Suitability of isotope kinetic approach to assess phosphorus status and bioavailability of major acidic soils in subtropical China

A ³²P isotope kinetic approach was used to describe the chemical status and bioavailability of phosphorus in 32 acidic soils from subtropical China. By determining the residual radioactivity, r_t, in soil solution at different time, t, after introduction of the isotope in an amount of R into the steady soil-water system, a well-defined isotope kinetic model was established, and upon this model the decrease rate, n, of log(r_t/R) with respect to logt, the mean sojourn time of phosphate ions in solution, the mean exchange rate and the mean flux of phosphate ions between soil solid and solution phases were calculated. Other parameters, such as the exchangeable P within the first minute of isotope exchange (E₁), and P in various compartments that could be exchanged with solution phosphate ions at different periods of time, were also obtained. For these acidic soils, the r₁/R had a significant correlation with the contents of clay and free Al₂O₃ where r₁ is the radioactivity in solution 1 minute after introduction of the isotope into the system. Parameter n also had a significant correlation with clay content and a negative correlation with soil pH. E₁ values and C_p, the P concentration in soil solution, also significantly correlated with clay and sesquioxide contents of the soils. These indicated that these isotope kinetic parameters were largely influenced by P-fixing components of the soils. For the soils with strong P-fixing ability, the E₁ values overestimated labile P pools and hence their correlations with A values and plant P uptake were not significant. The other isotope kinetic parameters also had no significant correlation with plant P uptake. On the other hand, the conventional chemical-extracted P correlated better with plant P uptake. It was concluded that the isotope kinetic method could assess the P chemical status yet it would be inappropriate in predicting plant available P for soils with a high P-fixing ability as the problem of an overestimation of soil labile P in these soils was inevitable.