Stable carbon isotope fractionation during microbial turnover of sucrose in different media: Soils, carbon-free quartz and liquid media

Using vegetation sources (C3 vs. C4) with different stable carbon (C) isotope ratios (δ¹³C) is an important approach for revealing ecosystem functioning and soil organic carbon (SOC) dynamics. However, differences in δ¹³C across different C pools (substrate, microbial biomass, respired CO₂) indicate isotopic fractionation causing interpretive challenges. This calls for a deeper understanding of the isotopic fractionation processes taking place during the initial microbial turnover of substrates. We addressed this issue by incubating soil extracted microbial consortia in a C-free liquid media and quartz along with soils providing C3 (−26.6 ‰) and C4 (−12.2 ‰) sucrose as sole C-source. Regardless of media type, respired CO₂ showed consistent ¹³C depletion of 3.0 ‰ for C3 and 4.3 ‰ for C4 compared to the sucrose substrate. While δ¹³C of the SOC remained unchanged, the soil microbial biomass was enriched in ¹³C by 3–7 ‰ compared to the sucrose. This rather consistent initial fractionation around 4 ‰ between substrate and respired CO₂, suggests that ignoring microbial fractionation in short-term studies of specific substrates may lead to over- or underestimations of C exchange among different pools. Furthermore, it highlights potential flaws in studies assessing short-term C turnover in soil and priming effects using ¹³C natural abundance. Further research is needed to explore the governing factors of microbial fractionation during turnover of complex substrates and refine our understanding of longer-term SOC dynamics, ultimately improving accurate interpretations of δ¹³C data.