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Prediction of microbially mediated selenium (Se) transfers in Se-contaminated soils is required for accurate assessment of their potential use in bioremediation techniques. A mathematical model was developed to describe short-term, microbially mediated Se reactions linked to the C cycle in a straw-amended soil, and the model was then validated. Straw decomposition was assumed to follow first-order kinetics and the amount of microbial biomass produced to be determined by the initial amount of readily available decomposable C (Cdpm). Microbial biomass decay was considered to occur when Cdpm became limited. The Se reactions considered were the sequential reduction of Se(VI) to Se(IV) and, thereafter, to Se(0) as well as the methylation of Se from Se(VI) and Se(IV) sources. These reactions were assumed to depend on both the Se concentrations and the amount of microbial biomass. The latter was introduced into the equations expressed as a fraction of the maximum amount of microbial biomass produced during the experiment. Values of the parameters used in the model were estimated from data available in the literature. The differential equations were solved simultaneously with a numeric scheme using the 4th order Runge-Kutta method followed by the 4-step Adams-Bashforth-Moulton predictor-corrector method. The results obtained indicate that the links between the cycles of the two elements must be taken into account in order to reflect the influence of changes in the amount of microbial biomass on the different processes of Se transfer. Overall, the model was shown to be useful in establishing the relative importance of the different parameters under study. However, for longer-term experiments, the model should include other pools and fluxes of these elements as well as environmental responses.