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

1. The carbon content and δ¹³C 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 δ¹³C value of C(mic) and respired CO₂ reflects gross changes in the δ¹³C value of SOC in the corresponding sample. On average, C(mic) is ¹³C-enriched by c. 2‰ compared with SOC, while respired CO₂ is ¹³C-depleted by c. 2.2‰ compared with C(mic). Thus, the observed ¹³C-enrichment in C(mic) 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.