Spatial patterns and multi-drivers of water and sediment carbon components along the Minjiang River continuum, China

Rivers are vital conduits linking terrestrial and marine ecosystems, playing a pivotal role in global carbon (C) cycling and climate regulation. However, scarce data spanning river orders and ecosystem types limit our ability to accurately assess riverine C transport and storage. Here, we performed a large-scale field sampling across the Minjiang River Basin in southeastern China, covering eight river orders and six dominant ecosystem types. We used one-way analysis of variance, piecewise structural equation modeling (SEM), and SHAP (Shapley Additive Explanations) analysis based on Random Forest (RF) models to quantify the spatial distributions and the associated drivers of total C (TC), total organic C (TOC), dissolved organic C (DOC), and/or dissolved inorganic C (DIC) in both water and sediments. Results showed significant spatial heterogeneity in C components across the Minjiang River Basin, including a clear spatial decoupling between water and sediment C. Higher concentrations of water TC and DIC, as well as sediment DIC were observed in the northeastern and southwestern regions, whereas sediment TC and TOC reached their highest levels in the southeastern estuarine area. Mean concentrations of TC, DOC, and DIC in water were 2.03, 0.89, and 1.14 mg/L, respectively, while averaged sediment TC, TOC, DOC, and DIC were 3315.77, 2876.31, 29.45, and 6.61 mg/kg, respectively. Carbon concentrations generally increased from upstream to downstream, with the highest values observed in the fifth (water) and eighth order rivers (sediment). Cropland rivers (water) and town dominated rivers (sediment) exhibited significantly higher values than forested rivers. Water C components were primarily driven by environmental factors (precipitation and soil erosion) and water quality variables (electrical conductivity and water temperature), while sediment C components were mainly controlled by sediment quality (clay and sand contents). Overall, our findings highlight the strong spatial heterogeneity and downstream accumulation of C in the Minjiang River Basin and the dominant role of precipitation, temperature, and physicochemical conditions. These results provide new evidence on riverine C dynamics in southeastern China and offer practical implications for watershed management, regional ecological protection, and carbon budgets.