The determination of δ13C in soil microbial biomass using fumigation-extraction

Abstract The determination of the isotopic composition of the microbial biomass C in soil is an important tool to study soil microbial ecology and the decomposition and microbial immobilization of soil organic C. We discuss advantages and disadvantages of different methods to determine 13 C/ 12 C in soil microbial biomass and propose a new procedure that is based on the UV-catalyzed liquid oxidation of fumigated and non-fumigated soil extracts combined with trapping of the released CO 2 in liquid nitrogen and subsequent determination of δ 13 CO 2 -C by a gas chromatograph connected with an isotope ratio mass spectrometer (IRMS). This method was evaluated using test solutions with known isotopic composition and soil extracts. Additionally, the method was compared with an off-line sample preparation technique combined with isotope analysis by a dual-inlet IRMS and an on-line analysis using an elemental analyser connected with an IRMS. All methods applied obtained comparable results and there were no significant differences between the δ 13 C values measured. The off-line preparation procedure had the highest precision but it was also the most labour-intensive. The choice of the most suitable method depends mainly on the number of samples that have to be analysed, the salt concentration of the extracts and the differences of δ 13 C that have to be detected. The application of this method with liquid oxidation and subsequent GC-IRMS analysis showed that microbial biomass C of a grassland soil was 13 C-enriched by 2‰ δ 13 C PDB compared with the total soil organic C. The addition of maize straw resulted in a rapid immobilization of maize C in the microbial biomass.