Figure 3: CO2 flux calculated with diffusion technique by soil temperature at the control and fire s

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 3 CO2 flux (calculated with diffusion
technique) by soil temperature at the control and
fire sites
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 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 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 lt0.01
  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
  Plt0.0000, R20.8699 Fire Plt0.0000, R20.7480
  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. 121032-1046. 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.