Influence of clay content and salinity on the bioavailability and toxicity of metals (copper and zinc) to soil organisms

[摘要] ENGLISH ABSTRACT: Metal pollution is a problem of increasing global concern. It could arise fromindustrial activities, as well as pesticide use in agriculture, among other sources. Foradequate protection of the soil ecosystem from metal toxicity, the bioavailability ofmetals must be properly evaluated. A plethora of soil factors affect the bioavailabilityof metals to soil organisms. These include pH, clay and organic matter contents,salinity among others. While much is known about the influence of some of theseparameters, little is known on how clay content and salinity modify the bioavailabilityof metals to soil organisms.This study investigated the influence of clay content and salinity onpartitioning, uptake and toxicity of two essential metals (Cu and Zn) to the earthwormEisenia fetida in separate laboratory trials. Partitioning of the metals was evaluatedwith 0.01 M CaCl2, DTPA (di-ethylene-triamine-penta acetic acid), and nitric acidextractions. The metal content of worms was determined by acid digestion, whilegrowth, cocoon production, and mortality were used as endpoints showing toxicity tometals and/or salinity. To test the validity of some of the laboratory results, a fieldstudy was undertaken, using the earthworm Aporrectodea caliginosa. Further, thestudy assessed the effect of salinity, using a battery of laboratory tests (acute, chronicand avoidance tests) with natural and/or artificial soils on four species of organisms (acollembolan Folsomia candida, a potworm Enchytraeus doerjesi and two earthwormspecies E. fetida and A. caliginosa), representing different feeding patterns andecological roles in soil.Results showed that with increased clay content, there was increasedavailability of Cu in the substrate, and increased toxicity to E. fetida as shown by datafor mortality and growth. The situation with Zn was less significant at sub-lethalconcentrations but much so at lethal concentrations. DTPA and CaCl2 extractedmetals revealed changes in partitioning of Cu and Zn with changes in clay content,but this trend was not always consistent. Both DTPA and CaCl2 revealed increasedavailability of Zn in substrates with increased salinity. Salinity had an additive tosynergistic effect with Zn in toxicity to E. fetida. When combined with Cu, salinityalso increased the availability of Cu as shown by CaCl2 extracted fraction, and hadadditive effect on toxicity of Cu to the earthworm. The field study did not succeed in inivconfirming the results of the laboratory study due to confounding role of floodingafter heavy rainfall and subsequent leaching of salts and Cu. The results of theexperiment on acute and chronic toxicity tests for NaCl on E. fetida showed LC50 of5436 mg/kg NaCl and EC50 for growth and cocoon production of 4985 and 2020mg/kg NaCl. These values showed that earthworms might be negatively affected inmany soils containing fairly moderate concentrations of salts. Similarly, A. caliginosacould not survive in natural soil containing relatively low salt concentrations (EC =1.62 dS/m) while reproduction was severely affected at lower EC value of 0.52 dS/m.F. candida and E. doerjesi could survive in the highest salinity soil (EC = 1.62 dS/m)used in this study but their reproduction was severely affected from 1.03 dS/m.Overall, it appears that of all the taxa used, earthworm species were the most sensitiveto saline stress and could proof useful in determining 'safe levels' of salt incontaminated soils. The results of the avoidance test showed that A. caliginosaavoided both natural and artificial saline soil containing concentrations lower thanthose avoided by E. fetida.The conclusion is that the influence of clay content and salinity on thebioavailability of Cu and Zn depends largely on the metal in question, but generallyspeaking, bioavailability and toxicity of the metals were reduced with increased claycontent while the opposite was true for salinity. If the species used in this study can beseen as fairly representative of a wide range of soil organisms, the conclusion is thatsalinisation of soil will be detrimental to most soil organisms at relatively low salineconcentrations. Given the role of beneficial soil organisms in several soil processeswhich in turn contribute to soil fertility and sustainable use of land, it is recommendedthat any farming practices that may lead to an increase in salt content of agriculturalsoils should be discouraged.

[发布机构] Stellenbosch University