| | In 1997, Canada became a participant in the Kyoto Accord on Greenhouse Gases (GHG). As a participant, Canada agreed to reduce GHG emissions to 6% below 1990 levels between 2008 and 2012. Based on increases from 1990 to 1997 and assuming business as usual, the reduction now required is estimated to be more than 20%. Alberta’s agricultural industry is responsible for 12% of Alberta’s GHG emissions. This represents 3% of Canada’s total GHG emissions.
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Carbon dioxide (CO2) emission estimates from agricultural soils in Canada are based on model estimates from 15% of the agricultural area. Alternative estimates in Alberta can be made with databases containing soil information, landscape variability and land management data characteristic of a given Ecodistrict. Agriculture is unique because it not only emits carbon (C) but it can store C in soil and vegetation. Carbon emissions from agricultural soils have declined due to increases in conservation practices. However, an accounting system is needed to determine the net flux of C from the agricultural soils as a function of management practices across Alberta.
Nitrous oxide (N2O) is another important GHG listed in the Kyoto protocol. Globally, agriculture accounts for 70% of the N2O released via human activity, whereas CO2 accounts for 25%. Nitrous oxide emissions are spatially variable at all scales. Current N2O estimates by the Intergovernmental Panel of Climate Change (IPCC) is based upon linear extrapolation between N2O emission and fertilizer N application and does not consider different crops, soils, or climates.
In order to assess agriculture’s impact upon GHG emissions and the potential to sequester C across Alberta, a five compartment, first order decay model consisting of two modules: the soil (S) module and residue (R) module, was developed to estimate gross C emission and net C change from agricultural soil. The model incorporated two tillage systems (conventional (CT) and zero (ZT) tillage), and four cropping systems (fallow, forage, cereals, oilseeds). Sequestering C may affect N2O emissions. The global warming potential of a molecule of N2O is 310 times more effective than a molecule of CO2 over a 100-year period. A model consisting of three modules: (1) crop (2) fertilizer and (3) soil was developed to represent gross N emission from agricultural soils. Landscape position and differences in grain yields characteristic of different soil types were incorporated into the model.
Net change of C can be either positive (+) or negative (-) and is the summation of C inputs (crop residue) and C outputs (gross C emission). A positive number indicates a gain of C and a negative number indicates a loss of C. Carbon sequestration occurs when there is a gain of C. Full system accounting of agricultural soil emissions is important so net gains or losses can be identified. In general, net change of C after five years was greatest in cereal cropping systems under ZT and lowest from fallow systems under CT management.
Carbon sequestration potential did not occur in all Ecodistricts and was dependent on soil type, landscape, and the cropping and tillage systems implemented. The predicted C sequestration rates were lowest in the Black soil zone and highest in the Luvisol soil zone. Using the C sequestration rates predicted by this model, if Kyoto accepts C sinks in the emission inventory for agriculture, the agricultural sector in Alberta would have a C sequestration potential between 4700 and 9000 Gg CO2 y-1. However, the ability of the soil to store C is not infinite and these numbers should be used with caution for any long-term predictions. If the government is truly committed to a long-term reduction in GHG emissions, other means of reducing GHG emissions are needed.
Gross C emission for all cropping systems and both tillage systems ranged from -249 kg C ha-1 y-1 to -3760 kg C ha-1 y-1. Gross C emission from all cropping systems (except fallow) increased over time. Conventional till systems had greater annual gross C emissions than did ZT systems. The mean gross emission rate of CO2 for Alberta was -4157 kg CO2 ha-1 y-1 for CT and -3674 kg CO2 ha-1 y-1 for ZT. In general, gross CO2 emissions were highest from Ecodistricts within the Aspen Parkland Ecoregion. Depending on the tillage system used, gross emission of CO2 from Alberta’s agricultural soils is between -38 000 to -43 000 Gg CO2 y-1. If 50% of Alberta’s farmers practice zero till, then Alberta could have an economic gain of $9.5 to $95 million per year (6850 Gg CO2 y-1 would be sequestered).
Model results predicted that gross emissions of N2O from Alberta were -14 000 Gg CO2 equivalent y-1. The majority of the N2O emissions were from the soil and crop residue modules and not from the fertilizer module. This may have implications for an overall GHG budget because in order to sequester C farmers are encouraged to leave crop residue on the surface, however this may have negative results due to the higher emissions of N2O from the decomposition of crop residue. N2O emissions were highest from Ecodistricts located in the Aspen Parkland. If Kyoto accepts C sinks of agricultural soils in the emission inventory, and all other things remain equal, the amount of C sequester by soils in Alberta is still not enough to offset the N2O emission. |
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