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), mi crobial biomass (C-mic) and respired CO2 were measured in a range of grassl and 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 u sing the chloroform fumigation extraction method, while CO2 was measured in a closed system after 3 and 10 days of incubation. Two soils, temperate an d tropical, were used for a long-term experiment, in which measurements wer e performed after 3, 10 and 40 days of incubation. 2. SOC and C-mic decrease exponentially with increasing mean annual tempera ture. C-mic decreases more slowly than SOC, resulting in a higher proportio n 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 respi red 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 respire d 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 gr owth stage of the soil microbial population.