Forest and Grassland Cover Increased Greenhouse Gas Emissions during Spring Thaw in the Agricultural Landscape in Western Canada.

The Prairie provinces in western Canada are a significant source of greenhouse gases (GHG) due to intensive agricultural practices. Agroforestry systems, which integrate trees into the agricultural landscape, can mitigate GHG emissions from soils. Emissions of GHG during spring thaw may account for a large proportion of annual GHG emission but have not been studied in agroforestry systems. In this study, gas fluxes of CO₂, CH₄ and N₂O were measured in treed area and neighboring herbland (areas without trees) soils at 36 agroforestry sites (12 hedgerows, 12 shelterbelts and 12 silvopastures) over two spring thaw seasons (early April to mid-May in 2014 and 2015). Research sites were distributed along a 270 km long north-south soil/climate gradient in central Alberta, Canada. Mean CO₂ emission in silvopasture (3.3 and 3.7 g C m^-2 day^-1 in 2014 and 2015, respectively) was greater (p<0.001) than that in hedgerow (2.3 and 1.8 g C m^-2 day^-1, respectively) and shelterbelt (2.0 and 1.3 g C m^-2 day^-1, respectively). Mean CO₂ emission in treed areas were 2.6 times greater (p<0.001) than in herblands within the hedgerow and shelterbelt systems during 2015. Methane uptake rates were greater (p<0.05) in silvopasture than the others by 201 to 206%. Methane uptake across all 3 systems was also generally greater (p<0.01) in treed areas (0.186 and 0.239 mg C m^-2 day^-1 in 2014 and 2015, respectively) than herblands (0.065 and 0.102 mg C m^-2 day^-1, respectively). Mean N₂O emission was lower (p<0.01) in silvopasture than other systems by 23 to 38% and also overall greater (p<0.001) in herblands (0.216 mg N m^-2 day^-1) than treed areas (0.031 mg N m^-2 day^-1) in 2015. Total global warming potential, the CO₂ equivalent of GHG emission, was greater (p<0.05) in silvopasture than in hedgerow or shelterbelt, and was also greater (p<0.05) in treed areas than herblands in 2015, likely due to greater soil organic carbon (SOC) content. Tree incorporation into agricultural landscapes increased GHG emissions during spring thaw; however, global warming potential per SOC content was smaller in treed areas than herblands (0.28-0.33 and 0.44-0.45 g CO₂-C kg SOC^-1 in treed areas and herblands, respectively), indicating that GHG emission during spring thaw was relatively smaller compare to C sequestration by tree incorporation. Furthermore, the overall impact on global warming potential is likely small considering the short duration of spring thaw (6 weeks) and GHG emissions during the main growing season (2.9 and 3.2 times greater for herblands and treed areas, respectively, during growing season than spring thaw).