Sequestration and Characterization of Soil Organic Carbon for Shelterbelt Agroforestry Systems in Saskatchewan

The increase in atmospheric concentration of carbon dioxide (CO₂) is contributing to global climate change. Agroforestry systems, such as shelterbelts, can contribute to the mitigation of increasing CO₂ levels, through carbon (C) sequestration in plant biomass and soils. However, little information is available on the storage and dynamics of soil organic carbon (SOC) for shelterbelt systems. The objective of this research was to examine the effect of shelterbelt plantings on the storage, physical stabilization and chemical composition of SOC for major shelterbelt species across Saskatchewan compared to adjacent agricultural fields. Soil and litter samples were collected for six major shelterbelt species including green ash (Fraxinus pennsylvanica), hybrid poplar (Populus spp.), Manitoba maple (Acer negundo), white spruce (Picea glauca), Scots pine (Pinus sylvestris) and caragana (Caragana arborescens) and the adjacent agricultural fields at 59 sampling sites across the agricultural region of Saskatchewan. Measurement of SOC concentration for soil samples was preceded by fumigation with concentrated HCl (12N), which was determined to be the efficient method for SOC determination in carbonate-rich soils. Physical stabilization of SOC was characterized by using the density fraction technique to separate SOC into uncomplexed, plant-derived debris (i.e. light fraction) and mineral-associated organic matter (i.e. heavy fraction). Changes in SOC composition due to shelterbelt plantation were studied using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and synchrotron based carbon K-edge X-ray absorption near edge structure (XANES) spectroscopy. Concentration of SOC for shelterbelts was significantly higher compared to agricultural fields throughout the soil profile (0-50 cm). Sequestration of SOC for shelterbelts varied from 6-38 Mg C ha⁻¹ under different shelterbelt species, along with 3-8 Mg C ha⁻¹ stored in the litter layer. Shelterbelts led to an increase in SOC content for both the labile light fraction and the mineral-associated heavy fraction. The increase in the heavy fraction was higher in coniferous shelterbelt species including white spruce and Scots pine, while the increase in the light fraction C was higher in hybrid poplar, Manitoba maple, green ash and caragana. These trends were attributed to differences in quality and decomposition rate of litter among shelterbelt species. Maximum amount of SOC was sequestered at the 10-30 cm soil depth, and the majority (70%) of it was in the stable mineral-associated form. Light fraction C was predominant in the surface layer (0-10 cm), where it accounted for 92% of the total sequestered C. Younger shelterbelts tended to lose SOC in the early years of shelterbelt establishment, but eventually resulted in net addition of C after about 20 years of age. SOC sequestration potential of shelterbelts was positively related to shelterbelt characteristics including stand age, tree height, diameter and crown width and density of litter layer. These variables together explained 56-67% of the inter-site variability in the amount of SOC sequestered. Analysis of molecular composition of SOC revealed shelterbelts had higher abundance of processed forms of C such as aromatic and conjugated carboxyl groups for hybrid poplar and white spruce shelterbelts and aromatic and aliphatic C moieties for Manitoba maple shelterbelts. In contrast, agricultural field soils were enriched in easily degradable C forms such as polysaccharides. These results revealed a strong effect of initial litter quality and extent of decomposition on SOC composition. Together, these findings indicate that shelterbelt planting leads to sequestration of SOC, resulting in the decrease of atmospheric CO₂ concentration. Additionally, shelterbelts also influence organo-mineral association and molecular composition of SOC, which may affect stabilization and dynamics of sequestered SOC.

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