Bacterial populations within copper mine tailings: Long-term effects of amendment with Class A biosolids

Aim: This study evaluates the effect of surface application of dried Class A biosolids on microbial populations within copper mine tailings. Methods and Results: Mine tailing sites were established at ASARCO Mission Mine close to Sahuarita Arizona. Site 1 (December 1998) was amended with 248 tons ha^-1 of Class A biosolids. Sites 2 (December 2000) and 3 (April 2006) were amended with 371 and 270 tons ha^-1, respectively. Site D, a neighbouring native desert soil, acted as a control for the evaluation of soil microbial characteristics. Surface amendment of Class A biosolids showed a 4 log₁₀ increase in heterotrophic plate counts (HPCs) compared to unamended tailings, with the increase being maintained for 10-year period. Microbial activities such as nitrification, sulphur oxidation and dehydrogenase activity were also sustained throughout the study period. 16S rRNA clone libraries obtained from community DNA suggest that mine tailings amended with biosolids achieve diversity and bacterial populations similar to native soil bacterial phyla, 10 years postapplication. Conclusion: Addition of Class A biosolids to copper mine tailings in the desert south-west increased soil microbial numbers, activity and diversity relative to unamended mine tailings. Significance and Impact of the Study: The amended tailings resulted in a functional soil with respect to microbial characteristics, which were sustainable over a 10-year period enabling the development of appropriate vegetation.