A novel method for simultaneous quantification and isotope analysis of labile soil carbon directly from liquid extracts

Quantifying labile soil carbon (C) pools and their stable isotope composition (δ¹³C) is fundamental for elucidating microbially mediated C cycling, soil organic matter turnover, and isotope fractionation during biogeochemical transformations. Extractable C and microbial biomass C are commonly obtained using salt solutions (e.g., 0.25–0.5 M K₂SO₄); however, subsequent determination of C concentrations and isotope ratios typically requires labor-intensive sample preparation steps, including freeze-drying, oven-drying, or desalting by dialysis. These procedures are time-consuming and may result in substantial losses of dissolved organic C, potentially biasing isotopic signatures.Here, we present a novel analytical method that enables the simultaneous determination of C concentrations and stable isotope composition (δ¹³C) directly from liquid 0.5 M K₂SO₄ soil extracts without any prior sample preparation. This approach allows direct quantification of extractable and microbial biomass C, substantially reducing sample handling and associated analytical uncertainty.Method validation across contrasting soil types demonstrates high precision and reproducibility for both elemental concentrations and isotope ratios, while avoiding C losses associated with dialysis or concentration procedures. The method facilitates rapid, high-throughput analysis and enhances the temporal and mechanistic resolution of studies on microbial turnover, rhizosphere processes, and soil C dynamics.Overall, this approach provides a robust new tool for biogeoscience research, enabling integrated assessments of labile C pools and their isotopic signatures and supporting improved process-based understanding of soil biogeochemical cycling.