Temperature effects on the diversity of soil heterotrophs and the δ13C of soil-respired CO2.

Abstract We measured the respiration rates, δ 13 C of respired CO 2 , and microbial community composition in root-free bulk soils incubated at 4, 22 and 40°C. The soils were obtained from the Duke Forest Free-Air CO 2 Enrichment (FACE) experiment where organic carbon in soils sampled from the elevated CO 2 plots contained a unique 13 C label that was derived from FACE fumigation. The CO 2 produced by soil heterotrophs at 4°C was 2.2 to 3.5‰ enriched in 13 C relative to CO 2 respired at 22 and 40°C and was similarly enriched relative to bulk soil carbon. There was no isotopic difference between CO 2 produced at 22 and 40°C. Respiration rates increased exponentially with temperature from 0.25 mg CO 2 g soil −1 d −1 at 4°C to 0.73 mg CO 2 g soil −1 d −1 at 40°C. Microbial community composition, as measured by the differences in populations of morphology types, differed across the temperature range. Only eight of 67 microbial morphology types were common to all three incubation temperatures, while six types were unique to 4°C soil, 17 to 22°C soil and 18 to 40°C soil. Species richness, approximated from morphology type, was significantly lower at 4°C than at 22 and 40°C. This change in microbial community structure from 4 to 22 and 40°C caused a shift in mineralizable carbon pools, resulting in a shift in the isotopic composition of CO 2 respired at the low temperature.