Soil Organic Carbon Stability, Storage and Response to Heat Wave Events in Agroforestry Systems in Central Alberta, Canada

  • Author / Creator
    An, Zhengfeng
  • Agroforestry systems are a common land-use type in western Canada and play an essential role in mitigating greenhouse gas (GHG) emissions and soil organic carbon (SOC) sequestration. The stability of SOC within agroforestry systems is critical to achieving the long-term goal of C sequestration in agroecosystems. Increasing extreme climate events (e.g., summer heat waves) may reduce the potential of agroforestry systems to mitigate GHG emissions. Information on past C losses, current C storage, and future potential for C sequestration in agroforestry systems on a regional scale is critical to understand better the full biological and economic impact of agroforestry systems. This thesis research investigated SOC structural, thermal and biological stability and the influence of heat wave events and their frequencies on GHG emissions and soil labile C in the forest and cropland components (or land-uses) of common agroforestry systems (hedgerow and/or shelterbelt). This thesis also evaluated the current regional C storage (in hedgerow, shelterbelt and silvopasture systems), historical C loss (in hedgerow and silvopasture) and the future potential for forest land-uses to sequester C (in shelterbelts).
    In the hedgerow system, cropland SOC was of a lower quality, which led to higher structural and thermal stabilities than SOC in the forest land-use (p < 0.05). Both SOC structural and thermal stabilities increased with soil depth regardless of the land-use type (p < 0.01), while SOC quality decreased with soil depth (p < 0.01). The SOC in the hedgerow system had higher biological stability than that in the shelterbelt system (p < 0.1), and the biological stability of SOC was higher in the cropland than in the forest land-use (p < 0.01). The thermal stability of SOC was affected by the interaction between the agroforestry system and land-use type.
    Heat wave events increased CO2 and N2O emissions from both cropland and forest soils within the hedgerow system (p < 0.01), and more frequent heat wave events led to higher cumulative N2O emissions. Forest soils consistently had higher CO2 and N2O emissions than cropland soils, regardless of the heat wave treatment (p < 0.02). Emissions of CO2 and N2O fell quickly after the heat wave treatment ended, and the heat wave treatment did not significantly decrease soil labile C (Permanganate oxidizable C, microbial biomass C and hot-water extractable organic C).
    The forest land-use in the three agroforestry systems (hedgerow, shelterbelt and silvopasture) stored 699.9 Mt C in a 9.5 M ha area across central Alberta, corresponding to a value of $102.7 billion based on a C tax rate of $40 ton-1 CO2 equivalent in Alberta, Canada, in 2021. The forest land-use in the silvopasture system had the highest C stocks (645 Mt C) in central Alberta, storing 14.2 and 67.2 times the C held in the forest land-use in the hedgerow and shelterbelt systems, respectively. Projected increases in C stocks with the expansion of shelterbelt systems in this region, to include all roadside-cropland interfaces, are estimated to be 21.8 Mt C in total, or 2.3 times the C stocks in current shelterbelt forests (9.6 Mt C). The coverage of forest land-use in hedgerow and silvopasture systems in the 9.5 M ha agricultural land area in central Alberta declined at an average rate of 2425 ha yr-1 from 2001 to 2020, corresponding to a decline of 8.5 Mt of C in the past twenty years.
    Overall, incorporating trees to form agroforestry systems altered the quality and stability of SOC, and SOC stability in forests was not as stable as croplands within agroforestry systems despite having more C, likely as a result of the large labile C content in the forest land-use. In addition, maintaining hedgerow systems can increase the abundance of stable C more than planted shelterbelts. Particular attention should be paid to addressing the influence of extreme climate events on agroforestry systems, which may turn agroforestry systems into GHG sources. Maintaining existing hedgerow, shelterbelt and silvopasture systems and establishing new shelterbelt forests are critical for increasing C sequestration in western Canada.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • 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.