『Abstract
The characteristic feature of the Prairie Pothole Region is a
complex assemblage of mineral soil wetlands embedded in the dominantly
agricultural landscape. Soils in these wetlands are loci of high
potential greenhouse gas (GHG) emissions, and our objective was
to provide estimates of greenhouse gas emissions and the controls
on these emissions for typical wetlands of this region. Three
years (2004-06) of N2O and CH4
emissions were taken from a large semi-permanent pond and five
ephemeral freshwater mineral soil wetlands at the St. Denis National
Wildlife Area (SDNWA) near Saskatoon, Saskatchewan, Canada. Methane
emissions from the semi-permanent pond were low (ranging from
0.04 to 3.33 g CH4 m-2 yr-1)
but emissions from landscape elements of the ephemeral ponds were
substantially higher, with a maximum of 138.6 g CH4
m-2 yr-1 (or approximately 110 g CH4 m-2 yr-1 when corrected
for mid-day sampling bias) from basin centers of these ponds in
2005. The average annual CH4 emissions averaged
across the three elements of the ephemeral ponds at SDNWA were
54.8 g CH4 m-2 yr-1
in 2005 and 32.4 g CH4 m-2 yr-1
in 2006. Methane emissions were significantly inversely correlated
to SO42- concentrations of the
pond water, which are in turn related to the balance between surface
runoff and groundwater inputs into the ponds. The semi-permanent
pond consistently had low annual N2O emissions
(＜0.4 kg N2O-N ha-1 yr-1).
N2O emissions from landscape elements within
the ephemeral ponds showed considerable inter-annual variation,
ranging from 0.09 to 1.0 kg N2O-N ha-1
yr-1 for riparian grass elements, 0.3 to 0.6 kg N2O-N ha-1 yr-1 for riparian
tree, and 1.0 to 2.1 kg N2O-N ha-1
yr-1 for basin centers. Major N2O
emission events in the wetland elements were associated with periods
of rapid drainage (i.e., from greater than 80％ to less than 60％
water-filled pore space) in the upper 15 cm of the soil. Within-year
patterns of N2O and CH4
emissions from soils of the ephemeral ponds were closely related
to a second hydrological control, the area and duration of inundation
in the ponds but negligible differences were observed between
riparian grass and tree elements. The strong interactions between
hydrology, water chemistry, and emissions of N2O
and CH4 demonstrate the need for a landscape-scale
assessment of GHG processes in these landscapes.
Keywords: Nitrous oxide; Methane; Greenhouse gas; Grassland; Topography;
Land use; Denitrification; Nitrification』
1. Introduction
2. Methods and materials
2.1. Field site and sampling design
2.2. Wetland sampling design
2.3. Gas sampling design
2.4. Gas measurement and flux calculations
2.5. Measurement of soil moisture, soil temperature and climate
data
2.6. Water depth and chemistry
3. Results
3.1. Precipitation and air temperature
3.2. Hydrology of wetlands
3.3. Water chemistry of wetlands
3.4. N2O emissions from wetlands
3.5. CH4 emissions from wetlands
3.6. Diurnal effects on emissions
3.7. Relationships between water chemistry and N2O
and CH4 emissions
4. Discussion
4.1. Pond hydrology and chemistry
4.2. Annual emissions of CH4
4.3. Annual N2O emissions
4.4. Effect of wetland vegetation type on emissions
5. Conclusions
Acknowledgements
References