Molecular activity mediates the composition and assembly of dissolved organic matter in lake sediments

Lake sediments are hotspots for carbon transformation and burial, where the turnover of dissolved organic matter (DOM), influenced by molecular activity, regulates global carbon cycling. However, the composition of sediment DOM and how its assembly mechanisms are related to molecular activity remain poorly understood. Here, 63 freshwater sediments were collected from tropical to cold temperate climatic zones in China. We explored the molecular composition and assembly of sediment DOM and the underlying mechanisms driven by climate, physicochemical factors, and microbes along the gradient of molecular activity, indicated by the number of potential biochemical transformations in which a molecule is involved. Sediment DOM was dominated by lipid‐ (35%) and lignin‐like compounds (33%), and the latter were enriched as the molecular activity of DOM increased. Besides, the DOM assemblage with higher potential biochemical transformations had greater compositional similarity across different climatic zones and tended to assemble deterministically. Specifically, as the average number of potential biochemical transformations of molecules increased from 0.4 to 14, the assembly of these molecules was structured by a shift from stochastic to deterministic processes, with the latter accounting for ≥ 75% thereafter. Overall, DOM assemblages were primarily structured by physicochemical factors, including sediment total organic carbon and electrical conductivity. As molecular activity increased, however, assemblages were increasingly affected by climate and bacterial communities, consistent with the enhanced complexity of bacterial–molecular networks. Collectively, our study highlights that the turnover of DOM regulated by biotic and abiotic factors is further constrained by the intrinsic molecular activity.