Soil-to-soybean transfer of 99Tc and its underground distribution in differently contaminated upland soils

• Soybean TF values of 99Tc were markedly lower for the seed than for the other aerial parts.
• A growing-period deposition (GPD) led to a higher transfer of 99Tc than pre-sowing one.
• Post-harvest 99Tc activities in the GPD soils showed logarithmic decreases with depth.
• We stressed the importance of using empirical aggregated TF data for the GPD 99Tc.
• Soybean 99Tc uptake appeared substantial but the fallen leaves likely took most of it.

Pot experiments were performed in a greenhouse to investigate the soil-to-soybean transfer of 99Tc in two different upland soils labeled with 99TcO4− in two contrasting ways. One was to mix the soil with a 99TcO4− solution 26 d before sowing (pre-sowing deposition: PSD), and the other was to apply the solution onto the soil surface 44 d after sowing (growing-period deposition: GPD). The soil-to-plant transfer was quantified with the transfer factor (TF, ratio of the plant concentration to the average of at-planting and at-harvest soil concentrations) or the aggregated transfer factor (TFag, ratio of the plant concentration to the deposition density). For both the depositions, the transfer of 99Tc to aerial parts decreased in the order of leaf > stem > pod > seed. TF values (dimensionless) from the PSD were 0.22 and 0.27 (no statistically significant difference) for mature dry seeds in the respective soils, whereas a 600-fold higher value occurred for dry leaves. The post-harvest concentrations of the PSD 99Tc in the top 20 cm soils as a whole were about half the initial concentrations. Around 25% of the total applied activity remained in the GPD soils after the harvest. The post-harvest depth profiles of the GPD 99Tc in the two soils showed similar patterns of logarithmic activity decrease with increasing soil depths. Only 1.5–4.3% of the total applied activity was removed through the harvested biomass (seeds, pods and stems), and it was estimated that a great part of the total pant uptake returned to the soil through the fallen leaves. TFag values (m2 kg−1) were about 2–4 times higher for the GPD than for the PSD. This finding and generally high root uptake of Tc may indicate that the use of empirical deposition time-dependent TFag data is particularly important for predicting the plant concentrations of Tc after its growing-period deposition.