Increased soil pH and enhanced microbial activity stimulate the gradual immobilisation of selenate added to soils

In recent years, a global interest in selenium (Se) has arisen, both in the light of crop biofortification and risk assessments of ⁷⁹Se present in nuclear waste. In both cases, a profound understanding of the fate of Se in soils is required. The objectives of this study were to evaluate the fate of selenate (Se(VI)) added to soil and to relate the rate and extent of its immobilisation in the months after soil spiking, termed ageing, to soil properties. The underlying hypothesis is that Se mobility can be reduced by incorporation in microbial biomass and by pH-dependent adsorption to oxyhydroxides. Ageing of Se was studied in 14 soils with contrasting properties after spiking with an enriched ⁷⁷Se(VI) isotope tracer. During six months of incubation, subsamples of the soils were collected and extracted to monitor the mobile, adsorbed and NaOH-extractable fractions of soil-native Se and spiked ⁷⁷Se. After 182 days, the mobile concentration of the ⁷⁷Se spike was reduced by a factor 2-300, with the largest factors consistently found in soils with a pH above 6. The decrease in Se availability with time was described by first-order kinetics, which allowed to derive a rate and extent of Se ageing in soils. Distinct but gradual ageing was mainly promoted by high soil pH, whereas Se immobilisation was faster but less pronounced in low pH soils. Amendment of five soils with a carbon source enhanced microbial activity, thereby increasing the rate and/or extent of Se ageing. Also among the unamended soils, the immobilisation rate constant increased with increasing measured soil respiration rates. This study showed a pronounced effect of both soil pH and biochemical reactions on the time-dependent solid:liquid distribution of Se, which should be considered in biofortification practices and risk assessments.