Soil water–organic carbon–depth interactions and stand age are linked to carbon sequestration efficiency in rainfed and irrigated apple orchards

Soil organic carbon (SOC) is strongly affected by water scarcity, yet how soil moisture regulates SOC dynamics within deep-soil profiles remains poorly documented in apple production systems. We collected 60 soil profiles to a depth of 6 m from cropland, and 8-, 17-, and 25-year-old rainfed apple orchards, and 16 soil profiles to a depth of 13 m from cropland and 10-, 20-, and 30-year-old irrigated apple orchards on China’s Loess Plateau. We quantified changes in soil water content (ΔSWC), SOC (ΔSOC), and SOC sequestration efficiency (ΔSOC/ΔSWC) following cropland-to-orchard conversion. Soil water deficit occurred in 17- and 25-year-old rainfed orchards but not in 8-year-old rainfed orchards. Irrigation alleviated but did not eliminate soil desiccation at depths of 6–11 m. Negative ΔSOC values were detected across the 6-m profiles of rainfed orchards, whereas SOC accrual occurred in 20- and 30-year-old irrigated orchards but not in 10-year-old irrigated orchards. Significant relationships between ΔSWC and ΔSOC were observed at most soil depths in rainfed and irrigated apple orchards, highlighting the crucial role of soil water in modulating SOC sequestration. Pooled data revealed an inverted bell-shaped ΔSWC-ΔSOC relationship for different-aged rainfed orchards, and ΔSOC was positively and linearly correlated with ΔSWC in 20- and 30-year-old irrigated orchards. Those distinct water-carbon coupling patterns indicated that soil water deficit constrains SOC sequestration in both orchard systems, whereas severe drought facilitates SOC accrual in rainfed orchards. The ΔSOC/ΔSWC ratio exhibited a quadratic relationship with stand age in the 0 -1 m soil layer of rainfed orchards and the 0 -2 m soil layer of irrigated orchards, with critical thresholds of ~16 years for rainfed orchards and ~18 years for irrigated orchards. Partial least squares path modeling revealed that stand age had significant positive effects on ΔSOC/ΔSWC in both rainfed and irrigated apple orchards, whereas soil depth had divergent effects on ΔSOC/ΔSWC between the two orchard types. Our findings highlight the importance of soil moisture in regulating SOC sequestration in rainfed and irrigated apple orchards and elucidate deep-soil water–carbon interactions.