1. Amanda Brennan 1, Elise Pendall 1, Dave Risk 2, Yolima Carrillo 3 1 University of Wyoming, Department of Botany and Program in Ecology, Laramie, WY 82071, contact: abrenna3@uwyo.edu 2 Dept. of Earth Sciences, St. Francis Xavier University, Antigonish, NS, Canada; 3 University of Sydney, Australia Acknowledgements We thank Dan LeCain and Jack Morgan for technical support. This project was supported by USDA-CSREES Soil Processes Program, DOE and NSF, with travel support from UW Program in Ecology and Department of Botany. B51B-0547 Continuous soil respiration at the Prairie Heating and CO 2 Enrichment site using forced diffusion chambers Introduction Variation in soil respiration is regulated by plant and microbial responses to environmental changes, such as soil moisture. Our first objective was to determine the reliability of forced diffusion chambers housing non-dispersive infrared CO 2 sensors for continuous fluxes in semi-arid grasslands (Risk et al. 2011). We also compared the response of soil respiration to environmental drivers under different climatic treatments. We hypothesized that elevated CO 2 would stimulate both total soil and microbial respiration, due to increased belowground allocation, whereas warming would reduce soil and microbial respiration as a result of soil drying. We further hypothesized that total soil respiration would be more sensitive to soil moisture, whereas microbial respiration in the plant-free plots would be more sensitive to changes in soil temperature. Study site Prairie Heating and CO 2 Enrichment (PHACE) is located in northern mixed-grass prairie near Cheyenne, WY, USA (http://www.ars.usda.gov/npa/rrru/phace) Vegetation dominated by cool season (C3) grasses with an important warm season (C4) species Mean annual precipitation is 370 mm; mean temperature is 7°C PHACE rings are 3-m diameter, elevated [CO 2 ] is raised to 600 ppm by direct injection in daytime during the growth season, and temperature is warmed to +1.5/+3°C day/night year-round with ceramic heaters. [CO 2 ] treatment started in 2006, warming in 2007. [CO 2 ] disparities between air sensors and drift over time indicate a need for frequent sensor calibrations to accurately estimate fluxes. For instance, [CO 2 ] in the ambient treatment is unrealistically high in winter months. Soil CO 2 efflux and microbial respiration tends to be reduced by warming. Elevated CO 2 had no apparent effect on total soil respiration yet enhanced microbial respiration in the plant-free plot. Seasonal fluctuations in soil temperature affected mixed grass prairie soil respiration more than microbial respiration. During the growing season, soil water content was a good indicator of soil respiration. Microbial respiration was generally less affected by pulses in moisture. Observations Conclusions and Future Work Prairie soil respiration was not consistently affected by climate change treatments, but was largely influenced by environmental drivers. Treatment effects were stronger on microbial respiration with less effects of environmental drivers. The drift in CO 2 concentration over time needs to be addressed before flux-driver relationships can be better understood. Infra-red Heaters Free Air CO 2 Enrichment Ring PHACE Treatments ct = ambient [CO 2 ] and temperature cT = ambient [CO 2 ] and elevated temperature Ct = elevated [CO 2 ] and ambient temperature. Soil Efflux and Environmental Controls CO 2 Concentrations Continuous Timeseries- Forced Diffusion Soil Emissions pass into a dynamic chamber along a slightly restrictive membrane with known diffusivity; background air is monitored in a chamber sealed off from the soil Vaisala GMP-222 IRGAs inside the chambers continuously measure [CO 2 ] Plant-free plots had glyphosate added in May 2008 and a root barrier to 25 cm is maintained by manual removal. References Risk D, Nickerson N, Creelman C, McArthur G, Owens J (2011). Forced Diffusion soil flux: A new technique for continuous monitoring of soil gas efflux. Agricultural and Forest Meteorology 151: 12, 1622-1631. Risk et al. 2011 Mixed-grass chamber Plant-free chamber Air Chamber