Nature uses carbon to store energy. In the air, carbon exists mostly as carbon dioxide (CO2). Through photosynthesis, green plants invest the sun's energy in this CO2, building from it first sugars and then other energy-rich forms. Plant materials are then eaten by other organisms-microbes, cows, and humans, among others-who, in effect, burn the material back to CO2, using the solar energy it contains to live and grow.
Some of the energy-rich carbon materials can be stored for thousands or millions of years before being converted back to CO2. For example, soils contain vast amounts of carbon held in organic matter (humus), and the carbon in fossil fuels such as coal, oil and natural gas, which is solar energy trapped by plants aeons ago.
Farms and other ecosystems can be likened to batteries; building carbon stocks is like charging the battery and losing carbon like discharging it. On Canadian farms, carbon is stored mostly in the organic matter of soils. Changes in amounts stored depend on the rate of carbon coming in as plant litter, compared to the rate of carbon lost through decay.
If the rate of carbon input exceeds the rate of loss, carbon accumulates. This is called a carbon sink. If the rate of carbon added is less than the rate of the loss, carbon is depleted. This is called a carbon source.
Soil Carbon is dynamic. Changes in the amount of carbon stored in soil organic matter depend on the relative rates of carbon input from plant litter and carbon emitted as CO2 via decomposition. If carbon inputs are greater than carbon loss, then the amount stored increases; if carbon input is less than carbon loss, the amount of carbon stored decreases. To increase stored carbon, practices must either:
- increase plant yield (photosynthesis);
- increase the proportion of fixed carbon added to soil; or
- slow the rate of organic matter decomposition.
Figure shows a simplified Carbon system. Plants grow utilizing solar energy and atmospheric carbon dioxide. Plant tissue is removed from the system in the harvest or decomposes to add organic matter to the soil. Biological decomposition completes the cycle by returning CO2 to the atmosphere.
What Farmers Can Do to Enhance the Carbon Sink
By managing soils for growing crops and raising livestock, the world's farmers are unconsciously, managing a soil carbon reservoir that is roughly equivalent to the total carbon that would be released after 100 years of fossil fuel burning at the current world rate. In recent years, the size of this immense carbon pool has changed very little.
Despite the apparent stability of this carbon reservoir, there is nothing permanent about any of the carbon which it contains, nor of the agricultural practices that promote this apparent stability.
Historically, when lands were first cropped, large amounts of carbon were lost because cultivation accelerated decay and removal of harvests meant less carbon was returned to soil. But today, farmers can rebuild some of the lost carbon through improved practices.
By increasing the amount of carbon stored in soils, these practices make soils more productive and resilient for use by future generations, while continuing to remove CO2 from the air.
If land management practices are changed in ways that increase the soil organic carbon, CO2 is effectively removed from the atmosphere and stored or 'sequestered' in the soil. The size of the 'sink' is increased. Farm practices that contribute to the carbon sink are:
- Reduction in tillage
- Restoring degraded land, improving pasture management
- Reducing fallow periods
- Adding animal manures to the soil
- Crop residue management
- Using legumes and/or grasses in crop rotations
- Converting marginal crop land to perennial grass or trees
- Using rotational grazing and high-intensity/short duration grazing
- Planting shrubs and trees as shelterbelts
- Restoring wetlands
In addition to sequestering carbon in the soil, these practices also increase soil productivity, enhance the quality of water running off or draining from agricultural land, and provide a more hospitable environment for wildlife inhabiting agricultural lands.
These practices can also help improve profitability. For example, minimum tillage increases energy efficiency by reducing machinery use. Improved crop varieties and crop fertilization can increase yields and soil carbon.
In Canada, one-quarter of a million farmers manage about 68 million hectares of land. Overall, these farmers have considerably improved the sustainability of their soil management practices on land used for crops and grazing.
In the year 2000, for the first time in Canada's history, agricultural soils sequestered more carbon than was emitted. This achievement was the result of a strong commitment to address soil degradation, in response to desertification risk and devastating erosion during much of the 20th century.
At current adoption rates of carbon sequestering agricultural practices in Canada, soil carbon accumulation can continue for at least until 2040. After this period, the continuance of sound farming practices that conserve the soil will maintain this sink rather than increase it.
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