Decoupling of carbon burial from eutrophication in chinese lakes reveals a weakened sink.

Lakes are integral to the global carbon cycle, yet the long-term impact of eutrophication on sedimentary organic carbon (OC) burial remains highly debated. Here, we present a multi-proxy analysis of Lake Dian, a representative shallow eutrophic plateau lake in China, integrating ²¹⁰Pb-¹³⁷Cs chronology, OC accumulation rate (OCAR) quantification, and stable isotope source apportionment (MixSIAR) to trace centennial-scale carbon dynamics. We further validate our findings across three major Chinese lake regions (Northeast, Eastern Plain, and Yunnan-Guizhou Plateau). Our results show that allochthonous OC (soil organic matter and sewage) dominates Lake Dian core sediments (>60% mean), with algal-derived OC ≤20% in surface sediments and <10% pre-1980. The post-1980 OCAR increase is statistically decoupled from eutrophication indicators, driven instead by watershed land-use change and long-term precipitation trends. Eutrophication appears to impair long-term carbon burial via three synergistic mechanisms: (1) rapid mineralization of labile algal OC under conditions of high temperature and sediment resuspension; (2) priming effect where fresh algal carbon accelerates allochthonous OC decomposition; (3) suppression of inorganic carbon (IC) precipitation due to pH reduction from algal bloom decomposition. Regional comparisons reveal consistent decoupling of OCAR from eutrophication across all three lake regions, with OCAR increases uniformly linked to terrestrial rather than autochthonous inputs. Our findings challenge the prevailing view that eutrophication enhances lake carbon sinks, and highlight watershed management as the primary lever for regulating lake carbon burial. These results provide critical insights for refining global carbon budgets and implementing "dual-carbon" ecosystem management strategies.