Effects of introduced macadamia on soil organic carbon and water stocks in subtropical agroforestry systems of southwestern China

Macadamia ( Macadamia integrifolia )-based agroforestry systems,widely adopted in Yunnan Province, China since 1981, are critical for soil organic carbon (SOC) sequestration. However, their effects on SOC stocks and coupling mechanisms with soil water dynamics remains poorly understood. This study quantified changes in SOC and soil water stocks following the conversion from primary forest to macadamia monoculture and intercropping systems (dasheen, konjac, and maize) in Yongde County, southwestern Yunnan province. Soil profiles (0–100 cm, 0–20 cm, and 20–100 cm) were analyzed to assess vertical SOC and soil water stocks interactions and driving factors. Results revealed that macadamia-based agroforestry systems reduced significantly SOC stock compared to primary forest (261 t/hm 2 ) but enhanced soil water stock. Among intercropping systems, macadamia+dasheen plantation exhibited the highest recovery of SOC and soil water stock. Surface soil layers (0–20 cm) showed strong SOC-water coupling, whereas this relationship weakened in deeper layers (20–100 cm). Simultaneously, woody above-ground biomass directly increased SOC stock, while specific leaf area and leaf phosphorus content regulated SOC stock directly or indirectly via woody above-ground biomass. Conversely, herb above-ground biomass negatively influenced SOC stock in surface soil layer, but had no significant effect in the deeper layer. Woody above-ground biomass exhibited a consistent negative influence on soil water stock across all soil layers, with the standardized path coefficients of −1.23 (0–100 cm), −1.153 (0–20 cm), and −1.23 (20–100 cm). This relationship was further modulated by indirect effects mediated through specific leaf area and leaf phosphorus content. This study provides mechanistic insights into the interdependencies of SOC and water stock in the subtropical macadamia-based agroforestry systems, providing valuable guidance for sustainable carbon management practices in China’s subtropical mountainous region. • Macadamia-based agroforestry significantly reduced SOC stock but increased soil water stock compared to primary forest. • SOC and soil water stocks exhibited coupling relationships in surface soil layers (0–20 cm) but decoupled in deeper soil profiles (20–100 cm). • Woody aboveground biomass drove SOC and soil water stock via specific leaf area and leaf phosphorus.