Could Nature Successfully Trigger Soil Functional Processes in Metal Mine Tailings or Do We Always Need a Manmade Restoration Intervention?

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 how and to whatextent technically reclaimed (capping and afforestation) and spontaneously colonized metal(loid) mine tailings soils differ infunctional aspects. For this, a complete seasonal monitoring program (physical, chemical, physicochemical, and biological soilparameters) was carried out during 2022 in two mine tailings from SE Spain: a tailing technically reclaimed by capping andafforestation, and a tailing spontaneously colonized by plants directly growing in mine wastes. Both tailings store similar type ofmine wastes and were restored/abandoned at the same time. In each tailing monitoring plots were establised in areas with bothshrub-herbaceous and tree vegetation. Among others, in each study season, a battery of biological parameters (in)directly relatedto soil functionality was measured: respiration (in situ soil CO2 emission), fauna feeding activity (bait lamina), microbial biomasscarbon/nitrogen, dehydrogenase activity, enzymes activity related to soil biogeochemical cycles (e.g., β-glucosidase, N-acetyl-βglucosaminidase, protease, urease, acid phosphatase).