Carbon cycling in a native grassland exposed to elevated CO2 and warming: A role for priming

Background/Question/Methods

Terrestrial ecosystem carbon cycling has the potential to mitigate or exacerbate climate change, depending on relative responses to rising temperatures and atmospheric CO2 concentrations, and possible indirect effects mediated through soil moisture and nutrient availability. The Prairie Heating and CO2 Enrichment (PHACE) experiment is being conducted in a native semiarid grassland near Cheyenne, Wyoming, to examine the independent and combined effects of elevated atmospheric CO2 (380 vs. 600 ppm during the growing season) and warming (+1.5/3C day/night above ambient) on ecosystem processes. Net primary production (NPP) is a commonly measured component of the C cycle but it does not reflect the full C balance. Measurement of net ecosystem exchange (NEE) of CO2 using a canopy gas exchange chamber system allows estimates of net C storage as well its underlying processes of gross primary production (Pe) and ecosystem respiration (Re). We measured NPP, NEE, Pe, Re, microbial respiration (Rh), litter decomposition, and soil C stocks over four years to assess potential for C cycle feedbacks at PHACE. We hypothesized that C cycling would be stimulated by elevated CO2, diminished by warming, and that C losses would be greatest in the combined elevated CO2 and warming treatment, owing to moisture-related effects.

Results/Conclusions

Aboveground biomass production and Pe were stimulated by elevated CO2 in years of average or below-average precipitation. Belowground biomass was stimulated by elevated CO2 after a 2-year lag, and Re was stimulated by the combined elevated CO2 and warming treatment during the first four years at PHACE.  Despite stimulation of NPP in most years, net C uptake measured with the canopy gas exchange chamber was reduced in elevated CO2 treatments, especially when combined with warming. The net C losses with elevated CO2 were consistent with stimulated Rh on root-exclusion plots. Soil C pools in the top 15-cm increased with warming but were not altered significantly by elevated CO2. Numerous lines of evidence therefore point to a potential role for priming enhanced decomposition of soil organic matter in the elevated CO2 treatment: increased input via NPP is offset by increased Rh under elevated CO2, leading to a lack of soil C storage. In this semiarid grassland, soil C accumulated with warming, probably due to soil drying, but soil C storage was compromised by elevated CO2 due to stimulated heterotrophic respiration. Ecosystem biogeochemistry models should incorporate these effects to improve forecasts of climate-carbon cycle feedbacks.