Soil carbon accrual in Opuntia ficus-indica orchards despite texture-related constraints

Soil organic carbon (SOC) dynamics in dryland agroecosystems are commonly assumed to be constrained by mineral stabilization capacity, particularly in coarse-textured soils. However, the extent to which sustained biological inputs can compensate for texture-related constraints under long-term perennial management remains poorly resolved. Here, we quantified SOC storage dynamics along a 30-year chronosequence of Opuntia ficus- indica (L.) Mill. agroecosystems, a perennial crassulacean acid metabolism (CAM) system, under Mediterranean semi-arid conditions, comparing clay loam and sandy loam soils. We integrated fixed-depth, depth-normalized, and equivalent soil mass approaches with stable isotope analyses ( 13C and 15N) to evaluate SOC accumulation, vertical distribution, and isotopic evidence of organic matter turnover. SOC storage increased consistently with orchard age across all estimation approaches. Profile-scale gains reached approximately 30–40 Mg C ha1 over 30 years, with relative increases of 50–75% in clay loam soils and 90–120% in sandy loam soils. SOC accu- mulation was strongly concentrated in surface layers, indicating a dominant role of localized organic inputs. Notably, relative SOC increases were greater in sandy loam soils despite their lower expected mineral stabili- zation capacity, suggesting that sustained biological inputs can partially compensate for texture-related limita- tions on carbon accrual. Carbon isotope signatures showed progressive enrichment consistent with the incorporation of cactus-derived carbon into soil organic matter, whereas 15N patterns were mainly site dependent, indicating partially decoupled carbon and nitrogen dynamics. Together, these results indicate that SOC accrual in perennial CAM dryland agroecosystems is governed by the interaction between sustained carbon inputs, soil properties, and stabilization processes, rather than by mineral protection alone. Perennial cactus systems may therefore represent functionally important agroecosystems for enhancing soil carbon storage and resilience under water-limited Mediterranean conditions.