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Modeling the kinetics of the solution phosphate concentration during sorption and desorption experiments

Abstract : The concentration (Ct) of phosphate ions (Pi) in soil solution is the primary variable on which plant nutrition with phosphorus and eutrophication depend. The description of soil mobile Pi (Pm) is a necessary step in the modeling of Ct kinetics. The description of the change in the content of soil mobile Pi (ΔPm) must take into account the initial amount of mobile Pi (Pm,0). The change in the content of soil mobile Pi has been described by ΔPm =a (Cbt −x) tc, where a, b, x, and c are fitted parameters. We tested the suitability of a derived simplified equation where the native sorbed Pi, i.e., x, is calculated from the measured initial Pi concentration in solution (C0): ΔPm =a (Cbt −Cb0) tc. Both equations were compared for their suitability for describing the kinetics of Ct. Two sorption and one sorption-desorption experiments were performed on soil samples taken from plots having different P fertilizer histories in two long-term field trials located on Luvisols. The differences in conditions between experiments included (i) number of Pi addition rates (up to 120 mg P kg−1), (ii) soil-to-solution ratio, and (iii) period of contact between soil and solution (up to 7 days). In the adsorption domain, both equations fitted experimental Ct values (r2>0.99) closely. In the desorption domain, however, some slight discrepancies were observed between measured Ct values and those calculated by the simplified equation. They were probably the result of overestimation of C0. Thus, the proposed simplified equation accounts for the participation in the soil solution Pi dynamics of the initial mobile Pi content of the soil that determines the initial Pi concentration in solution (C0). PLANT phosphorus (P) nutrition and eutrophication both depend on the concentration of phosphate ions (Pi) in the soil solution (Barber, 1984). Thus, agronomists and soil scientists are mainly interested in the change of the solution Pi concentration during the time after a supply, after a rainfall event, or during plant uptake. Pi concentration in the soil solution depends on the intensity of the constraint applied to the soil solution system, i.e., the dilution, the plant absorption flux, or the level of supply, on the content in the mobile Pi of the soil solid phase, i.e., the pool that participates in the soil solution dynamics when the soil solution system is put out of balance, and on time. A necessary step in the modeling of the kinetics of the soil solution Pi concentration is, therefore, the description of the fate of the soil mobile Pi.
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André Schneider, Alain Mollier, Christian Morel. Modeling the kinetics of the solution phosphate concentration during sorption and desorption experiments. Soil Science, Lippincott, Williams & Wilkins, 2003, 168 (9), pp.627-636. ⟨10.1097/01.ss.0000090803.06903.f5⟩. ⟨hal-02677471⟩

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