The isotope exchange kinetic technique: A method to describe the availability of inorganic nutrients. Applications to K, P, S and Zn

This paper presents the interests and the limits of the isotope exchange kinetic method to assess soil K, P, S and Zn availability for plants. This method allows to quantify the amount of nutrient present on the soil's solid phase which can arrive in the solution of a soil/solution system at a steady state. Three parameters characterize this exchange in addition to exchange time: R/r(1) which is the ratio between the total introduced radioactivity (R) and the radioactivity remaining in the soil solution after 1 minute of exchange (r1), n which describes the rate of disappearance of the radioactivity from the solution for exchange times longer than 1 minute, and C which is the concentration of the studied element in the soil solution. These three parameters can be used to calculate the amount of element isotopically exchangeable with time (E(t) values). This approach gives access to the three parameters characterizing the availability of inorganic nutrient: the intensity factor (the concentration of nutrient in the solution), the quantity factor (the quantity of nutrient which can be taken up by a plant E(t)), and the capacity factor which describes the relationship between the intensity and the quantity factors (R/r(1) and n). Results obtained by this approach are relevant to areas such as fertilizer management and organic matter mineralization. This method bears also a large potential for assessing the bioavailability of heavy metals, such as Zn, either in agricultural soils where those nutrients may be limiting or in polluted soils where they may be present in excessive concentrations. However, results obtained by this approach may be properly interpreted if and only if the basic assumptions underlying its use are fulfilled, and if the various parameters (C, R/r(1), n) are measured with enough precision.