There is tremendous capacity for storage of carbon in soil, both in the United States and globally. In fact, it is estimated that about 100 billion tons of carbon that was originally in the soil has been lost due to human activity, especially through agriculture practices. Most of this carbon could be returned to the soil, improving the soil in the process. With mankind’s annual carbon emissions estimated to be just 6 to 8 billion tons, this signifies huge storage potential.
World soils contain about 3.2 trillion tons of carbon within the top six feet. An estimated 2.5 trillion tons is in the form of soil organic carbon. This is the organic matter in the soil that makes it fertile. The remaining 0.7 trillion tons is soil inorganic carbon.
These are very large numbers. In fact the soil carbon pool is 4.2 times the entire atmospheric pool – and 5.7 times the biotic pool. Thus, even a relatively small increase in soil carbon taken from the air could provide a significant reduction in atmospheric carbon. Moreover, since plants feed on carbon dioxide in the air, the primary way to store carbon in soil is to grow plants. Improved agriculture is the key to soil storage of carbon. Soil carbon storage is probably the ultimate "no regrets" climate policy.
Acknowledgements:
Much of this report’s technical content is abstracted from The Potential of U.S. Cropland to Sequester Carbon and Mitigate the Greenhouse Effect by R. Lal, J.M. Kimble, R.F. Follett, and C.V. Cole [Lewis Publishers, CRC Press, Boca Raton, FL, 1999]. I have also benefited from attending two symposia: "Agricultural Practices and Policies for Carbon Sequestration in Soil" at Ohio State University, July 19–23, 1999, and U.S. Department of Energy Workshop on Carbon Sequestration Research Needs in Gaithersburg, MD, September 14–15, 1999.
Many thanks to the following for useful conversations. I recommend their expertise to anyone seeking additional information: Rattan Lau at Ohio State University (lal.1@osu.edu), John Kimble of the USDA National Soil Survey Center (john.kimble@nssc.nrcs.usda.gov), Earl Kulp of Agroforest Group (ekulp@email.msn.com), and Fred Palmer, President of Greening Earth Society (fredpalmer@greeningearthsociety.org). All opinions expressed are my own.
There is tremendous capacity for storage of carbon in soil, both in the United States and globally. In fact, it is estimated that about 100 billion tons of carbon that was originally in the soil has been lost due to human activity, especially agriculture. Most of this carbon could be returned to the soil, improving the soil in the process. With mankind’s annual carbon emissions estimated to be just 6 to 8 billion tons this signifies huge storage potential.
World soils contain about 3.2 trillion tons of carbon within the top six feet. An estimated 2.5 trillion tons is in the form of soil organic carbon. This is the organic matter in the soil that makes it fertile. The remaining 0.7 trillion tons is soil inorganic carbon.
These are very large numbers. In fact the soil carbon pool is 4.2 times the entire atmospheric pool, and 5.7 times the biotic pool. Thus, even a relatively small increase in soil carbon, if it is taken from the air, could provide a significant reduction in atmospheric carbon. Moreover, because plants feed on carbon dioxide (CO2) in the air, the primary way to store carbon in soil is to grow plants. Improved agriculture is the key to soil storage of carbon.
Soil organic matter is concentrated in the upper 12 inches of the soil. So it is readily depleted by anthropogenic (human-induced) disturbances such as land use changes and cultivation. The magnitude of soil carbon depletion is increased by soil degradation, especially due to erosion. (See figure – Soil Carbon Storage Replaces Lost Carbon)
Land use changes in forests, grasslands and wetlands have transformed large areas of the earth from relatively stable ecosystems to agro-ecosystems under extensive and intensive use. The introduction of agriculture involves land clearing, draining, sod breaking, cultivation, replacing perennial vegetation with annual crops, and fertilizing. These changes have had major impacts on carbon pools and fluxes around the globe. In the initial phases of these transformations, major losses of CO2 to the atmosphere occurred as soil carbon levels adjusted to reduced carbon inputs and increased soil disturbance.
Intense pressure for production also has led to serious soil degradation through erosion and nutrient losses. These trends continue in many areas of the world. In the U.S. and other industrialized nations with available energy and technology, agricultural productivity has steadily increased, land degradation has slowed or reversed, and soil carbon pools have stabilized or increased. However, soil carbon levels are still well below pre-agricultural levels.
