Water deficit limits soil organic carbon sequestration under old apple orchards in the loess-covered region

The water and organic carbon stored within unsaturated zones are crucial in terrestrial ecosystems; however, the coupling effects of deep soil water and organic carbon are still poorly documented. To explore the water-carbon coupling effects within soils under tree plantations, we investigated the water and organic carbon content to 20 m deep under cultivated farmland and orchards with apple trees of varying ages (5, 10, 15, 20, and 24 years) in the loess-covered region of China. A conditional process model was used to explore the impacts of vegetation and edaphic factors on the water-carbon coupling processes. Compared with farmland, young apple orchards (0-10 years) showed lower water deficit, but the soil water storage reduced by 21%- 32% under old apple orchards (15-24 years). The organic carbon content exhibited negligible variation among apple trees of different ages. However, the water-carbon coupling effects varied with apple tree ages. A positive correlation existed between water storage and organic carbon density under young apple orchards (p < 0.05), but a negative correlation was found between water deficit and organic carbon sequestration under old apple orchards (p < 0.01). The conditional process model suggested that the water-carbon coupling relationship under young apple orchards was primarily controlled by clay content due to lower root biomass, while that under old apple orchards was concurrently shaped by silt content and coarse roots because of the developed root system of apple trees. This study offers novel insights into the sustainable management of agriculture and tree plantations in regions with deep-rooted vegetation and thick unsaturated zones.