Leaching impacts Ni toxicity differently among soils but increases its predictability according to nitrification assay

Abstract

Deriving toxicity thresholds through bioassays has become the scientific basis for the risk assessment of metal contamination in soils, but few studies have been reported for Ni toxicity in Chinese soils. In addition, although leaching has been recommended to increase the ecological relevance of the output of soil toxicity bioassays, its impacts on Ni threshold and soil solutions in soils remained to be shown. Therefore, in this study, 17 soils were collected throughout China and were assessed for Ni toxicity using substrate-induced nitrification (SIN) assay for both un-leached and leached treatments. Meanwhile, soil solutions of all Ni treatments were extracted and analyzed. This study provides information for the development of a terrestrial biotic ligand model (TBLM) for Ni in Chinese soils. Soils were artificially amended with NiCl2 solution to generate nominal Ni contamination levels, and aliquot of each Ni treatment was leached with artificial rain to prepare leached soil samples. All the 17 un-leached and the 17 leached soils were subjected to the standard SIN protocol. A total of 816 microcosm tubes were established. Soil solution was extracted, and the solution physiochemical properties were analyzed for each Ni treatments. Both the total-Ni-based and the solution-Ni-based median effective concentrations (EC50) in un-leached soils and leached soils were calculated from dose–response curves. Free-Ni2+-based EC50 was also calculated based on solution chemical properties. Leaching effect was assessed based on these results. Ni EC50 varied largely among soils for both the total-Ni-based and the solution-Ni-based EC50 in un-leached soils. In un-leached soils, the total-Ni-based EC50 values ranged from 52.8 mg kg−1 (Hulunber soil) to 2,385.4 mg kg−1 (Wulumuqi soil), and the solution-Ni-based EC50 ranged from 0.70 mg L−1 (Jiaxing soil) to 293.35 mg L−1 (Wulumuqi soil). Stepwise multiple regressions showed that soil pH and total Ca were the best predictors for the total-Ni-based EC50. Protective effects of Ca2+ and Mg2+ but not H+ against Ni toxicity in soil solution were found. Leaching impacted Ni EC50 differently among soils, with leaching factor ranging from 0.2 (Haikou soil) to 19.7 (Hulunber soil) for the total-Ni-based EC50 and from 0.1 (Dezhou soil) to 35.7 (Jiaxing soil) for the solution-Ni-based EC50. In addition, predictability of Ni EC50 was higher in leached soils. The variations in Ni EC50 were in agreement with the findings in previous studies. Soil pH and Ca may play important roles in the partition of Ni in Chinese soils and thus impact the Ni toxicity. Protective effects of Ca2+ and Mg2+ in soil solutions against Ni toxicity may exist, but the role of H+ remained to be justified. The inconsistent impact of leaching on the free Ni2+ concentration at low Ni doses and the two competing effects of loss of salts in soil solution on Ni EC50 can partly account for the variation in leaching factor among soils. Ni toxicity varied largely among Chinese soils, and this variation can be largely explained by soil parameters. Leaching-impacted soil solution compositions and Ni EC50 values heavily. The impacts, however, were apparently different among soils. The results in this study can be used to develop a TBLM for Ni in Chinese soils. Furthermore, a need exists to quantitatively investigate the leaching effect on soil solution chemistry before leaching is used as a standard protocol in toxicity assays.