It is estimated that a large part (75 to 80%) of the lost carbon can be re-sequestered in the soils of the earth. Of course the soil carbon storage capacity is finite. Moreover, ecological factors and management practices limit the rate of storage. Nevertheless, it is thought possible to achieve this storage over the next 25 to 50 years. Since most of the original productive capacity of the earth would be restored in the process, this is indeed the ultimate "no-regrets" climate policy.
The following table shows estimates of annual global soil storage of carbon that might be sustained over the next 25 to 50 years. [Source: Carbon Sequestration – State of the Science, U.S. Department of Energy, Office of Science and the Office of Fossil Energy, February 1999 available on-line at http://www.fe.doe.gov/coal_power/sequestration/index.html.
(carbon in billion tons per year)
Agricultural lands
0.85 – 0.90
Biofuel croplands
0.50 – 0.80
Grasslands
0.50
Rangelands
1.20
Forests
1.00 – 3.00
Deserts and degraded lands
0.80 – 1.30
Terrestrial sediments
0.70 – 1.70
Boreal peatlands and other wetlands
0.10 – 0.70
Total
5.65 – 10.1
These numbers compare favorably with total human carbon emissions due to fossil fuel combustion, which are presently estimated to be six billion tons per year.
One of the key research questions is how long these rates of carbon storage could be maintained. Also, there clearly is some maximum capacity for soil storage, but that capacity is far from certain. Refining such estimates should be one of the central R&D tasks in any soil-carbon-storage program.
While perhaps surprisingly large, these relatively high levels of potential carbon storage are not unreasonable. For example, a five percent increase in total carbon contained in global terrestrial ecosystems and agro-ecosystems over a 25-year period would store over 100 billion tons of carbon. Storing 100 billion tons over 25 years requires increasing the rate of storage by an average of only 0.2 percent per year – roughly one half of the amount estimated above.
Scientific study of the potential for soil storage of carbon is relatively recent, because widespread concerns about climate change have only arisen in the last decade or so. This is also true with regard to research into practices to increase soil storage of carbon.
A good "best place to start" in investigating the science of soil carbon storage is the recently-published book The Potential of U.S. Cropland to Sequester Carbon and Mitigate the Greenhouse Effect; by R. Lal, J.M. Kimble, R.F. Follett, and C.V. Cole [Lewis Publishers, CRC Press, Boca Raton, FL, 1999]. Most of the data referenced in this report comes from the book. CRC Press also has available many additional titles in this area.
Nevertheless, there is a large literature on the role of carbon in soil fertility and on ways to improve or retain the quality of soil for agriculture. The above-referenced book includes several hundred scholarly references. Among the principle U.S. soil science journals are:
Advanced Agronomy
Crop Science
Crops and Soils
Journal of Soil and Water Conservation
Journal of Sustainable Agriculture
Soil & Tillage Research
Soil Science Society of America Journal
Soil Technology
Additionally, every county in the United States has an agricultural extension agent who is familiar with this body of science.
Soil is an ultimate storehouse for carbon. Carbon dioxide taken out of the air by photosynthesis can be stored in soil either as living organisms (biomass carbon) or in their residue, especially humus. Soil humus is relatively resistant to biological degradation and has a long storage time, ranging from decades to centuries. Moreover, the poorer the soil, the greater is its potential for increased storage. So, the greatest carbon storage potential is in the countries with the poorest soils.
Numerous practices are available to increase carbon storage in soil. Many of these improve soil quality or have direct ecological benefits. Some also decrease carbon dioxide emissions from the soil. Basic strategies include:
Soil erosion management
Land conversion and restoration, including
Conversion of marginal cropland to grassland, forest or wetland
Restoration of degraded lands
Restricting use of organic soils
Restoration of wetlands
Reclamation of mined land and toxic soils
Restoration of salt-affected soils
Intensification of prime agricultural cropland, including
Erosion control through conservation tillage, buffers, conservation reserves, etc.
Supplemental irrigation
Soil fertility management
Improved cropping systems and winter cover
Elimination of summer fallow
Biofuels, including
Energy crops (including blending with fossil fuels)
Biogas from liquid manure
Soil carbon storage in land for biofuel
Improved fertilizer-use efficiency, including
Crop rotation and tillage methods
Nitrification inhibitors
Mixed farming
Recycling of organic material
Biological nitrogen fixation
Management of rice paddies, including
Residue management and tillage methods
Water management
Fertility management
The options directly related to CO2 emission reduction can be grouped into three categories: Land conversion and restoration, biofuels that reduce fossil fuel combustion, and intensification of prime agricultural land.
