Soils of high latitudes store approximately one-third of the global soil carbon pool. Decomposition of soil organic matter (SOM) is expected to increase in response to global warming, which is most pronounced in northern latitudes. It is, however, unclear if microorganisms are able to utilize more stable, recalcitrant C pools, when labile soil carbon pools will be depleted due to increasing temperatures. Here we report on an incubation experiment with intact soil cores of a frost-boil tundra ecosystem at three different temperatures (2 degrees C, 12 degrees C and 24 degrees C). In order to assess which fractions of the SOM are available for decomposition at various temperatures, we analyzed the isotopic signature of respired CO2 and of different SOM fractions. The delta13C values of CO2 respired were negatively correlated with temperature, indicating the utilization of SOM fractions that were depleted in 13C at higher temperatures. Chemical fractionation of SOM showed that the water-soluble fraction (presumably the most easily available substrates for microbial respiration) was most enriched in 13C, while the acid-insoluble pool (recalcitrant substrates) was most depleted in 13C. Our results therefore suggest that, at higher temperatures, recalcitrant compounds are preferentially respired by arctic microbes. When the isotopic signatures of respired CO2 of soils which had been incubated at 24 degrees C were measured at 12 degrees C, the delta13C values shifted to values found in soils incubated at 12 degrees C, indicating the reversible use of more easily available substrates. Analysis of phospholipid fatty acid profiles showed significant differences in microbial community structure at various incubation temperatures indicating that microorganisms with preference for more recalcitrant compounds establish as temperatures increase. In summary our results demonstrate that a large portion of tundra SOM is potentially mineralizable.

The aim of this study was to assess initial effects of warming on the CO 2 balance of a lichenrich dwarf shrub tundra, a widespread but little studied ecosystem type in the Arctic. We analyzed whole ecosystem carbon exchange rates as well as nutrient dynamics, microbial and plant community composition and biomass after 2 years of experimental temperature increase. Plant biomass increased significantly with warming, mainly due to the strong response of lichens, the dominant plant group within this ecosystem. Experimental warming also increased soil nitrogen pools and nitrogen turnover rates. Major changes in soil microbial and plant composition, however, were not detected. Although experimental warming increased gross ecosystem productivity, the higher plant biomass did not compensate for the much greater increase in C losses. Ecosystem respiration and net ecosystem CO 2 losses were significantly higher in warmed plots compared to control ones. We suggest that this was due to increased soil respiration, since soil carbon pools were lower in warmed soils, at least in the upper horizons. Our study thus supports the general hypothesis that tundra ecosystems turn from a carbon sink to a carbon source when temperatures increase in the short-term. Since lichens, which produce low quality litter, increased their biomass significantly with warming in this specific ecosystem type, CO 2 losses may slow down in the long-term.

Cryoturbation (mixing of soil layers due to repeated freeze‐thaw processes) is a major soil forming process in arctic regions, which may contribute to long‐term storage of C in soils of northern latitudes. Our goal was to determine the effect of subduction of organic matter by cryoturbation on microbial decomposition processes in tundra soils. Buried layers were situated at 30–60 cm depth, between Bg and B horizons, but exhibited a C and N content highly similar to present‐day A horizons. Radiocarbon dating revealed, however, that the mean age of C in the buried layer was three times higher (∼1300 years BP) than in the A horizon (∼400 years BP), suggesting that decomposition rates in the buried layer were delayed. The observed microbial processes support this result: gross C and N mineralization rates were substantially lower in the buried layers than in the respective A horizons. The amount of C stored in the buried layer still doubles the amount of C stored in topsoil horizons (O and A). Assuming that the buried layer originates from both O and A horizons, this indicates that O and A horizon at time of burying (800–1300 years BP) must have been significantly thicker and present‐day O and A horizon at this site may still have the capacity to accumulate additional C. Cryoturbation therefore may lead to additional long‐term storage of carbon in the system by (1) retarding decomposition processes of buried organic material and (2) enabling the soil to restart C accumulation in topsoil layers.

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