Elevated CO₂ and warming are both known to stimulate soil respiration rates, leading to concerns regarding soil-related feedback effects on climate change. We investigated soil C cycling at the Prairie Heating and CO₂ Enrichment (PHACE) experiment near Cheyenne, WY, a factorial experiment combining FACE (ambient and elevated [600 ppm] CO₂ concentration), experimental warming (1.5ºC daytime, 3ºC nighttime) and irrigation to evaluate direct, indirect and interactive effects of global changes on native grassland structure and function. We measured soil respiration rates and applied two methods (vegetation removal by herbicide and stable isotopes) to partition the total flux into root respiration and decomposition components, during the growing season of 2008. We hypothesized that soil respiration and decomposition would be stimulated by elevated CO₂ and experimental warming, and that vegetation removal would provide more comprehensive results in comparison with stable isotope partitioning. 

Results/Conclusions

Experimental warming did not alter soil respiration or decomposition rates during the study period. Elevated CO₂ did not alter soil respiration rates on undisturbed mixed-grass prairie, but it stimulated decomposition on non-vegetated plots by ~40% at ambient temperature and 60% at elevated temperature. These results are consistent with a priming effect due to increased labile C allocation belowground under elevated CO₂ that enhances decomposition. Vegetation removal suggested that decomposition from ambient CO₂ plots was 35% of total soil respiration, and that from elevated CO₂ plots was 52% of soil respiration. The δ¹³C value of soil respiration in non-vegetated, elevated CO₂ plots was similar to that in undisturbed plots, showing that labile substrates remained available for decomposition for several months following herbicide application.  Stable isotopes suggested that warming in combination with elevated CO₂ enhanced decomposition of older soil organic matter, in comparison to unwarmed, elevated CO₂ plots. This research suggests that although vegetation removal causes disturbance, it can be applied in situations that do not allow stable isotope partitioning, and is reasonably straightforward to interpret. Uncertainties in stable isotope partitioning owing to determination of “end member” isotope values and non-steady state respiration conditions make it more challenging to apply. Additional research to quantify effects of elevated CO₂ and warming on decomposition of soil organic matter will reduce uncertainties in model predictions of future climate.