Constructing soils for climate-smart mining

Brazil’s mining sector is economically pivotal but environmentally harmful, leading to pollution, biodiversity loss, and catastrophic waste management failures. As global demand for minerals grows to support clean energy technologies, mining-related greenhouse gas emissions need comprehensive accounting beyond direct fuel and electricity use to include emissions from vegetation and soil removal. Surface mining, prevalent in Brazil, removes soil and vegetation and can release substantial carbon, as soils hold up to three times more carbon than vegetation. This study aims to estimate ecosystem carbon stocks (soil and above-ground biomass) at legally active mine sites (LAMS) in Brazil and quantify potential losses from future surface mining. It further evaluates the extent to which constructing soils (Technosols) from mine and other wastes could recover soil organic carbon (SOC) stocks and mitigate mining-related emissions. The hypothesis is that Technosols used for post-mine reclamation can significantly offset future CO₂ emissions by rebuilding SOC stocks and supporting vegetation recovery.

Prior assessments of mining emissions have focused largely on direct operational emissions, neglecting indirect emissions from vegetation and soil removal. Soils store vast amounts of carbon and can be significant carbon sinks, particularly when stabilized by mineral-organic interactions. Technosols—constructed soils using mine and other wastes—have emerged as a promising reclamation strategy with demonstrated benefits for soil quality, biodiversity, and potential carbon accumulation. Evidence indicates that poorly crystalline minerals, polyvalent cation bridging (e.g., Ca²⁺, Mg²⁺), and iron oxides (e.g., goethite, hematite) enhance stabilization of soil organic matter. However, the capacity of Technosols to restore ecosystem carbon stocks at scale had not been evaluated, motivating this study.

• Mining site delineation: The geolocation and areas of legal active mining sites (LAMS) in Brazil were obtained from the National Mining Agency’s SIGMINE platform. Entries representing active stages (mining concession, licensing, extraction register, artisanal/small-scale mining) were selected. Duplicates (by process ID/number) and offshore processes were excluded. From approximately 205,000 raw entries (~1.8 million km²), about 33,000 entries (54,000 km²; 5.4 million ha) were retained as LAMS.
• Carbon datasets and processing: Above-ground biomass carbon (AGBC) was estimated using a high-resolution nationwide aboveground carbon map for Brazil; soil organic carbon (SOC) used SoilGrids v2. LAMS polygons were uploaded to Google Earth Engine, and original-resolution raster values were sampled within polygons. Pixel-level carbon mass was computed by multiplying carbon density (kg m⁻²) by pixel area (m²), then summed across LAMS for tropical, subtropical (temperate), and dry climatic regions.
• SOC depth conversion and CO₂eq: SOC data were available for 0–30 cm. SOC stocks for 0–100 cm were estimated using a factor of 1.887 (assuming 0–30 cm represents 53% of 0–100 cm). Carbon was converted to CO₂eq by multiplying by 3.67.
• Technosol SOC dataset and calculation: An updated dataset of Technosol carbon stocks provided climate type, SOC or SOM, bulk density (BD), and depth. Observations labeled as Technosols or constructed soils with data to 0–30 cm were selected. When SOC was missing, SOM was converted to SOC using 0.58. SOC stock (0–30 cm) was computed as SOC (%) × BD (g cm⁻³) × layer thickness (cm). Sample sizes: tropical n=9, dry n=15, subtropical n=25 (profiles 0–30 cm).
• Statistical treatment: As distributions were non-normal, a Box-Cox transformation (maximum likelihood) was applied; normality was checked with the Shapiro–Wilk test. Means and 95% confidence intervals were back-transformed. Regional SOC stocks (mass units) were obtained by multiplying per-area SOC stocks by the area of each climate region and converted to CO₂eq (×3.67).
• Reporting: SOC and AGBC stocks within LAMS were aggregated nationally and by climate region; potential Technosol-based recovery was quantified by region and overall.

