Landscape controls on N 2O and CH 4 emissions from freshwater mineral soil wetlands of the Canadian Prairie Pothole region

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 N 2O and CH 4 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 CH 4 m − 2 yr -1) but emissions from landscape elements of the ephemeral ponds were substantially higher, with a maximum of 138.6 g CH 4 m − 2 yr − 1 (or approximately 110 g CH 4 m − 2 yr − 1 when corrected for mid-day sampling bias) from basin centers of these ponds in 2005. The average annual CH 4 emissions averaged across the three elements of the ephemeral ponds at SDNWA were 54.8 g CH 4 m − 2 yr − 1 in 2005 and 32.4 g CH 4 m − 2 yr − 1 in 2006. Methane emissions were significantly inversely correlated to SO 4 2 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 N 2O emissions (< 0.4 kg N 2O–N ha − 1 yr − 1 ). N 2O emissions from landscape elements within the ephemeral ponds showed considerable inter-annual variation, ranging from 0.09 to 1.0 kg N 2O–N ha − 1 yr − 1 for riparian grass elements, 0.3 to 0.6 kg N 2O–N ha − 1 yr − 1 for riparian tree, and 1.0 to 2.1 kg N 2O–N ha − 1 yr − 1 for basin centers. Major N 2O 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 N 2O and CH 4 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 N 2O and CH 4 demonstrate the need for a landscape-scale assessment of GHG processes in these landscapes.