High carbon burial fluxes in lakes do not equate to stable carbon sinks: Evidence from typical large river-connected lake systems

Lakes play a pivotal role in the global carbon cycle and climate change mitigation by sequestering carbon. However, under combined human and climate pressures, it remains uncertain whether increased organic carbon burial flux (OCBF) necessarily corresponds to stable carbon sequestration. Here, we utilized MAOC% as the key indicator of organic carbon pool stability (OCPS), complemented by grain size and isotopic signatures from high resolution sedimentary records, to investigate the mechanisms underlying the decoupling between OCBF and OCPS in Dongting Lake. Results showed that OCBF and OCPS responded nonlinearly to sedimentary transitions, explaining 72% and 46% of their respective variance (p < 0.001). In East Dongting, OCBF peaked at 350.30 ± 253.51 g C m⁻² yr⁻¹, primarily driven by eutrophication enhanced autotrophic production. However, the rapid burial of labile organic matter likely overwhelmed the available capacity for stable mineral association, contributing to a marked decline in the proportion of mineral associated organic carbon (MAOC%). This imbalance ultimately weakened the OCPS. In contrast, West Dongting maintained a consistently high MAOC% (94.63%) due to sustained inputs of fine minerals from upstream channel scouring. This mineral supply promoted persistent organic mineral associations, resulting in continuously enhanced OCPS despite a relatively lower burial flux (250.60 ± 61.00 g C m⁻² yr⁻¹). These findings provide robust evidence that high OCBF does not necessarily indicate a stable carbon sink. We therefore emphasize that a multidimensional assessment framework integrating carbon flux and stability is essential for developing spatially differentiated and effective lake carbon management strategies.