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

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 (PO4)-P-32 ions in the presence of a carrier (25-50 mg P-31 kg-1 soil). To determine the E value, the isotopic exchange kinetic experiment was carried out on each soil. Carrier-free (PO4)-P-32 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 = 131 040 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.