Ciprian Stroia

Effect of pooling soil samples on the diffusive dynamics of phosphate ionic species

Limited information is available on the effect of pooling subsamples of soils on the dynamics of diffusive phosphorus (P), the phosphate ions in the soil solid phase that can equilibrate with the solution under a gradient of ionic P concentration. We examined the change in the parameters of the kinetic Freundlich equation, which accounts for the interactive effects of time (t, min) and soil solution P on the gross amounts of diffusive soil P. A common procedure was used to prepare a single pooled sample from four individual soil samples taken from each block of the same agronomic treatment of a grassland experiment. The pooling procedure consisted of combining four individual soil subsamples and mixing them on orbital shaker (100 r.p.m.) with agate balls. One of the individual samples was mixed without pooling. The kinetic Freundlich parameters were estimated for all individual and pooled samples by coupling a sorption experiment with a subsequent 32P labeling and dilution analysis in soil suspensions at steady state. The pooling procedure of soils, more specifically, the mixing step, was found to significantly change the kinetic Freundlich parameters that characterize both the immediate and the slow reactions. The immediate sorption capacity increased by 50%, whereas the slow process decreased by about 10%. This pooling procedure should, therefore, be avoided because it leads to a large and variable effect on the dynamics of diffusive soil P. We recommend determining the kinetic Freundlich parameterization on individual soil samples.