High stability of autochthonous organic matter in inland aquatic ecosystems

Recalcitrant dissolved organic matter (RDOM) is a key component of ocean carbon sinks and it can be preserved in seawater for thousands of years. However, the fate of RDOM derived from the primary production in inland lake is unclear. In this study of Erhai lake in China, used the combination of δ13C, carbon/nitrogen (C/N) ratio, and optical spectroscopy analyses to constrain the variation of organic matter in the vertical direction (i.e., lake water → trap sediments → surface sediments → core sediments), to trace the burial process of RDOM. Two autochthonous components (C1, C2) and two allochthonous components (C3, C4) were identified in the water. High concentrations of C2 in winter and spring indicate that these two seasons provide favourable burial conditions for autochthonous dissolved organic matter. Three autochthonous components (C1, C2, C5) and three allochthonous components (C3, C4, C6) were identified in the sediments. The contributions of autochthonous organic matter were ∼49.9 ± 5.84 % (based on PARAFAC analysis) and ∼56.7 ± 5.62 % (based on the C/N ratio) in the sediment trap samples; 43.4 ± 8.84 % (based on PARAFAC analysis) and 40.8 ± 14.26 % (based on the C/N ratio) in the surface sediments; and 44.5 ± 14.4 % (based on PARAFAC analysis) and 48.4 ± 6.04 % (based on the C/N ratio) in the core sediments. Additionally, C2, which is the autochthonous component after microbial mineralisation, was preserved as a significant part of the RDOM in the water. Our results suggest that the mineralisation of autochthonous organic matter (OM) in the core sediments did not promote bacterial mineralization, and that the burial of OM on long timescales dependents primarily on its concentration rather than on its origin. Our results provide a new perspective for studying the stability of autochthonous OM and highlight a new direction for the study of the carbon sink of inland lakes.