Use of transfer factors to characterize uptake of selenium by plants

The radioactive selenium isotope 79Se can be a component of radioactive waste produced in nuclear power plants. The accidental release of 79Se in the environment from power plants or nuclear waste repositories, and subsequent transfer of 79Se into soils, plants and food chain, are relevant concerns in Europe. In environmental risk assessment models, the ability of plants to take up Se is often characterized by means of soil-to-plant transfer factors (TF). However, these recommendations take little or no account of the variability in plant Se accumulation among plant species and differences in Se distribution between plant parts. Also, such factors as soil type, form of Se, climatic conditions, as well as the chemical form in which Se is present in the soil, are not taken into consideration.This paper reviews the current knowledge on Se uptake by plants and compiles published data on the transfer of Se from soils and nutrient solutions into plants. The data were categorized according to the form of Se supply, i.e. when added as either selenate or selenite to nutrient solutions or soils, or whether plant Se accumulation was studied on soil containing only natural-occurring Se (native Se).Plant Se accumulation can vary more than two orders of magnitudes at a given soil Se concentration for a specific form of soil Se (i.e. “native” soil Se, Se added as selenate, or as selenite) among different plant taxa. Differences observed in the transfer of Se from soil into plants appear to result primarily from differences in the solubility of Se species in soil and only to a minor degree from differences in plant uptake efficiency among these species.Values of the TF for Se were found to lie between 0.01 and 100 with few exceptions. Transfer factor values derived from studies in which Se was added to soil as selenate or selenite were generally found to be one or more orders of magnitude higher for a given plant species and plant part than TF values derived for “native Se”. Situations where radioactive Se has been accidentally released into the environment may be better represented by TF values for experimentally added Se than by TF values computed for native soil Se.The uncertainty in selecting a TF value for modeling Se transfer from soil into plants can be substantially reduced by identifying the predominant Se form in the soil, the plant species, and the target plant parts. The large variability in TF values suggests that it is still important to understand the complexity of the soil-plant system when selecting TF values to assess risks arising from the transfer of accidentally released 79Se from soil into plants.

Research Highlights
► Plant Se accumulation can vary more than two orders of magnitude depending on form of Se.
► Transfer factors are between 1 and 10 depending on plant, soil and climatic conditions.
► Transfer factors derived from selenate additions are 1 or more orders of magnitude higher than native Se or released radioactive Se.
► Variability in transfer factor values clearly show that it is essential to understand soil-plant system.