Juan L. Celis-Diez

Thresholds of arsenic toxicity to Eisenia fetida in field-collected agricultural soils exposed to copper mining activities in Chile

Several previous studies highlighted the importance of using field-collected soils-and not artificially-contaminated soils-for ecotoxicity tests. However, the use of field-collected soils presents several difficulties for interpretation of results, due to the presence of various contaminants and unavoidable differences in the physicochemical properties of the tested soils. The objective of this study was to estimate thresholds of metal toxicity in topsoils of 24 agricultural areas historically contaminated by mining activities in Chile. We performed standardized earthworm reproduction tests (OECD 222 and ISO 11268-2) with Eisenia fetida. Total soil concentrations of Cu, As, Zn, and Pb were in the ranges of 82-1295 mg kg(-1), 7-41 mg kg(-1), 86-345 mg kg(-1), and 25-97 mg kg(-1), respectively. In order to differentiate between the effects of different metals, we used regression analysis between soil metal concentrations and earthworm responses, as well as between metal concentrations in earthworm tissues and earthworm responses. Based on regression analysis, we concluded that As was a metal of prime concern for Eisenia fetida in soils affected by Cu mining activities, while Cu exhibited a secondary effect. In contrast, the effects of Zn and Pb were not significant. Soil electrical conductivity was another significant contributor to reproduction toxicity in the studied soils, forcing its integration in the interpretation of the results. By using soils with electrical conductivity ≤ 0.29 dS m(-1) (which corresponds to EC50 of salt toxicity to Eisenia fetida), it was possible to isolate the effect of soil salinity on earthworm reproduction. Despite the confounding effects of Cu, it was possible to determine EC10, EC25 and EC50 values for total soil As at 8 mg kg(-1), 14 mg kg(-1) and 22 mg kg(-1), respectively, for the response of the cocoon production. However, it was not possible to determine these threshold values for juvenile production. Likewise, we were able to determine EC10, EC25 and EC50 of earthworm tissue As of 38 mg kg(-1), 47 mg kg(-1), and 57 mg kg(-1), respectively, for the response of the cocoon production. Finally, we determined the no-observed effect concentration of tissue As in E. fetida of 24 mg kg(-1). Thus, earthworm reproduction test is applicable for assessment of metal toxicity in field-collected soils with low electrical conductivity, while it might have a limited applicability in soils with high electrical conductivity because the salinity-induced toxicity will hinder the interpretation of the results.

Zinc alleviates copper toxicity to symbiotic nitrogen fixation in agricultural soil affected by copper mining in central Chile.

According to the Terrestrial Biotic Ligand Model, other cations might compete with Cu+2 for biotic ligand sites and provide a protective effect. In particular, evidence suggests Zn may alleviative Cu toxicity. No study, to the best of our knowledge, has focused explicitly on the alleviating effect Zn might have on Cu toxicity to soil microorganisms in field-contaminated soils. The aim of this study was to investigate the alleviating effect Zn might have on Cu toxicity to symbiotic nitrogen fixation in agricultural soils affected by copper mining in central Chile. The bioassay estimated the symbiotic nitrogen fixation capacity of a population of rhizobia in a specified soil, using the soil as inocula for Phaseolus vulgaris L. grown in a soil-less system (pots with perlite) irrigated with a sterile nitrogen-free nutrient solution. Among all soil physicochemical characteristics, the Cu/Zn ratio best explained changes in symbiotic nitrogen fixation. The effective concentration 50% (EC50) of Cu/Zn ratio for symbiotic nitrogen was equal to 1.2, with 95% confidence interval of 1.0-1.3. Symbiotic nitrogen fixation decreased with increased Cu/Zn ratio, thus suggesting that Zn alleviates Cu toxicity to nitrogen fixing microorganisms.