There is a high degree of uncertainty in the response of soil organic matter (SOM) dynamics to interacting global change factors. Elevated CO₂ can increase soil C pools, but might also enhance decomposition by increasing soil moisture and/or via priming. Warming can enhance decomposition but might also decrease it by reducing soil moisture. The magnitude of the interacting effects of these factors will determine the direction of soil C feedback on climate change. We conducted laboratory incubations of soils from two different depths at the PHACE (Prairie Heating and CO₂ Enrichment) experiment site in the northern mixed grass prairie in Wyoming, USA. These soils had been exposed to elevated CO₂ (600 ppm) and temperature (1.5/3 °C increase during the day/night) in a factorial arrangement. Two-pool decay model fits were used to estimate active C pool sizes and mean residence times, and slow C pool decomposition rates. Isotopic composition of respiratory CO₂ was used to detect changes in the sources of respiration and shifts in δ¹³C of soil organic matter pools.

Results/Conclusions After two years of CO₂ enrichment the active C pool increased and this effect was stronger in surface soils. The mean residence time of active C also increased under elevated CO₂ and this effect was consistent across depths, which, together with greater observed root biomass suggests that the greater active C pool size was a result of both greater plant C inputs and lower turnover times (as indicated by greater mean residence times) under elevated CO₂. There was no indication of change in the mineralization rate of the slow pool, which suggests that the extra input of new carbon did not enhance the decomposition of older soil carbon. We found no temperature or CO₂*warming effects after one year of exposure. Active pool δ¹³C was lower than slow pool δ¹³C and this corresponded with the isotopic composition of plant residues and soil organic matter respectively. There was no effect of temperature on δ¹³C of the active or slow C pools indicating no shift in C source associated with a temperature increase. The proportion of new C (incorporated to soil since the beginning of the experiment) utilized by microbes increased with depth, decreased as the incubations progressed and also tended to decrease with experimental warming.