Thin technosols mature to agronomically functional soils on a reclaimed ash landfill

The long-term functionality of thin (≤ 30 cm) technosols – anthropogenic soils constructed from homogenized transported soil over reclaimed lignite fly ash landfills – is evaluated here. This study investigates three distinct parcels (defined as contiguous land-use areas reclaimed with same initial substrate material in different phases) on a landfill in central Slovenia: P1 (28 years old), P2 (25 years old), and P3 (15 years old). Soil sampling was conducted during a single campaign in 2024 to assess pedogenic stabilization. All parcels demonstrated plant-available water (PAW) above the 20% vol. agronomic threshold, with P1 reaching 26.1% vol. and P3 23.8% vol. Strong aggregate stability (>90% water-stable aggregates) was observed across the site. The oldest parcel (P1) exhibited the most favorable bulk density (~1.11 g cm - ³), stable pH and uniform saturated hydraulic conductivity (K s ), while younger parcels showed localized compaction exceeding 1.50 g cm - ³. Chemical analysis via pXRF confirmed that trace elements (e.g., Cr: 84–111 mg kg - ¹, Ni: 45–57 mg kg - ¹) remain within Slovenian regulatory limits. While topsoil nutrients are adequate (OM 5.5–6.0%), a sharp decline in the subsoil (OM ~2.1%) indicates limited vertical nutrient cycling. These findings prove that even a thin technosol can develop essential functions for agriculture, such as water retention and structural stability, within three decades. However, limited soil depth restricts drought resilience to approximately 7–10 days in younger covers versus 28 days in mature ones.