Effect of selenite additions on microbial activity and dynamics in three soils incubated under aerobic conditions

Three soils were incubated for 4-day periods with selenite (0-5 mg Se kg⁻¹) and the effect of straw amendment was also studied for one of these soils. The same soil was also incubated for up to 6 weeks, with and without straw and selenite (0-2 mg Se kg⁻¹). Changes in microbial activity (oxygen respiration) and dynamics (total, fungal biomass; bacterial density number) were assessed. Selenium fractionation and volatilization were also quantified. Selenium addition had no marked effect on respiratory activity, total and fungal biomass whatever the incubation period, but inhibited the growth of heterotrophic bacteria in the longer term. When straw was added, the same trend was observed with more variability. The volatilization losses of Se differed between soils but did not vary significantly over the 6-week period and were not affected by straw addition. Freshly added Se was more easily extracted than native soil Se and its extractability decreased with time, whereas no change in the fractionation of native Se was observed. This study shows that moderate additions of Se may influence microbial dynamics in aerated soil but could not establish a causal link with changes in Se fractionation.