This report identifies anthropogenic sources of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) released to the environment as emissions. In addition, emission estimates have been developed for the five major chlorofluorocarbons (CFC-11, -12, -113, -114, -115) and some of their substitutes. Emissions have been defined as gaseous releases to the atmosphere and have been expressed on a full molecular basis.Estimates of emissions of CO2, CH4, N2O, and CFCs in Canada for 1990 are summarized in Emissions of CO2 by sector and province are shown in. The emission factors used to develop the estimates are summarized in Figures to illustrate emissions of each gas by major source. In 1990, Canadians contributed about 461 Mt of CO2 to the atmosphere, or about 2% of global emissions. Approximately 94% of these emissions were attributable to the combustion of fossil fuels. Transportation accounted for about 32% of CO2 emissions, electricity production 20%, industrial sources 17%, and the remainder was attributed to miscellaneous heating and industrial processes. Landfills were the major source of CH4 and accounted for about 38% of total emissions of about 3.7 Mt. Other major sources of CH4 were: oil and gas operations 29%, livestock 27%, coal mining 4%, and miscellaneous 1%. Total emissions of N2O were estimated to be about 92 kt. Fuel consumption accounted for 52% of N2O emissions, adipic acid production and nitric acid production 34%, fertilizers 12%, and miscellaneous 2%.
Nitrous oxide can be released from the soil under either anaerobic or aerobic conditions. Liberation of N2O from soils is associated with the oxidation of mineral nitrogen. When either organic or inorganic nitrogen fertilizers are applied, most of the nitrogen is oxidized to nitrates before it is taken up by the plants. This oxidation process is known as nitrification. If soils become anaerobic, i.e., waterlogged or poorly drained, the nitrates can be reduced by facultative anaerobic bacteria to N2O, which can be further reduced to N2 before it is lost to the atmosphere. This process is called denitrification. The amount of N2O formed depends on a number of soil factors, such as oxygen supply, water content, temperature, structure, organic matter content, and nitrate concentration. Most of the sampling periods used to generate the N2O emissions from fertilizers in Canada were less than one year; therefore, the estimate of total annual atmospheric emissions might be low. On the other hand, the estimates of emissions of N2O from the application of anhydrous ammonia are probably high because the application rates in Canada are generally low. More research is necessary to improve the estimates (Canadian Fertilizer Institute, 1992).
There is little doubt that biomass sources of CO2emissions, both anthropogenic (prescribed burning, wood and wood product combustion and decay, landfills, human and animal respiration, and fermentation) and natural (wild fires and decay),add CO2 to the atmosphere. In fact, biomass contributes about 7% of Canada's energy needs (Boyle, 1992).However, it is extremely important to note the fundamental difference between biomass and fossil-fuel sources of emissions. Bio-related sources have a sink term, whereas fossil sources do not (at least not on human timescales). The magnitude of this sink term is directly related to the size of the source term, assuming that sustainable practices are used. Furthermore, to a large extent, bio-related emissions would occur even in the absence of human intervention; the effect is largely on the timing of emissions (Apps, 1990; Taylor, 1990; Apps and Kurz, 1991). It is misleading to include the absolute biomass-related emission estimates in the totals. What is more appropriate is to treat fossil-fuel sources and bio-related sources and sinks separately and to examine the anthropogenically driven changes in these terms. A first attempt to develop a carbon budget for the forest sector has been made in Canada (Apps and Kurz, 1991) and this report includes the results of that model. Information on the net fluxes of carbon from agricultural practices and other thropogenically driven fluxes is scarce. Estimates of carbon losses, mainly as CO2, from agricultural soils are not available
(Benzing-Purdie, 1992), What is known is that converting land to agricultural practices can result in substantial losses of soil carbon, and that most soils currently under cultivation in Canada have lost about 50% of their original carbon content. It is highly probable that most of the carbon loss occurred within the first few years after cultivation and that most of the soils are now in a stable state (Agriculture Canada, 1990). Results of long-term studies at Lethbridge, Alberta, provide evidence for the positive effect of fertilizer
application on organic matter reserves (Janzen, 1986; 1992). Soil analysis in 1990 suggested that nitrogen fertilizer application since 1967 had increased the organic carbon content in the surface soil layer by about 3 t/ha in a continuous wheat cropping system. The CO2 emitted during the manufacture of the nitrogen fertilizer was approximately 0.5 t/ha, which resulted in a net
sequestration of carbon in the soil. However, in fallow-wheat cropping systems (where response to nitrogen is much lower), the contribution of fertilizers to organic matter may be minimal, and the use of fertilizers in these unsound practices may result in a net increase in CO2 emissions (Janzen, 1992). A carbon budget for the agricultural sector has not been developed because of a lack of data. Efforts are now being made to better understand the role played by agriculture (Benzing-Purdie, 1992). In summary, the use of good management practices can sequester carbon and, in certain cases, offset any emissions caused by the production of fertilizers. Not to use nitrogen, when it is required, would in most cases be environmentally harmful because it would reduce the production of protective biomass. Other sources of CO2 released to the atmosphere include human and animal respiration and activities involving fermentation, such as the production of wines. Because the source of carbon is plant matter (foods and other farm crops), the net flux of carbon to the atmosphere from these sources is assumed to balance the carbon being sequestered annually by photosynthesis. Therefore, no estimates of either the sources or the sinks were made in this report.