Dynamics of carbon dioxide, methane and nitrous oxide fluxes in planted shelterbelts and adjacent cropped fields
For more than a century, over 600 million shelterbelt trees have been distributed to land owners in the Canadian Prairies mainly to protect farms from soil erosion and extreme wind events. In Saskatchewan, there exists over 60,000 km of planted shelterbelts; however, there is a lack of data quantifying the role of shelterbelts in mitigating greenhouse gas (GHG) emissions in agricultural landscapes. These limited estimates of carbon (C) sequestration and GHG mitigation potential for shelterbelts are needed for regional C budgets and GHG inventories. The objective of this research was to quantify the role of shelterbelts on the mitigation of CO2, CH4 and N2O in cultivated fields. Chamber-based GHG monitoring and modeling approaches were employed. Nitrous oxide emissions were lower in shelterbelts (0.65 kg N2O-N ha-1 yr-1) than in cultivated soils (2.5 kg N2O-N ha-1 yr-1), attributed to the capability of deep rooting trees to remove excess available N and soil water. Both shelterbelt and cultivated soils were small sinks for CH4, though the sink potential was 3.5-times greater for the shelterbelt soils. Soil-derived CO2 emissions were greater in the shelterbelts (4.1 Mg CO2-C ha-1 yr-1) than in the adjacent fields (2.1 Mg CO2-C ha-1 yr-1). Nevertheless, cumulative emissions of non-CO2 GHGs was reduced by 0.55 Mg CO2e ha-1 yr-1 in the shelterbelts and soil C storage (0–30 cm soil depth) was 27% greater, representing an increase of 28 Mg ha-1 in the shelterbelts than in the cropped fields, attributed to long-term inputs from tree litter. Holos model simulations of GHG fluxes in a cereal-pulse rotation indicated that a shelterbelt planting occupying 5% of the farmland resulted in total farm emissions being reduced by 8.2 – 23% during a 60-year period, depending on selected tree species. Between 90 – 95% of GHG mitigation by shelterbelts was through C sequestration in tree biomass and in stable SOC pools, while the reduction in N2O emissions and increased oxidation of soil CH4 totalled 5.1 – 9.8% of the overall GHG mitigation by shelterbelts. Faster growing trees (e.g. hybrid poplar) were more effective in accumulating C in tree biomass and soil and in mitigating soil GHG emissions. This study provides evidence that farm shelterbelts function as net biological sinks of CO2 and can play a role in mitigating soil-derived GHG emissions in agricultural landscapes.
Shelterbelts, carbon dioxide, methane, nitrous oxide, greenhouse gas, cropped field, agroforestry, Holos model, modelling
Doctor of Philosophy (Ph.D.)