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

Soil organic carbon (SOC) and soil inorganic carbon (SIC) are major carbon pools in semi-arid agroecosystems, yet their coupled dynamics and controlling mechanisms remain poorly understood, particularly in long-managed perennial systems. This study investigated the spatial distribution and stabilization mechanisms of SOC and SIC in Moroccan olive orchards under contrasting long-term management regimes, including mineral fertilization (NPK), organic amendment (sheep manure), and non-fertilized conditions. Soils were sampled from the rhizosphere, 1-3 m, and 3-5 m from tree trunks in old and young orchards.

Carbon partitioning varied strongly with soil compartment and management history. Fertilized orchards exhibited higher SOC dominance in rhizospheric soils, whereas non-fertilized orchards retained larger SIC pools, particularly in bulk soils. Synchrotron-based micro-FTIR mapping revealed pronounced co-localization of calcite with polysaccharide- and carboxylate-rich organic functional groups in fertilized rhizospheres, indicating enhanced organo-mineral interactions. Calcium K-edge XANES spectroscopy showed a shift in Ca speciation from carbonate-dominated forms in bulk soils to clay-bound and organically complexed pools (Ca-phytate) near roots, highlighting Ca redistribution as a key mechanism linking SOC and SIC dynamics. In contrast, organic-amended systems were characterized by persistent carbonate-bound Ca, consistent with geochemical stabilization of SIC.

Microbial community structure exhibited limited long-term sensitivity to fertilization, with diversity patterns primarily driven by root proximity rather than management. Overall, these results demonstrate that carbon stabilization in olive orchards is controlled mainly by calcium-mediated organo-mineral interactions, driven by rhizosphere processes and long-term management. This study highlights the importance of integrating SOC/SIC dynamics when assessing carbon sequestration potential in semi-arid, calcareous agroecosystems.