The basic strategies for land conversion and restoration are:
Conversion of marginal agricultural land to non-agricultural restorative uses such as grassland, forest or wetland;
Restoration of lands that have been drastically disturbed by processes such as erosion, salinization, or fertility depletion; and
Restoration of wetlands and organic soils to their natural vegetation.
These strategies allow some marginal land to be used to produce biomass for conversion into biofuels.
Such approaches imply intensification of production from prime agricultural lands through widespread adoption of best management practices (BMP). A BMP can be both soil- and ecoregion-specific, and therefore must be specified locally. However, a BMP typically involves using conservation tillage, also called "no till" farming. A BMP can also involve crop residue management, increased fertilizer and water-use efficiencies, increased crop intensity, and frequently uses cover crops suited to the region and soil.
Many of the listed strategies are discussed more fully elsewhere in this report.
A large proportion of soil organic carbon content is concentrated near the soil surface. Therefore it is highly vulnerable to the processes associated with soil erosion, particularly oxidation. As a result, accelerated erosion increases greenhouse gas emissions.
Accelerated erosion is a severe problem on a large proportion of U.S. farmland, especially cropland and rangelands. Soil erosion by water is considered severe in Illinois, Indiana, Kansas, Kentucky, Texas and parts of other states where row crops are grown. Wind erosion is ranked severe in many arid and semiarid areas including Arizona, Colorado, New Mexico, Texas and Wyoming.
Like erosion by water, wind-blown soil often winds up in waterways. As an indicator of the magnitude of the erosion problem, an estimated 400 million cubic yards of sediment are dredged each year as part of the maintenance of U.S. waterways and harbors. Soil erosion has become a major water quality problem in many parts of the U.S.
Overall it is estimated that, in the U.S. alone, water erosion removes about 1.5 billion tons of soil each year. Wind erosion is even worse, removing an estimated 2.5 billion tons. Average erosion is estimated to be about seven tons per acre, but almost 25 percent of U.S. cropland erodes at a rate three times that great, or more.
Soil erosion control can be achieved using a combination of techniques, depending on local conditions. These include:
Conservation tillage (basically a technique of not plowing to control weeds and mix the soil);
Management of crop residues and cover crops to maintain soil coverage; and Water management systems, including runoff management.
Land conversion and restoration converts marginal agricultural land into ecologically enhanced land-use systems. Taking fragile lands out of production curtails soil degradation, sets soil restorative processes in motion, and leads to significant carbon storage. Under suitable conditions, this storage is permanent.
The U.S. already has a policy of land conversion through the Conservation Reserve Program. Lands eligible for this program include those with highly erodable soil that cannot be protected with best management practices. Incentives include the government sharing the cost of converting cropland to alternative uses such as woodland or cover crops, as well as annual payments. This program could be expanded and could be exported.
Conservation buffers are another form of conversion and restoration. Conservation buffers consist of restored vegetative strips and riparian wetlands used in conjunction with other recommended practices. In addition to storing carbon, such strips also can help control flooding and reduce water pollution. Depending on the characteristics of the biomass, these buffers can be extremely effective in absorbing water, sediments, and chemicals transported from agricultural lands. The USDA has a voluntary program to develop two million miles of conservation buffers by 2002. This program could be expanded and it could be exported.
Wetland maintenance and restoration is extremely important for soil carbon storage. This principally is due to peat accumulation. Wetlands include the swamps, bogs marshes, mires, fens and other wet ecosystems that cover about six percent of the earth’s land area. Of these, about 15 percent (or 100 million acres) are within the U.S.
Peat accumulation is the result of reduced oxidation of the biomass produced in wetlands. This effect is so pronounced that wetlands have an important impact on the global carbon cycle. In fact, while wetlands comprise only six percent of the land, they hold an estimated 15 percent of the soil organic carbon pool. They are also extremely important for promoting biodiversity and improving water quality.
The U.S. has a Wetlands Reserve Program intended to restore and protect wetlands that have been converted to agricultural use. The WRP places accepted land areas under 30-year or permanent easements that prohibit drainage. The WRP pays compensation based on the type of easement and the appraised land value, up to the full value of the land. This program could be expanded and could be exported.
Cropland and grazing land that is irrigated in arid and semiarid regions is prone to salinization when there is insufficient drainage to carry away salts leached from the soil. Salinization retards plant growth and inhibits soil carbon storage. The total area of salt-affected soils in the U.S. is estimated to be 50 million acres. These soils are located primarily in the following water resource regions: Missouri (44%), Souris-Red-Rainy (16%), California (8%), