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This article in SSSAJ

  1. Vol. 58 No. 3, p. 846-851
     
    Received: July 8, 1993
    Published: May, 1994


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doi:10.2136/sssaj1994.03615995005800030031x

Soil Isotopically Exchangeable Phosphorus: A Comparison between E and L Values

  1. E. Frossard ,
  2. J. L. Morel,
  3. J. C. Fardeau and
  4. M. Brossard
  1. LAE-INRA-ENSAIA, BP 172, F-54505, Vandœuvre-lès-Nancy Cedex, France
    DPVE CEN Cadarache, CEA, F-13108 St Paul-lez-Durance, France
    ORSTOM c/o Centre de Pédologie Biologique-CNRS, BP 5, F-54501 Vandoeuvre-lès-Nancy Cedex, France

Abstract

Abstract

This study was designed to explain the apparent discrepancies often reported in the literature between E and L values, two parameters obtained from isotopic exchange experiments and commonly used to quantify available soil phosphate. The E and L values of the surface horizons of 10 soils from tropical, mediterranean, and temperate regions were determined. The L value was measured from a 13-wk pot experiment with common bentgrass (Agrostis capillaris L.) where the available soil P was labeled with 32PO4 ions in the presence of a carrier (25–50 mg 31P kg−1 soil). To determine the E value, the isotopic exchange kinetic experiment was carried out on each soil. Carrier-free 32PO4 was added to the soil-solution system at a steady state and the quantity of isotopically exchangeable soil phosphate at time t, E(t), was calculated from the kinetic equation describing the decrease of radioactivity in solution with time. Results showed that L values determined after 13 wk were not significantly different from E(t) values extrapolated to the same period (t = 131040 min). It was concluded that (i) the L value is a particular point of the kinetic equation and (ii) isotopically exchangeable phosphate is the available P for common bentgrass. A strict equality between E(13wk) and L values was not, however, reached for all samples. Possible causes for the differences were: (i) an overestimation of the water-soluble phosphate due to the presence of silica and (ii) disturbance of the steady state following a too large uptake of phosphate by the crop or the application of too large quantities of carrier compared with the initial quantity of exchangeable soil phosphate.

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