• Extent of mining areas: Brazil has approximately 5.4 million hectares of legally active mine sites (LAMS), widely distributed, mostly in tropical and subtropical regions.
• Potential carbon stock losses if fully exploited:
  • SOC (0–100 cm) within LAMS: 1.68 Gt CO₂eq.
  • AGBC within LAMS: 0.87 Gt CO₂eq.
  • Total organic carbon (TOC = SOC + AGBC): 2.55 Gt CO₂eq, equivalent to about 5% of current total annual anthropogenic emissions worldwide.
• SOC by climate region within LAMS (0–100 cm):
  • Tropical: 1.05 Gt CO₂eq.
  • Subtropical (temperate): 0.59 Gt CO₂eq.
  • Dry: 0.04 Gt CO₂eq.
• Technosol SOC recovery potential (0–100 cm equivalent):
  • Overall: 31–60% of soil-related emissions (0.52–1.00 Gt CO₂eq) could be offset.
  • Tropical: 48–90% (0.50–0.94 Gt CO₂eq).
  • Subtropical: 23–56% (0.14–0.34 Gt CO₂eq).
  • Dry: 17–63% (0.01–0.03 Gt CO₂eq).
• Mechanisms for enhanced SOC stabilization in Technosols include formation of poorly crystalline minerals during weathering, mineral–organic associations, and cation bridging (notably with Ca²⁺ and Mg²⁺); Fe oxides further promote SOC sorption.
• Resource and risk context: Iron mining in Brazil (>240,000 ha) generates ~184 Mt iron tailings annually stored in >200 dams, many high-risk; Fe-rich tailings are promising Technosol parent materials with SOC sequestration potential. Projected mine waste production in Brazil could reach 11 billion tons by 2030, underscoring the waste management opportunity.
• Co-benefits: Properly designed Technosols can restore soil-related ecosystem services and may be cost-effective due to proximity and availability of waste materials.

Findings confirm that potential CO₂ emissions from soil and vegetation removal in Brazil’s LAMS are substantial (2.55 Gt CO₂eq), and that Technosols can meaningfully mitigate a large share of soil-related emissions (up to 60%), especially in tropical regions where weathering promotes mineral–organic stabilization. The climate dependency of SOC recovery reflects differences in mineral weathering and formation of reactive mineral phases that protect organic matter. Designing Technosols with Ca/Mg-rich or Fe-rich wastes can enhance cation bridging and sorptive stabilization, increasing SOC accumulation. While certain mine wastes may contain toxic elements, combining waste selection with amendments (e.g., compost, sewage sludge, ashes, biochar) and remediation approaches (e.g., phytoremediation) can mitigate pollution risks and improve soil function. Technosols also support rapid vegetation establishment, suggesting above-ground biomass carbon can recover, further increasing total organic carbon beyond soil sequestration alone. Economically, using locally available wastes lowers reclamation costs and reduces risks associated with waste storage (e.g., tailings dams). Overall, constructing climate-smart Technosols provides a nature-based solution that addresses carbon mitigation, land reclamation, and waste management simultaneously.

Constructing Technosols from mine and compatible industrial/domestic wastes can offset a sizeable fraction (31–60%) of soil-related CO₂ emissions anticipated from surface mining across Brazil’s legally active mine sites, with the highest potential in tropical regions. This approach leverages natural pedogenic processes to stabilize SOC, supports vegetation and ecosystem service recovery, and improves waste management safety and economics. The study highlights Technosols as a promising nature-based solution for carbon-neutral mining and recommends their consideration in environmental policies of major mining countries. Future work should optimize Technosol design tailored to climate and waste mineralogy, rigorously assess pollutant risks and remediation combinations, quantify above-ground biomass recovery trajectories, and evaluate large-scale implementation, monitoring frameworks, and cost–benefit analyses.

• Estimates assume full future exploitation of all legally active mining sites; actual development may vary.
• AGBC recovery in reclaimed scenarios was not quantified (data unknown), so total TOC recovery is conservative regarding vegetation.
• SOC depth scaling (0–30 to 0–100 cm) relies on a uniform conversion factor (1.887), which may not capture spatial variability.
• Technosol SOC dataset comprises heterogeneous Technosol types, many not designed for sequestration, potentially underestimating optimized outcomes; sample sizes per climate zone were modest.
• Non-normal data required transformation; despite back-transformation, uncertainty remains.
• Potential contamination in some mine wastes necessitates site-specific assessments; results depend on appropriate material selection and management.