C and N Mineralization in Metal Mine Tailings Technically Recovered versus Spontaneously Colonized by Vegetation as Indicator to Assess Soil Functionality

Resumen

Metal mine wastes from ore processing are among the most hazardous residues (e.g., high metal(loid) levels, extreme pH, highsalinity, organic matter/nutrients deficiency). These wastes are often deposited in open-air piles (mine tailings). Classicrestoration approaches (technical reclamation) usually involve topsoil capping aiming to create a “new soil” above wastes andthen afforestation. These options have been questioned in (semi)arid regions due to ecosystems inability to be self-sustaining. Analternative, based on nature-based solutions principles, is to promote the growth of plants directly in mine wastes(phytomanagement by phytostabilization). This contributes to create the so-called fertility islands that may promote plant-soilfeedbacks and ecosystem functioning. If tailings pose immediate risks (e.g., structural collapse, close to urban areas), thenecessary financial/technical means should be made available for immediate technical restoration. However, if conditions are notso extreme, the spontaneous succession of plants directly growing in mine wastes has some advantages (e.g., plants adaptation tolocal climate/substrate conditions, high natural value, improved long-term sustainability). Previous works showed that the directgrowth of native plants in tailings can provide functional soil-vegetation systems. This study aimed to evaluate whethertechnically reclaimed (capping and afforestation) and spontaneously colonized metal(loid) mine tailings’ soils differ on carbonand nitrogen transformation activities. For this, C and N mineralization rates (following OECD guidelines) were evaluated in soilsamples collected in spring 2022 in areas with both shrubs and pine trees in two mine tailings from SE Spain: a tailing technicallyreclaimed by capping and afforestation, and a tailing spontaneously colonized by plants directly growing in mine wastes. Theresults showed that both tailings differed in their C transformation activity, showing the technically reclaimed tailing greatercapacity to mineralize C. However, in the case of N, both tailings showed similar mineralization rates. Furthermore, the soilsunder shrubs were the ones reaching greater mineralization scores in the technically reclaimed tailing, while the contrary occurredin the tailing spontaneously colonized by vegetation with greater mineralization scores in soils under pine trees. The latter maysuggest that the type of vegetation is modulating soil functioning