Coupled organic and inorganic carbon dynamics mediated by calcium in semi-arid olive orchards

Soil organic carbon (SOC) and soil inorganic carbon (SIC) are key carbon reservoirs in semi-arid agroecosystems, yet the mechanisms governing their spatial distribution and interaction in perennial cropping systems remain poorly understood. This study investigated the spatial heterogeneity of SOC and SIC, calcium (Ca)-mediated carbon stabilization, and associated bacterial communities in the rhizosphere and soils located 3–5 m from olive trees subjected to long-term mineral and manure fertilization. The spatial distribution of carbon functional groups and Ca species was characterized using synchrotron-based micro-FTIR mapping and Ca K-edge XANES spectroscopy, while bacterial community composition was assessed by 16S rDNA gene sequencing. Total carbon content was highest in manure-only (old) orchards (8.11%), where SIC presented the dominant carbon pool, and it decreased by 47% from the rhizosphere to distal soils. Mineral-fertilized systems exhibited significantly higher rhizosphere SOC concentrations than organically fertilized and control systems. Micro-FTIR mapping revealed greater co-localization of calcite with polysaccharide- and carboxylate-rich organic functional groups in manure + NPK rhizospheres, suggesting stronger organo-mineral associations. Ca K-edge XANES analyses showed that while distal manure-only (old) soils were dominated by calcite (75% of total Ca), manure + NPK (old) rhizospheres contained higher proportions of organically associated Ca (18% Ca-phytate) and clay-bound Ca (29% Ca-kaolinite). Bacterial communities differed significantly between rhizosphere and distal soil compartments, reflecting strong root-driven selection, whereas fertilization had comparatively limited effects on community structure. The enrichment of copiotrophic and rhizosphere-associated taxa in root-influenced soils coincided with shifts in SOC accumulation and Ca-associated carbon fractions. Together, these findings indicate that rhizosphere effect and Ca redistribution interact to increase organo-mineral associations, influencing SOC stabilization and SOC-SIC partitioning in semi-arid Mediterranean olive agroecosystems.