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Chemodiversity mediates temperature sensitivity of organic carbon decomposition in lake sediments

chemodiversity,carbon-climate feedback,carbon quality temperature hypothesis,global warming,lake ecosystems

Theme 2: Geobiology of Modern Habitable Earth > Session 2a: Microbe-mediated Carbon Cycling on Modern Earth

Organic carbon decomposition in lake sediments contributes substantially to the global carbon cycle and is strongly affected by temperature. However, the magnitude of temperature sensitivity (Q₁₀) of decomposition and the underlying factors remain unclear at the continental scale. Carbon quality temperature (CQT) hypothesis asserts that less reactive and more recalcitrant molecules tend to have higher temperature sensitivities, but its support is challenged by complex composition of organic matter and environmental constraints. Here, we quantified Q₁₀ of the sediments across 50 freshwater ecosystems along a 3,500 km north-south transect, and characterized the quality of sediment dissolved organic carbon with chemodiversity reflected in molecular richness, functional traits (i.e., molecular weight, bioavailability, etc.) and composition. We further included classic environmental variables, such as climatic, physicochemical and microbial factors, to explore how Q₁₀ is constrained by these factors or carbon quality. The composition of sediment DOM was dominated by lipid- (35%) and lignin-like compounds (33%), and was primarily structured by physicochemical factors, including sediment total organic carbon and electrical conductivity. As molecular activity increased, however, sediment DOM was increasingly affected by climate and bacterial communities. We found that Q₁₀ varied greatly across lakes, with the mean value of 1.78 ± 0.62, but showed nonsignificant latitudinal pattern. Q₁₀ was primarily predicted by chemodiversity, and showed an increasing trend with the biochemical recalcitrance indicated by traits such as aromaticity and standard Gibb’s Free Energy at both molecular and compositional levels. This suggests that carbon quality is the crucial determinant of Q₁₀ in lakes, supporting the CQT hypothesis. Moreover, Q₁₀ decreased linearly with the increase of molecular richness, implying that the resistance of decomposition to warming is associated with higher molecular diversity. Compared with the structural equation model containing only environmental variables, inclusion of chemodiversity increased 32.8% of the explained variation in Q₁₀, and chemodiversity was the only driver showing direct effects. Collectively, this study illustrates the importance of chemodiversity in shaping the pattern of Q₁₀, and has significant implications for accurately predicting the carbon turnover in lake ecosystems in the context of global warming.