Methodological approach to assess the effect of soil ageing on selenium behaviour: first results concerning mobility and solid fractionation of selenium

The aim of the study presented here is to determine the impact of short- and medium-term transformations (0-3 years) of the soil organic matter (SOM) on the major processes and parameters that enable or inhibit selenite, Se(+IV), transfers between the soil components ( solid, liquid or gaseous). Three types of soil of similar mineralogical origin but containing diverse quantities and qualities of SOM were first contaminated with Se(+IV) and incubated at 28 C. Soils were sampled throughout the incubation period to characterise the mobility of Se (batch and soil column experiments) and also its fractionation within the soil compartments (selective extractions and size-density fractionation). The following are the main results obtained within the first month of incubation. (a) Selenium was partly volatilized during soil incubation (< 0.1%), (b) Se extracted with CaCl2 (5 x 10(-4) M) was equally small for the three soil samples (similar to 1-5%), suggesting that Se was strongly sorbed on the solid phase and ( c) at least 10% of Se was associated to the particulate organic matter (POM (> 50 mu m)) whereas 60% of Se was extracted with soil humic substances. These results suggested that both SOM quantity and quality played a significant role in selenium retention. Furthermore, comparison between experimental and predicted variations of CO2 fluxes (due to C mineralisation) and soil biomasses are presented. By this way, we estimated the capacity of the RothC model as an experimental gauging tool in the prediction of C turnover on a laboratory scale.