Decomposition of aged sedimentary organic matter in a deep temperate lake under warming conditions: An experimental...

Although lakes are major reservoirs for organic matter (OM), global warming may accelerate the decomposition of sedimentary OM. Enhanced OM decomposition under warming intensifies carbon cycling in the biosphere and can lead to ecosystem degradation through deoxygenation. However, it remains unclear whether this temperature-driven increase in decomposition primarily involves modern OM or aged OM. In this study, we applied a novel approach that integrates radiocarbon (Δ¹⁴C) analysis with aerobic incubation experiments using undisturbed sediment cores collected from the largest lake in Japan. This approach enabled us to examine directly whether warming accelerates the decomposition of aged OM in aquatic sediments. The age of decomposed OM was estimated by measuring the Δ¹⁴C in the dissolved inorganic carbon (DIC) of the overlying water before and after incubation. At the current hypolimnetic temperature of 8 °C, the Δ¹⁴C value of decomposed OM was -43.6 ± 50.8‰, which is comparable to the surface lake water DIC value. However, the Δ¹⁴C values decreased with increasing temperature, reaching -182.3 ± 31.1‰ at 18 °C. Monte Carlo simulations estimating the temperature sensitivity of modern OM and aged allochthonous OM indicated that aged allochthonous OM exhibits higher temperature sensitivity. This finding supports the carbon quality to temperature hypothesis. These results suggest that reducing anthropogenic erosion and implementing other land-based management measures could help slow the progression of lake deoxygenation under climate warming.