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 (δ 13 C) is an important approach for revealing ecosystem functioning and soil organic carbon (SOC) dynamics. However, differences in δ 13 C 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 13 C depletion of 3.0 ‰ for C3 and 4.3 ‰ for C4 compared to the sucrose substrate. While δ 13 C of the SOC remained unchanged, the soil microbial biomass was enriched in 13 C 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 13 C 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 δ 13 C data. • Acknowledging C isotopic fractionation is crucial for interpretation of C cycling • Soils, C-free liquid and quartz with microbial consortium were incubated for 171 days • Media independent 13 C fractionation was recorded across the treatments • Average 3.0 ‰ and 4.3 ‰ depleted 13 CO 2 was respired post C3 and C4 sucrose application. • Soil microbial biomass carbon was 13 C enriched after adding 13 pulses sucrose.