Toxicity in lead salt spiked soils to plants, invertebrates and microbial processes: Unraveling effects of acidification, salt stress and ageing reactions.

The fate and effects of toxic trace metals in soil freshly spiked soluble metal salts do not mimic those of metals in the field. This study was set up to test the magnitude of effects of salinity, acidification, and ageing on toxicity of lead (Pb) to plants, invertebrates and soil microbial processes. Three soils were spiked with Pb2+ salts up to a concentration of 8000 mg Pb/kg and were tested either after spiking, after soil leaching followed by pH correction, or after a 5-year outdoor ageing period with free drainage followed by pH correction. Soil solution ionic strength exceeded 150 mmol/L in soils tested directly after spiking and this decreased partially after leaching and returned back to background values after 5-year outdoor equilibration. Chronic toxicity to two plants, two invertebrates, and three microbial endpoints was consistently found in all spiked soils that were not leached. This toxicity significantly decreased or became absent after 5 years of ageing in 19 of the 20 toxicity tests by a factor 8 (median factor; range: 1.4->50), measured by the factor increase of total soil Pb dose required to induce 10% inhibition. The toxicity of Pb in leached soils was intermediate between the other two treatments. The lowest detectable chronic thresholds (EC10) in aged soils ranged 350-5300 mg Pb/kg. Correlation analysis, including data of Pb2+ speciation in soil solution, suggests that reduced ionic strength rather than acidification or true ageing is the main factor explaining the soil treatment effects after spiking. It is suggested that future toxicity studies should test fine PbO powder as a relevant source for Pb in soils to exclude the confounding salt effects.

Lead phytotoxicity in soils and nutrient solutions is related to lead induced phosphorus deficiency.

This study was set up to relate lead (Pb) bioavailability with its toxicity to plants in soils. Tomato and barley seedlings were grown in six different PbCl(2) spiked soils (pH: 4.7-7.4; eCEC: 4.2-41.7 cmol(c)/kg). Soils were leached and pH corrected after spiking to exclude confounding factors. Plant growth was halved at 1600-6500 mg Pb/kg soil for tomato and at 1900-8300 mg Pb/kg soil for barley. These soil Pb threshold were unrelated to soil pH, organic carbon, texture or eCEC and neither soil solution Pb nor Pb(2+) ion activity adequately explained Pb toxicity among soils. Shoot phosphorus (P) concentrations significantly decreased with increasing soil Pb concentrations. Tomato grown in hydroponics at either varying P supply or at increasing Pb (equal initial P) illustrated that shoot P explained growth response in both scenarios. The results suggest that Pb toxicity is partially related to Pb induced P deficiency, likely due to lead phosphate precipitation.