Farmlands are currently viewed as opportunistic areas of carbon storage if they are managed properly. It is known that the tillage method used is a critical factor in determining the amount of carbon left in the soil. The more the soil is disturbed, the more carbon is released into the atmosphere. Therefore, the goal is to reduce soil disturbance by using no-till or low-till methods instead of traditional till methods. Low-till methods are called "carbon sequestration" methods because they retain carbon in the soil instead of releasing it into the atmosphere. Conserving carbon in the soil not only benefits the environment; it also benefits the farmer. It makes the soil more productive, reduces erosion, and benefits wildlife habitat by making the habitat more fertile.
According to the Pew Center on Global Climate Change, agricultural mitigation and sequestration offers a unique means of managing and reducing greenhouse gas emissions. The studies have identified several opportunities where soil carbon can be increased. One major method involves managing land more efficiently by choosing tillage methods that increase plant residues within the soil. Studies have also determined that if half of the original historic croplands of the United States could be regained, that tens to hundreds of millions of tons of carbon could be stored in the soils annually over the next several decades. In addition to increasing plant residues, slowing the rate of soil organic matter decay would also increase soil carbon reservoirs. Restoration of
Conservation tillage leaves at least 30 percent of the soil covered after planting with last year's crop residue, as shown in this farm in central Iowa. (Lynn Betts, NRCS)
degraded lands can also increase soil carbon. The Pew Center states that these processes of soil sequestration and storage will increase carbon content for about 20 to 30 years, after which time the carbon content stabilizes and cannot store additional carbon.
During farming practices, carbon inputs to soil can be increased by three major methods: (1) using crop rotations with high-residue yields, (2) reducing or eliminating the fallow period between successive crops in annual crop rotations, and (3) by using fertilizer efficiently.
High-residue crops and grasses are those that leave large amounts of plant matter in the soil after the crop is harvested. Good examples of this are corn, sorghum, hay, and pasture grasses, which have significant root systems in the soil.
The elimination of the practice of fallow periods between crops is another method that has proven successful in promoting higher carbon levels in areas such as the western United States. For instance, the practice of planting winter wheat along with summer season crops such as corn, millet, sorghum, and beans utilizing a no-till practice not only increases soil carbon but also improves soil moisture—an important and scarce resource for the American Southwest.
It is also important to apply manure, nitrogen fertilizers, and irrigation efficiently. If too much fertilizer is used, it offsets the benefits of the carbon stored in the soil because it increases the levels of N2O. Each area needs to be assessed for its specific needs so that a proper balance is reached, and an area gets only the fertilizer it will actually utilize—no more (which will simply be left behind in the soil or washed into another source by the irrigation).
Reducing the impact from tillage is also important. Soil tillage in general accelerates organic matter decomposition by warming up the soil and breaking it apart where it can decay more rapidly, releasing greenhouse gases such as CO2 into the atmosphere. Traditional tillage methods are the most invasive (such as moldboard plowing) because they completely invert (turn over) the soil. Reduced tillage methods are better because they plow to shallower depths, cause much less soil mixing, and keep more of the crop residue within the soil. The best alternative is the no-till practice, in which crops are sown by cutting a narrow slot in the soil for the seed and herbicides. This method results in the least amount of soil disturbance.
Pastures and rangelands can also retain large amounts of soil carbon. Not only do their extensive root systems store high proportions of carbon underground, but they protect the ground surface year round with plant cover, and they also help strengthen the soil, keeping the carbon fixed in place. One way to increase carbon storage on these lands is to increase irrigation, plant legumes, or plant improved grass species in humid areas. In the semi-arid western rangelands, there are fewer options available—usually limited to changing species composition. Restoration of serious degraded lands is also an option to increase carbon storage. Land-use change is another method. For instance, land that is currently used for crop production can be converted into hay or pasture lands.
Carbon sequestration rates vary geographically depending on climate, soil types, topography, past management history, the current condition of the land, and the current land management practices. Although they may vary, enough research has been done to date to enable the agricultural industry to make positive steps toward combating global warming.
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