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

Abstract

Abstract 1. The carbon content and δ 13 C value of soil organic carbon (SOC), microbial biomass (C mic ) and respired CO 2 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 CO 2 . The soil from a depth of 0–2 cm was analysed. C mic was measured using the chloroform fumigation extraction method, while CO 2 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 δ 13 C value of C mic and respired CO 2 reflects gross changes in the δ 13 C value of SOC in the corresponding sample. On average, C mic is 13 C‐enriched by c. 2‰ compared with SOC, while respired CO 2 is 13 C‐depleted by c. 2·2‰ compared with C mic . Thus, the observed 13 C‐enrichment in C mic is balanced by a corresponding 13 C‐depletion in respired CO 2 resulting in the δ 13 C value of respired CO 2 being approximately similar to the δ 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.