Patterns of water-extractable organic matter across management and depth in Mediterranean perennial cropping systems

Agricultural soils in Mediterranean semi-arid regions are increasingly affected by climatic variability, challenging the sustainability of perennial systems such as vineyards and olive groves. Conventional tillage promotes organic matter oxidation and limits carbon persistence, whereas the capacity of cover cropping to enhance carbon sequestration beyond surface accumulation remains uncertain, largely due to strong site-specific responses and intrinsic soil constraint

This study investigates the mechanisms controlling soil carbon stabilization at six paired vineyard and olive grove sites in central Spain under semi-arid conditions, comparing conventional tillage and cover-cropped systems. Soil samples were collected at 0–10 and 10–30 cm depths and analyzed for total soil organic carbon (SOC), water-extractable organic matter (WEOM), WEOM optical properties (E4/E6), texture, calcium carbonate (CaCO₃), porosity, and β-glucosidase activity. Depth gradients (30–10 cm) were calculated to explicitly focus on vertical processes, and carbon stabilization was assessed using the SOC/clay ratio as an indicator of mineral-associated organic carbon.

Across sites, all variables exhibited substantial variability. SOC contents were typical of Mediterranean agricultural soils, ranging from approximately 0.77–0.84% in the topsoil and from 0.51–0.64% at 10–30 cm depth, while SOC/clay ratios declined from 0.09–0.10 in the surface layer to 0.04–0.06 at depth. These values indicate a moderate but constrained potential for mineral-associated carbon stabilization, with management-related differences being modest and largely confined to surface horizons. WEOM concentrations ranged from approximately 15 to 21 mg L⁻¹, with slightly higher values under cover cropping at 0–10 cm, whereas differences at 10–30 cm were less pronounced. Textural composition was dominated by relatively high sand contents but showed strong site-to-site heterogeneity, reflecting contrasting edaphic contexts. WEOM E4/E6 ratios averaged 3.3–3.5 under cover cropping and 4.1–4.8 under tillage, although large variability resulted in substantial overlap between managements.

Linear models revealed that the vertical WEOM gradient (30–10 cm) was the most consistent predictor of increases in SOC/clay at depth, highlighting the importance of dissolved carbon transfer for subsurface carbon stabilization. Mean CaCO₃ content exerted a positive effect, indicating a strong mineralogical control on carbon retention. In contrast, β-glucosidase activity and WEOM-E4/E6 did not emerge as significant predictors.

Acknowledgements:

This work was supported by the European Joint Programme for SOIL (EJP SOIL), funded by the EU Horizon 2020 Programme (Grant Agreement No. 862695), within the SANCHOTHIRST project. J. González Canales was supported by grant PRE2021-097966 (MCIU/AEI/10.13039/501100011033) and the European Social Fund (ESF). We thank the farmers for field access and management information.