Salinity increases mercury mobilisation from soil but reduces its uptake in crops

Mercury (Hg) contamination of soils is a concerning issue worldwide. Source of contamination, chemical form and environmental conditions affect its mobility and bioavailability in soils and sediments. Contaminated coastal soils could potentially become hotspots of Hg re-mobilisation, because of flooding and salinization caused by the sea level rise expected as a consequence of climate change. Samples of a soil contaminated by mining activity and a sediment heavily contaminated by a chloro-alkali plant were collected from North-East Italy. Mercury speciation was performed either by Thermal Desorption (TD), or a Sequential Extraction Procedure (SEP). Bench-top simulations of salinity and flooding events was carried out using soil columns. Soils were kept submerged for different periods (1-30 days) and different salt concentrations (0-32.8 g L-1). After the flooding treatment, a rain event was simulated using freshwater. Leached solutions were collected and solubilised Hg quantified. Hg bioavailability was assessed in barley, grown in pots filled with contaminated soil. Hg mobility increased with increasing salinity and flooding time. The effect of salt concentration was more pronounced for longer flooding periods. After 1 day of inundation, the amount of solubilised Hg was negligible, but after 7 days flooding Hg levels in the highest saline solution reached up to 0.9 and 9.3 μg Hg L-1 in the soil and in the sediment, respectively. The mobility of Hg increased substantially after 30 days, and was about 22 times in the soil (19.8 μg Hg L-1) and 10 times in the sediment (89.8 μg Hg L-1) compared to the background level. Despite the potential risk of Hg re-mobilisation from contaminated soils and sediments, Hg accumulation in barley decreased either in shoots and roots as salinity increased. The mechanism can be possibly ascribed to a stronger ionic competition or a reduced physiological activity of the plants under high salt stress.