Soil amendments effects on radiocesium translocation in forest soils

We conducted an experiment to investigate the potential of phytoremediation by soil amendments in a forest area. To desorb radiocesium (¹³⁷Cs) from variable charges in the soil, ammonium sulfate (NH₄⁺) and elemental sulfur (S) (which decrease soil pH) were applied to forest soil collected from contaminated area at a rate of 40 and 80 g/m², respectively. A control condition with no soil treatment was also considered. We defined four groups of aboveground conditions: planted with Quercus serrata, planted with Houttuynia cordata, covered with rice straw as litter, and unplanted/uncovered (control). Cultivation was performed in a greenhouse with a regular water supply for four months. Following elemental sulfur treatment, soil pH values were significantly lower than pH values following ammonium sulfate treatment and no treatment. During cultivation, several plant species germinated from natural seeds. No clear differences in aboveground tissue ¹³⁷Cs concentrations in planted Q. serrata and H. cordata were observed among the treatments. However, aboveground tissue ¹³⁷Cs concentration values in the germinated plants following elemental sulfur treatment were higher than the values following the ammonium sulfate treatment and no treatment. Although biomass values for Q. serrata, H. cordata, and germinated plants following elemental sulfur treatment tended to be low, the total ¹³⁷Cs activities in the aboveground tissue of germinated plants were higher than those following ammonium sulfate treatment and no treatment in rice straw and unplanted conditions. Although no significant differences were observed, ¹³⁷Cs concentrations in rice straw following ammonium sulfate and elemental sulfur treatments tended to be higher than those in the control case. The results of this study indicate that elemental sulfur lowers the soil pH for a relatively long period and facilitates ¹³⁷Cs translocation to newly emerged and settled plants or litter, but affects plant growth in large concentrations and/or anaerobic conditions. Combining elemental sulfur application with forest management practices, such as mowing and thinning, could be a suitable method of decontamination of the forest environment. - Highlights: • Elemental sulfur maintained a lower soil pH than ammonium sulfate. • Elemental sulfur application increased ¹³⁷Cs concentrations in germinated plants and surface organic matter. • Excess elemental sulfur would decrease plant biomass.