Energy matters: Soil organic carbon fractions as soil health indicator or characterizing ecosystem property

Soil organic carbon (SOC) drives ecosystem functions, particularly climate change mitigation and soil biological activity. Due to its complex nature, SOC fractionation methods that can differentiate between functionally distinct carbon pools and reflect sustainable agricultural management are required for consistent soil health monitoring. We analysed topsoil samples (0–10 cm) from Phaeozem, Cambisol and Fluvisol soils under two contrasting farming systems: (i) conservation farming (minimum/no-till, diverse crop rotations, frequent cover cropping) and (ii) regional standard practice (conventional ploughing 25–30 cm depth, corn-dominated rotation). Size fractionation with and without ultrasonic pretreatment (0, 3, 60 J ml −1 ) was combined with elemental and isotopic analyses, thermogravimetry, and standard soil health indicators. Biochemical differences (SOC-to-TN ratio, δ¹⁵N) between size fractions were most evident when composite aggregate structures were disrupted at 60 J ml⁻¹ , indicating distinct mineral-associated vs. aggregate-related pools separating at a 50 µm size-threshold. Thermogravimetric analysis confirmed this qualitative distinction. However, farming system differentiation was better expressed in aggregate fractions at 0 J ml⁻¹ , where soil structure was conserved. In particular, the macroaggregate fraction (250–2000 µm) correlated best with biological soil health indicators. We propose a two-energy/two-size fractionation approach, using the key energies 0 and 60 J ml −1 , to obtain mineral-associated SOC (< 50 µm at 60 J ml −1 ) as climate change mitigation indicator, while retaining the aggregate fraction (> 50 µm at 0 J ml −1 ) to infer on management-sensitive biological properties. This four-fraction SOC characterization captures soil multifunctionality and provides a broadly applicable framework for sensitive monitoring of soil ecosystem health. • Two-energy, two-size fractionation optimizes soil organic carbon analysis. • Macroaggregate fractions ≥ 50 µm at 0 J ml −1 reveal farm system effects. • 60 J ml⁻¹ energy disrupts aggregates, highlighting mineral-associated SOC pools. • Thermogravimetric data and size fractions compared for varying stabilities.