RESUMO
The combined effects of salinity and organic amendments on lead (Pb) toxicity to earthworms as important components of soil invertebrates are still largely unknown. A mesocosm experiment was conducted to examine how the combined use of NaCl salinity and cow manure would affect the sublethal Pb toxicity to chronically exposed Eisenia fetida in natural soil. The response of life-cycle parameters of this earthworm species and biological properties to NaCl-induced salinity (0, 4 and 8 dS m-1) was determined in a Pb-contaminated clay loam soil amended or unamended with fresh cow manure. The NaCl salt and cow manure (4%, w/w) were added to the soil and the mixtures were incubated for 90 days under greenhouse conditions. The results showed that NaCl salinity increased soil Pb availability and toxicity, increased earthworm Pb concentration and uptake, and decreased earthworm survivorship, population (adults and juveniles), reproduction, wet weight, cocoon production, and cast activity. The detrimental effects of NaCl salinity on earthworms and biological properties were greater at high than low salinity levels. Addition of cow manure lowered the NaCl-induced Pb toxicity to earthworms at all salinity levels, suggesting the harmful effect of salinity-induced Pb toxicity was reduced due to the decreased Pb availability following manure application. This study demonstrated that soil salinity and animal manures can have a great impact on the life-cycle endpoints and activity of E. fetida, which requires attention when using saline waters for irrigation and organic amendments for soil amelioration in Pb-contaminated environments. It is concluded that (i) the multiple stresses induced by salinity and Pb mixtures may negatively affect earthworms and (ii) organic amendment application has a high potential for lowering the stronger negative effect of salinity in Pb-polluted environments and for improving earthworm population, reproduction and activity.