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

Abstract

Summary 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.

Mineralisation, leaching and stabilisation of <sup>13</sup>C-labelled leaf and twig litter in a beech forest soil

“…This hill range is situated about 20 km NW of Zurich (47 • 28 40.8 N, 8 • 21 55.2 E) and belongs to the easternmost part of the Jura mountain range. As a contribution to the CarboEurope IP, netecosystem CO 2 exchange and soil respiration have been measured routinely there for several years (Etzold et al, 2010;Ruehr et al, 2010). The mean annual temperature is 8.4 • C and mean annual precipitation is 930 mm.…”

“…How much the aboveground litter contributes to the soil C pool in the long term depends considerably on the rate at which its C is either mineralised to CO 2 or incorporated into mineral soils through soil fauna and dissolved organic C (DOC) (Rubino et al, 2010).…”

“…Bird and Torn, 2006;Fröberg et al, 2009;Rubino et al, 2010). Isotopic labels allow the estimation of litter contributions to soil respiration as well as the tracking of litter-derived C from the forest floor to mineral soils.…”

Microbial assimilation of plant-derived carbon in soil traced by isotope analysis

“…Little is known about isotopic effects of C transfer in soil through the microbial community. Recent studies revealed that δ 13 C values of microbial biomass were 13 C enriched by about −2‰ relative to that of the corresponding total soil C, indicating that an isotope effect due to microbial degradation may also be of importance in soil (Angers et al 1995;Santruckova et al 2000). The authors concluded that this isotope effect could be induced by selective assimilation of different soil C pools or 13 C fractionation during microbial metabolism, in particular during respiration where the released CO 2 is depleted in 13 C. It is extremely difficult to obtain microbial C from soil because of the likelihood of co-extracting material from indigenous soil C (Sparling et al 1990), resulting in an inaccurate determination of δ 13 C values.…”

Quantification of Sedimentary Organic Carbon Storage and Turnover of Tidal Mangrove Stands in Southern China Based on Carbon Isotopic Measurements

“…Third, many studies have indicated that fractionation by microorganisms results in SOC  13 C enrichment with depth (Agren et al 1996;Schweizer et al 1999;Santruckova et al 2000;Powers and Schlesinger 2002). Microorganisms preferentially metabolize molecules containing the lighter 12 C, so that the remaining SOC tends to be 13 C enriched.…”