Por: Gabriela Palomino Lucano, Compañía de Minas Buenaventura, y Jimmy López Pérez y Benoit Diringer, INCABiotec.

Abstrac

In the context of mine closure, Minera La Zanja and Compañía Minera Coimolache, both part of Buenaventura, have been conducting research initiatives aimed at optimizing mine closure operations in northern Peru (Cajamarca). This study employed metagenomics, a molecular biology tool, to characterize the microbial communities present in different soil samples collected from La Zanja and Coimolache. 

The primary objective was to identify the complete microbial communities (bacterial and fungal) associated with each soil type and to determine those that serve as indicators of soil fertility for subsequent application in the closure of mine components and soil restoration. To this end, different soil types were monitored, including soils from pristine areas, organic topsoil storage facilities (DMO, in Spanish), aerated topsoil, topsoil amended with pine biomass, organic residues, ash, limestone, and plant sludge (mature technosol), as well as areas previously reclaimed using technosol. Physicochemical parameters were also measured, and the results were evaluated using microbial diversity indices (Chao1, Shannon, and Simpson) and statistical analyses. 

The results showed that the pristine soils exhibited acidic characteristics (pH 4.5) and harbored acid-tolerant microorganisms involved in nitrogen and carbon cycling, as well as in humus formation. Two groups of organic topsoil were evaluated. The first group exhibited more acidic conditions (pH 3.9-4.7) due to its storage conditions (in DMO), lack of aeration, and a clay loam texture. Acidophilic and denitrifying microorganisms were identified in this group. The second group exhibited less acidic conditions (pH 6.53), had been aerated every other day for two weeks, and had a sandy loam texture. This group contained aerobic microorganisms, nitrogen-fixing microorganisms, organic matter degraders, nutrient-solubilizing microorganisms, and metabolite-producing microorganisms that enhance soil fertility. 

The mature technosol exhibited moderately alkaline characteristics (pH 7.7) and contained microorganisms involved in lignocellulose decomposition, nutrient mineralization, and the regeneration of soil fertility. Areas reclaimed using technosol had been under vegetation cover for eight months and exhibited slightly acidic conditions (pH 6.5). These areas also harbored a heterogeneous microbial community, dominated by genera associated with sulfur, carbon, and nitrogen cycling, as well as with heavy metal bioremediation and the degradation of organic compounds. 

The study results provide a basis for implementing new strategies focused on the use of beneficial microorganisms, including Bacteroides, Bradyrhizobium, Cellulomonas, Flavihumibacter, Flavobacterium, Geobacter, Nitrosococcus, Nitrosospira, Nitrosovibrio, and Rhizobium, to improve soil fertility during the closure of mine components. In addition, the results obtained for aerated organic topsoil demonstrated that physical aeration through periodic turning prior to use significantly improves its physicochemical and microbial characteristics by increasing soil pH and restoring the fertilization capacity lost during storage in DMOs. 

This latter finding is particularly significant, as many stockpiled topsoils are either discarded or applied directly in remediation activities without prior treatment, resulting in prolonged acid runoff following mine closure. The results indicate that this effect can be mitigated by aerating the topsoil before its application. 

This research was published in the Q1 journal PLOS ONE under the title “Assessing Microbial Diversity in Open-Pit Mining: Metabarcoding Analysis of Soil and Pit Microbiota Across Operational and Restoration Stages”. Available at: https://doi.org/10.1371/journal.pone.0320923