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.

Land application of Class B biosolids is routinely undertaken in the United States. However, due to public concern over potential hazards, the long-term sustainability of land application has been questioned. Thus, the objective of this review article was to evaluate the sustainability of land application of Class B biosolids. To do this we evaluated (i) the fate and transport of potential biological and chemical hazards within biosolids, and (ii) the influence of long-term land application on the microbial and chemical properties of the soil. Direct risks to human health posed by pathogens in biosolids have been shown to be low. Risks from indirect exposure such as aerosolized pathogens or microbially contaminated ground water are also low. A long-term land application study showed enhanced microbial activity and no adverse toxicity effects on the soil microbial community. Long-term land application also increased soil macronutrients including C, N, and, in particular, P. In fact, care should be taken to avoid contamination of surface waters with high phosphate soils. Available soil metal concentrations remained low over the 20-yr land application period due to the low metal content of the biosolids and a high soil pH. Soil salinity increases were not detected due to the low salt content of biosolids and irrigation rates in excess of consumptive use rates for cotton. Our conclusion, based on these studies, is that long-term land application of Class B biosolids is sustainable.

This study evaluated the influence of 20 annual land applications of Class B biosolids on the soil microbial community. The potential benefits and hazards of land application were evaluated by analysis of surface soil samples collected following the 20th land application of biosolids. The study was initiated in 1986 at the University of Arizona Marana Agricultural Center, 21 miles north of Tucson, AZ. The final application of biosolids was in March 2005, followed by growth of cotton (Gossypium hirsutum L.) from April through November 2005. Surface soil samples (0-30 cm) were collected monthly from March 2005, 2 wk after the final biosolids application, through December 2005, and analyzed for soil microbial numbers. December samples were analyzed for additional soil microbial properties. Data show that land application of Class B biosolids had no significant long-term effect on indigenous soil microbial numbers including bacteria, actinomycetes, and fungi compared to unamended control plots. Importantly, no bacterial or viral pathogens were detected in soil samples collected from biosolid amended plots in December (10 mo after the last land application) demonstrating that pathogens introduced via Class B biosolids only survived in soil transiently. However, plots that received biosolids had significantly higher microbial activity or potential for microbial transformations, including nitrification, sulfur oxidation, and dehydrogenase activity, than control plots and plots receiving inorganic fertilizers. Overall, the 20 annual land applications showed no long-term adverse effects, and therefore, this study documents that land application of biosolids at this particular site was sustainable throughout the 20-yr period, with respect to soil microbial properties.

Aim:  To evaluate the effect of long‐term annual land applications of Class B biosolids on soil bacterial diversity at University of Arizona Marana Agricultural Field Center, Tucson, Arizona. Methods and Results:  Following the final of 20 consecutive years of application of Class B biosolids in March 2005, followed by cotton growth from April to November 2005 surface soil samples (0–30 cm) were collected from control (unamended) and biosolid‐amended plots. Total bacterial community DNA was extracted, amplified using 16S rRNA primers, cloned, and sequenced. All 16S rRNA sequences were identified by 16S rRNA sequence analysis and comparison to known sequences in GenBank (NCBI BlastN and Ribosomal Database Project II, RDP). Results showed that the number of known genera (identifiable > 96%) increased in the high rate biosolid plots compared to control plots. Biosolids‐amended soils had a broad phylogenetic diversity comprising more than four major phyla: Proteobacteria (32%), Acidobacteria (21%), Actinobacteria (16%), Firmicutes (7%), and Bacteroidetes (6%) which were typical to bacterial diversity found in the unamended arid southwestern soils. Conclusion:  Bacterial diversity was either enhanced or was not negatively impacted following 20 years of land application of Class B biosolids. Significance and Impact of the Study:  This study illustrates that long‐term land application of biosolids to arid southwestern desert soils has no deleterious effect on soil microbial diversity.

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