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Curbing Climate Change
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As part of
a continuous soil improvement plan, SARE grant recipient, John
Vollmer, a North Carolina vegetable grower, mows a cover crop
of millet and soybeans to improve soil organic matter.
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Photo by Mary Vollmer |
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Specific agricultural and forestry practices can capture and store,
or “sequester”, carbon, which can be released into the
atmosphere as carbon dioxide (CO2), a primary greenhouse gas. According
to the Intergovernmental Panel on Climate Change (IPCC), roughly
100 billion metric tons of carbon could be sequestered in the world’s
soils during the next 50 years, offsetting 10–20 percent of
carbon emissions from fossil fuels.
Carbon storage is also good for soils. It increases organic matter,
improving soil structure and water infiltration, which can improve
yields and profits for growers. At Pennsylvania’s Rodale Institute,
the long-running Farming Systems Trial showed that after 23 years,
organic systems each stored about 1,000 pounds of carbon per acre
per year due to cover crops and crop rotations. The conventionally
fertilized system did not accumulate significant amounts of carbon.
To effectively store carbon in soils:
 Add
organic materials, such as manure and cover crops.
Reduce
or eliminate tillage.
Return
the maximum possible crop residue to the soil.
Plant
a permanent cover.
Finding the best approach to tilling to mitigate greenhouse gases
is a bit tricky, however. To store carbon, no-till is certainly
best. The more undisturbed the soil the better, as mixing and aerating
feeds otherwise dormant microorganisms that will hungrily metabolize
carbon and then release it as CO2. However, no-till systems can
also increase emissions of nitrous oxide (N2O) — a far more
potent greenhouse gas than CO2. Scientists are not completely sure
of the processes and conditions that increase N2O emissions but
believe it is due to the higher soil moisture levels and increased
fertilizer use that can occur under no-till.
To minimize release of N2O, no-till producers should use nutrient
management plans, nutrient testing and the “pre-sidedress
nitrate test” (PSNT) to help synchronize nitrogen application
and availability with crop demand.
Storing carbon may also offer financial benefits. Today, farmers
can buy and sell carbon credits on the Chicago Climate Exchange
and earn money for practices that reduce emissions of major greenhouse
gases. Carbon credits sell in 100-ton units, too large for individual
farmers. Both the Farmers Union and Farm Bureau, however, have organized
large blocks of farmers — representing about one million acres
from each organization — to sell credits.
Currently, all trading in the U.S. is voluntary. When the Climate
Exchange began, carbon traded at 50 cents per acre for land in no-till.
By 2007, the going rate on the same land was about $2 per acre.
Alfalfa and other grasses in 2007 traded for about $3 per acre;
forest land received $4–$20 per acre, depending on the type,
age and location of the trees; and dairy farmers capturing methane
earned about $20–$30 per cow per year.
Carbon trading is a new industry, its potential dependent on market
forces as well as government legislation and programs that could
impact the value of carbon by setting national, state or local limits
on the amount that can be released into the atmosphere.
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