The effect of slow adsorption of phosphate on its transport during the infiltration process in saturated agricultural soils

ABSTRACTAssessment of phosphorus (P)-infiltrating croplands is essential for the preservation of the water environment. It has been pointed out that a huge discrepancy lies in the different evaluation methods of P adsorption, such as batch experiments and column experiments, which makes it difficult to demonstrate P mobility under flow conditions. The objective of this study was to evaluate the applicability of the convective-dispersion equation using the parameters of the Langmuir-type isotherm obtained from batch experiments with different reaction times: the adsorption capacity of soil (qmax) = 0.112 (g kg−1) for a Gray lowland soil with 24 h reaction time, qmax = 0.484 (g kg−1) for an Andosol (volcanic ash soil) with 24 h reaction time, and qmax = 1.17 (g kg−1) for an Andosol with 32 d reaction time, for describing P mobility in typical Japanese agricultural soils under fast flow conditions. The breakthrough curves of P infiltrating the soil columns demonstrate nonequilibrium P adsorption by the soil. The chemical nonequilibrium model, with a kinetic adsorption rate of α = 0.40 (Gray lowland soil) and 0.098 (Andosol), succeeded in describing the observations in the column experiments. Compared with Gray lowland soil, which is relatively rich in iron oxide, P mobility was largely affected by kinetic sorption in Andosol, which is relatively rich in allophane. It is suggested that the P adsorption capacity of soils should be evaluated reflecting the soil composition in order to simulate the P mobility under flow conditions. In particular, the slow adsorption (long-lasting adsorption) of P by the soil should be considered in the estimation of the P transport.KEYWORDS: Adsorption isothermAndosolGray lowland soilchemical nonequilibrium modelconvection-dispersion equation AcknowledgmentsWe thank Dr. Takeo Shima (Kyushu-Okinawa Agricultural Research Center, National Agriculture and Food Research Organization) for providing us with soil samples. This work was supported by JSPS KAKENHI (grant number JP21H02305).Disclosure statementNo potential conflict of interest was reported by the author(s).Additional informationFundingThe work was supported by the Japan Society for the Promotion of Science [JP21H02305].