• Carbon isotopes;
  • isotopic fractionation;
  • microbial biomass;
  • microbial respiration;
  • soil organic carbon


1. The carbon content and δ13C value of soil organic carbon (SOC), microbial biomass (Cmic) and respired CO2 were measured in a range of grassland soils from tropical and temperate biomes to determine if isotope effect of microbial degradation can induce a shift in isotope composition of SOC and CO2. The soil from a depth of 0–2 cm was analysed. Cmic was measured using the chloroform fumigation extraction method, while CO2 was measured in a closed system after 3 and 10 days of incubation. Two soils, temperate and tropical, were used for a long-term experiment, in which measurements were performed after 3, 10 and 40 days of incubation.

2. SOC and Cmic decrease exponentially with increasing mean annual temperature. Cmic decreases more slowly than SOC, resulting in a higher proportion of Cmic in the SOC of tropical soils relative to temperate soils.

3. The δ13C value of Cmic and respired CO2 reflects gross changes in the δ13C value of SOC in the corresponding sample. On average, Cmic is 13C-enriched by c. 2‰ compared with SOC, while respired CO2 is 13C-depleted by c. 2·2‰ compared with Cmic. Thus, the observed 13C-enrichment in Cmic is balanced by a corresponding 13C-depletion in respired CO2 resulting in the δ13C value of respired CO2 being approximately similar to the δ13C of SOC.

4. The isotope effect of microbial degradation is of importance in soil. It can be induced by selective utilization of SOC and isotope discrimination during metabolism. Metabolic isotopic discrimination is dependent on the growth stage of the soil microbial population.