Microbial processes and carbon-isotope fractionation in tropical and temperate grassland soils

1. The carbon content and delta(13)C value of soil organic carbon (SOC), microbial biomass (C-mic) 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. C-mic 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 C-mic decrease exponentially with increasing mean annual temperature. C-mic decreases more slowly than SOC, resulting in a higher proportion of C-mic in the SOC of tropical soils relative to temperate soils. 3. The delta(13)C value of C-mic and respired CO2 reflects gross changes in the delta(13)C value of SOC in the corresponding sample. On average, C-mic is C-13-enriched by c. 2 parts per thousand compared with SOC, while respired CO2 is C-13-depleted by c. 2.2 parts per thousand compared with C-mic. Thus, the observed C-13-enrichment in C-mic is balanced by a corresponding C-13-depletion in respired CO2 resulting in the delta(13)C value of respired CO2 being approximately similar to the delta(13)C 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.