Forest land-use increases soil organic carbon quality but not its structural or thermal stability in a hedgerow system

How land-use and soil depth affect soil organic carbon (SOC) quality and stability, properties that are key to the long-term storage of C, is poorly understood in agroforestry systems. We examined the effects of land-use (forest vs. annual cropland) and soil depth (0–10 vs. 10–30 cm) on the quality (availability of C for decomposition), structural stability (composition of C functional groups), and thermal stability (energy yield and temperature characteristics during thermal oxidation) of SOC in a hedgerow system in central Alberta, using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and thermal analysis methods. The SOC quality (proportion of O-alkyl C) was higher, while the proportion of H- and C-substituted aromatic C and structural stability (hydrophobicity index (HB/HI), aromaticity index (ARM), and alkyl index (A/OA)) were all lower in the forest than in the cropland. The SOC also had lower thermal stability (higher energy density and lower temperature at which 50% of the mass was lost (TG-T50)) in the forest than in the cropland. Between the two soil depths, SOC quality (proportion of N-alkyl C, O-alkyl C and di-O-alkyl C) was higher, but the proportion of H- and C-substituted aromatic C and structural and thermal stabilities were lower, within the surficial 0–10 cm layer than in the 10–30 cm soil. We conclude that while the forest land-use within the hedgerow system had a higher SOC quality, it had lower SOC structural and thermal stabilities, suggesting that SOC under the forest land-use may be more susceptible to decline under climate change or other forms of anthropogenic disturbance. •Soil organic C (SOC) quality and structural and thermal stabilities in hedgerow were studied.•NMR and thermal analyses were used to study energy density and quality of SOC.•SOC quality was greater but structural and thermal stability was lower in forest than in cropland.•SOC thermal stability was lower in the surface than in the subsurface soil layer.•Land-use type and soil depth significantly affected SOC quality and stability.