The carbon isotopic signature of soil respiration has been demonstrated to be closely linked to the signature of recent photosynthate, suggesting autotrophs and autotroph-dependent organisms hve a large influence on soil CO₂ flux. Under low soil moisture, both plant phloem and soil respired CO₂ become enriched in ¹³C. However, it has not been demonstrated that the change in the isotopic signature of soil respiration under low moisture is due entirely to autotrophs. For example, it is not known whether the microbial component of soil respiration also becomes enriched with ¹³C with declining soil moisture.  It has also not been demonstrated how the relative contributions of autotrophic and heterotrophic respiration change as a proportion of total soil respiration as moisture declines.  To learn more about moisture effects on the isotopic composition of soil respiration, we tested the following two hypotheses: 1) heterotrophic respiration does not change carbon isotope composition under high and low moisture conditions, and 2) autotrophic respiration rates are more sensitive to soil moisture than heterotrophic respiration rates and will constitute a smaller proportion of total soil respiration under low moisture as compared with high moisture conditions.