considered as the most reliable method for quantifying the potential of P bioavailability. The aim of this study was to investigate the bioavailability of P in three paddy soils and its relationship with the growth of Chlorella sp. microalgae.

Soil samples were taken from a depth of 0-10 cm of paddy fields located in five regions of Pasikhan, Pirbazar, Siahdarvishan, Fakhab and Lakan, which had annually received phosphorus fertilizer for a long time. At first, different P fractions were determined by Hedley sequential extraction method in 5 soils, and 3 soils (Psikhan, Pirbazar and Lakan) based on the total P content, P in different fractions and clay content were selected and added to the Chlorella sp. culture medium as the only P supply source. The experiment was performed as repeated measures in a completely randomized design with three replications. Chlorophyll a content, microalga cell number, total dissolved P (TDP) and particulate P (PP) concentrations were measured at days 0, 7, 14, 21, and 28 of incubation.

Phosphorus fractions of Pirbazar and Fakhab soils were similar as followed in oxide >residual >carbonate> solution-exchangeable. While in the other three soils, the phosphorus fractions followed the trend of residual >oxide >carbonate> solution- exchangeable. The chlorophyll a content showed an increasing trend in all samples and reached its maximum on the 14th day and was then almost constant. The number of microalgae cells in Pirbazar treatment showed an increasing trend until the 14th day of incubation, while it remained increasing in Lakan and Pesikhan treatments until the end of the incubation period. TDP concentration increased during the incubation period in all treatments until the 7th day and then decreased. Particulate phosphorus decreased sharply on the 7th day of incubation and then increased. The chlorophyll a content and the number of microalgae cells showed a significant positive correlation with TDP at the first two weeks of incubation, while the microalga growth was affected by particulate P in the next two weeks of incubation.

Since soil as the only source of P can accelerate the microalgae growth in aquatic ecosystems, considering P originated from agricultural lands along rivers to manage the eutrophication phenomenon is necessary especially in wetlands catchment.