Distinct Fe- and Ca-Mediated Stabilization Pathways of Mineral-Associated Organic Carbon in Karst Spring-Fed River Sediments

River sediments are vital organic carbon reservoirs, yet the formation mechanisms of mineral-associated organic carbon (MAOC) in karst spring-fed rivers remain poorly understood. Here, we investigated sediments, water chemistry, submerged macrophytes, and sediment-extracted dissolved organic matter (DOM) from five typical karst spring-fed rivers in the Guilin and Jianshui regions of Southwest China. Results showed that the total organic carbon (TOC) in sediments averaged 32.21 ± 24.07 g/kg, and MAOC averaged 5.45 ± 2.68 g/kg. While TOC and MAOC concentrations were similar between Guilin and Jianshui, the dominant MAOC fractions showed clear differences between the two study areas. Guilin sediments were characterized by operationally defined higher iron-bound organic carbon (Fe-OC), whereas operationally defined calcium-bound organic carbon (Ca-OC) concentrations tended to be higher in Jianshui. Random forest (RF) analysis identified stable carbon isotope composition of dissolved inorganic carbon in water (δ13CDIC), fluorescent dissolved organic matter biological index (BIX), and specific conductivity of water (SpCw) as the most important predictors of MAOC variation. Operationally defined Fe-OC was primarily associated with SpCw, dissolved oxygen (DOw), spectral slope ratio (SR), and specific ultraviolet absorbance at 254 nm (SUVA254), whereas operationally defined Ca-OC was mainly related to δ13CDIC, dissolved organic carbon (DOC), and SUVA254. These findings indicate that similar MAOC concentrations in karst spring-fed river sediments were accompanied by contrasting contributions of operationally defined Fe- and Ca-associated carbon allocation patterns. Our study highlights the need to move beyond bulk carbon concentration assessment and consider carrier-specific MAOC composition when evaluating the composition and potential persistence of karst sedimentary carbon pools.