Soil Organic Carbon Content and Stability, and Greenhouse Gas Emissions in Three Agroforestry Systems in Central Alberta, Canada

• Author / Creator

  Baah-Acheamfour, Mark

• Western Canada’s prairie region is extensively cultivated for agricultural production, which is a large source of greenhouse gas (GHG) emissions. Agroforestry systems are common land uses across Canada, which integrate trees into the agricultural landscape and could play a substantial role in sequestering carbon (C) and mitigating increases in atmospheric GHG concentrations. This thesis research quantified soil C storage and stability, and CO2, CH4, and N2O emissions in forest and herbland (areas without trees) components of three agroforestry systems (hedgerow, shelterbelt, and silvopasture) over two growing seasons (May through September in 2013 and 2014). The study evaluated 36 sites (12 hedgerows, 12 shelterbelts, and 12 silvopastures) in central Alberta, Canada, distributed along a soil/climate gradient of increasing moisture availability. Within each agroforestry system, the areas under forest consistently had greater total soil organic C (SOC) and SOC in most soil fractions separated by particle-size (up to 10 cm) and density (up to 30 cm) fractionation than in herbland areas. The C stored in this forest cover is more stable, so less of it is expected to be lost as CO2 when the climate warms in the future. Soil CO2 emission and temperature (r2= 0.53, p < 0.01) and CH4 uptake and soil water content (r2 = 0.38, p < 0.01) were significantly related in the studied land uses. Soil temperature and water content are dominant controls on N2O emissions, and together explained 71% of the variation in N2O emissions. Over the two seasons, forest soils had 3.4% greater CO2 emission, 36% higher CH4 uptake, and 66% lower N2O emission than adjacent herbland soils. As a result, forested areas had a smaller global warming potential (129) than their herbland counterpart (157 kg CO2 ha-1) based on all three GHGs. Autotrophic respiration contributed more to total respiration in the forest than in herbland (p < 0.01), that, in turn, may be responsible for the high CO2 emissions in the forest. The SOC stock in the bulk soil (up to 30 cm) was greater in the silvopasture (201) than in either the hedgerow (178) or shelterbelt system (162 Mg C ha-1). Across particle-size fractions, SOC in the more stable fine fraction was in the order of: hedgerow >shelterbelt > silvopasture system. Similarly, the largest pool of SOC in the more stable heavy density fraction of both the 0-10 and 10-30 cm depth classes was in the shelterbelt (33 and 35 Mg ha-1, respectively), while the least SOC was in the silvopasture system (26 and 20 Mg ha-1, respectively). While ranked emissions of CO2 were silvopasture > hedgerow > shelterbelt, soils in the silvopasture system had 15% greater CH4 uptake and 44% lower N2O emission rates compared with the other two agroforestry systems. Silvopasture system can provide greater potential to induce soil C sequestration because it leads to a larger reduction in heterotrophic respiration (p = 0.03) than the hedgerow and shelterbelt systems. Overall, opportunities appear to exist for enhancing soil C storage and stability, while reducing GHG emissions by retaining and establishing perennial vegetation, both forest and grassland, within agricultural landscapes.

• Subjects / Keywords

  • Shelterbelt
  • Global warming potential
  • Hedgerow
  • Climate change
  • Agroforestry system
  • Silvopasture

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3VD6PG0K

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.