Adsorption of Phosphorus Infiltrating Andosol in Aso Caldera, Kumamoto

High concentration (more than 1.0 mgP/L) of phosphorus was detected in groundwater in Aso caldera, Kumamoto. The fact indicates phosphorus can be transported by groundwater flows. The objective of this study is to clarify the characteristics of paddy soil (Andosol) in phosphorus adsorption process. Adsorption capacity of the Andosol was evaluated by continuously-flowing system using the soil column. In addition, soil phosphorus was categorized into five fractions; water-extractable phosphorus (Water-P), phosphorus extracted by sodium bicarbonate and sodium dithionate (DB-P), phosphorus extracted by sodium hydroxide (NaOH-P), phosphorus extracted by hydrochloric acid (HCl-P) and residual phosphorus (Res-P). As a result, it is confirmed that the flow rate of water is an important factor to control the phosphorus adsorption by the Andosol. The lower the flow rate is, the more phosphorus is adsorbed on the upstream soil. On the other hand, when the flow rate was high, phosphorus was evenly distributed in column soil. In addition, the fraction of NaOH-P accounted for 52% of the adsorbed phosphorus, suggesting that NaOH-P is the most important fraction which controls phosphorus adsorption to Andosol.