Soil organic matter stabilization during early stages of Technosol development from Ca, Mg and pyrite-rich parent material

Abstract

The mechanisms underlying soil organic matter (SOM) accrual in Technosols are still poorly understood. Here we studied a 20-year chronosequence of three Technosols constructed with dolomitic limestone mining waste: two cultivated with sugarcane (for 3 and 7 years) and one under pasture (for ∼20 years). Through a combination of SOM physical fractionation and wet-chemical extractions, we aimed to unravel the processes driving SOM accumulation in these soils and developed a conceptual model for embryonic soils developed from Ca, Mg and pyrite-rich parent material. Our results revealed that the weathering of comminuted waste rocks (e.g., shale, siltstone) provided 2:1 minerals (especially smectite) to participate in mineral-organic associations. The weathering of primary minerals also promoted the formation of secondary minerals, which interacted with SOM and increased the amount of mineral associated organic matter (MAOM) up to 20 g SOC kg−1. Pyrite oxidation produced highly reactive secondary Fe phases, while the dolomite dissolution increased the concentrations of Ca2+ and Mg2+ and maintained near-neutral pH conditions, favoring the formation of cation-bridges. The progressive increase in the different operational Fe fractions (Fe in organomineral complex, poorly crystalline, and crystalline Fe oxides) along the Technosol chronosequence was strongly correlated with MAOM, highlighting the significant role of Fe in SOM accumulation. Furthermore, we observed substantial increase in particulate organic matter (POM), up to 22 g SOC kg−1, which was associated with increased aggregate formation and stability. These processes collectively contribute to the protection of SOM during early stages of soil development from in Ca, Mg, and pyrite-rich parent material, and thus indicate a high potential for carbon sequestration.