INTRODUCTION

1. Effects of wildfire on soil carbon dioxide and methane fluxes in a Southwest, U.S. ponderosa pine forest Sullivan, B.W.1, T.E. Kolb1, S.C. Hart1, S. Dore1, M. Montes-Helu1,B.A. Hungate2 1-Northern Arizona University School of Forestry, Flagstaff, AZ 2-Northern Arizona University Department of Biological Sciences, Flagstaff, AZ INTRODUCTION METHODS • CO2 diffusion probe technique: • Small solid-state infra-red gas analyzers (GMM 220, Vaisala Inc., Finland) were buried at three depths in the soil profile and measured CO2 concentration at ½ hour intervals every day • Soil volumetric water content (VWC) and temperature were measured with Decagon ECH2O probes and thermocouples, respectively • Using a model of soil diffusivity including VWC and temperature (Moldrup et al.,1999) the rate of CO2 diffusion between the different depths was used to calculate CO2 flux at the soil surface • 2) CO2 and CH4 Static-chamber technique: • 30-cm diameter PVC rings were permanently placed in the soil, distributed at 15 locations around each study site • Samples of gas headspace were taken at regular intervals • Gas samples were analyzed for CO2 and CH4 using gas chromatography. Changes in concentration over time were used to calculate fluxes Atmospheric levels of carbon dioxide (CO2) and methane (CH4) are currently at levels unseen in 450,000 years. Soil is a significant source and sink for CO2 and CH4. Worldwide, forest soils release to the atmosphere 10 times the CO2 of anthropogenic emissions. CH4 has 25 times the atmospheric warming potential of CO2. Every year thousands of hectares of ponderosa pine forest are consumed by stand-replacing wildfires in the western US. The soil in these burned areas are presumed to undergo changes in the production and consumption of CO2 and CH4. To accurately calculate carbon budgets for this region, the influence of fire on soil respiration and CH4 consumption must be quantified. We compare soil gas fluxes in a dense forest (unburned control site) to a previously forested site that burned catastrophically in 1996, killing all trees (fire site). The objective of this research is to explore how wildfire affects fluxes of CO2 and CH4 from soil in ponderosa pine forests on the Colorado Plateau in Northern Arizona. Figure 1: Changes in CO2 efflux between sites using probes (lines) and chambers (points) for all of 2006. Error bars indicate +/- one SE. Positive values are effluxes. Figure 2: Trend in CH4 uptake during the summer shows increased CH4 uptake at the fire site, and a reduction in CH4 uptake at the control site as VWC increases. Figure 3: CO2 flux (calculated with diffusion technique) by soil temperature at the control and fire sites Figure 4: CO2 flux (calculated with diffusion technique) by VWC at the control and fire sites DISCUSSION RESULTS • Using a combination of the CO2 diffusion technique and the static-chamber technique when probe data was unavailable, an annual flux of 417 g CO2-C m-2 and 210 g CO2-C m-2 was calculated for the control and fire site respectively. • Annual CH4 flux is -7.19 and -11.16 mg of CH4-C m-2 for the control and fire sites respectively, which has a warming potential equal to -177 and -279 mg of CO2-C m-2 yr-1. This is a <0.01% offset of the net CO2 flux at these sites. • Seasonal changes in temperature and precipitation influenced CO2 efflux (Figure 1). • CH4 consumption decreases with increasing %VWC at the forested sites but increases with increasing %VWC at the fire site (Figure 2); this inverse pattern is likely due to the wetter soils observed at the control site and the drier soils at the fire site. • There is a significant interaction of soil temperature and VWC on CO2 flux (Control: P<0.0000, R2=0.8699; Fire: P<0.0000, R2=0.7480; Figures 3 and 4) The annual CO2 flux values at the control site (417 g C m-2) are within the range of values reported in a proximate ponderosa pine forest in northern Arizona (225-565 g C m-2; Hart, S.C., 2006, Hart et al. 2006). The consumption of CH4 offsets less than 1/1000th of the warming potential of the annual soil CO2 flux. The control site releases twice the CO2 to the atmosphere than the fire site, but this may be mitigated by the weaker sink strength at the fire site than at the control site (see Dore et al. poster in this session). Soil respiration is strongly controlled by changes in soil temperature and soil moisture. Respiration rates increase with temperature in the spring, and increase again with VWC during the mid-summer monsoon season. CH4 uptake increased at the drier fire site, but decreased at the wetter control site. The combination of the diffusion profile and static chambers allows for accurate measurement of both spatial and temporal variation of soil respiration. An added advantage of the static-chamber method is the simultaneous measurement of CO2 and CH4. The results of these two techniques suggest that wildfire reduces soil respiration and increases CH4 consumption. No trees have established in the ten years since fire. These lower CO2 efflux rates and higher CH4 consumption rates could continue for a century until the forest regenerates and reaches a stand age similar to our control site (60-200 years). REFERENCES Hart, S.C., P. Selmants, S. Boyle, S. Overby, 2006. Carbon and nitrogen cycling in Southwestern ponderosa pine forests. Forest Science 52(6):683-693. Hart, S.C., 2006. Potential impacts of climate change on nitrogen transformations and greenhouse gas fluxes in forests: a soil transfer study. Glob. Change Biol. 12:1032-1046. Moldrup, P., T. Olesen, T. Yamaguchi, P. Schjonning, D.E. Rolston, 1999. Modeling diffusion and reaction in soils. IX. The Buckingham-Burdine-Campbell equation for gas diffusivity in undisturbed soil. Soil Science 164(8):5432-551.