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By Ian Cushon
Agriculture is, on a global scale, an important contributor to greenhouse gases. In Canada, it is estimated that agriculture accounts for 13.2 percent of all greenhouse gas production. The primary agricultural greenhouse gases, or GHGs, are carbon dioxide, nitrous oxide and methane. There is increasing evidence that organic agriculture can significantly reduce GHG emissions.
Carbon dioxide is captured by plants in the process of photosynthesis. It is primarily released by the burning of fossil fuels, animal respiration and the decomposition of plants and animals. Nitrous oxide is 310 times more potent than carbon dioxide. It is found in low concentrations and is increasing at about 0.3 percent per year. Global agriculture contributes up to 70 percent of man-made nitrous oxide emissions through nitrogen fertilizer, legumes and animal manures.
Methane is also a potent GHG found in low concentrations. It is 21 times more powerful than carbon dioxide. Ruminant animals are the largest contributor of methane. It is also produced from anaerobic (without oxygen) organic matter decomposition in wet soil. Animal manure also emits methane when decomposition occurs without oxygen. There are two primary sources of GHG emissions in agriculture: fossil energy used for the manufacture and use of production inputs, and emissions from soil that is affected by crop rotation and soil management.
Organic agriculture can dramatically reduce energy inputs. A recent University of Manitoba study indicates that some organic systems can be much more efficient at using fossil energy to produce crops than conventional agriculture. In his 2001 graduate thesis, Jeff Hoeppner compared energy use in conventional rotations with organic rotations and found that in a wheat-pea-wheat-flax, or WPWFx, rotation and in a wheat-alfalfa-alfalfa-flax, or WAAFx rotation, organic production used only 35 percent and 41 percent of the energy of conventional production, respectively. While organic yields can be lower, organic efficiency was nearly twice that of conventional systems when food energy output versus input energy was measured. The WPWFx organic rotation produced 13 units of food energy per unit of input energy versus 7.1 units in the conventional rotation. In the WAAFx rotation, the organic rotation produced 38.9units of food energy versus the conventional rotation's 19.8 units per unit of input.
In another University of Manitoba study, Martin Entz and Robert Gulden
compared energy inputs and carbon emissions from conventional and zero
tillage. The zero till system reduced energy use and carbon dioxide emissions
by 14 percent compared to the conventional system. An earlier study at
Indian Head, Sask., showed about a 30 percent decrease in energy use and
carbon dioxide emissions from the use of zero till in wheat production.
In organic systems, nitrogen and phosphorus are most often supplied
by nitrogen-fixing forages, legumes, animal manure and green manure crops.
Methods compared As for soil management emissions, organic agriculture
may not prove to be as dramatic at reducing GHG emissions. In organic
systems, farmers have traditionally tilled to control weeds and incorporate
green manure crops. Tillage encourages the breakdown of soil organic matter.
Soil management also affects the emissions of nitrous oxide and methane. For example, in conventional and zero-till systems, applying more nitrogen than the plants need increases nitrous oxide emissions. The use of nitrogen-fixing green manure in organic systems may reduce nitrous oxide emissions, because this form of nitrogen is less likely to be in oversupply and it is provided in a slower release form.
Legumes may be the key to reduced GHG emissions. They use solar energy to convert nitrogen and carbon dioxide from the air into plant material that enriches the soil. Compared to conventional nitrogen fertilizer production, legumes are much more efficient and also produce protein and food energy.
The main concern for organic farming comes down to soil organic matter content. If it is stable or rising because of the use of forages, green manure crops, cover crops and reduced tillage, then GHG emissions from organic systems may fare well compared to conventional and zero till systems. However, if soil organic matter is falling because farmers use too much tillage and black fallow without green manure crops, then emissions will increase.
GHG emissions vary depending on soil type, climatic zone and soil management practices. In Michigan, research comparing conventional, no-till, low-input, and organic farming systems found that no-till and organic had the lowest net global warming potential. But alfalfa had a much lower net global warming potential than either zero till or organic.
The issue of GHG emissions in agriculture is a complex scientific question. Research is still needed, especially on the effects of rotations and soil management in conventional, zero till and organic systems in various soil and climatic zones. Zero till is often seen as the only way to reduce GHG emissions, but there is increasing evidence that organic farming has as much or more potential. There is also evidence that organic farmers can use direct seeding, capturing some of zero tillage's benefits.
In the long term, as better farming methods increase carbon storage,
soil will reach its maximum carbon level. At this point the input-output
energy efficiency of farming systems likely will be the key factor in
determining which system produces less greenhouse gases. It would appear
that organic farming systems now has the lead in energy efficiency.
© 2012, Organic Agriculture Centre of Canada (OACC)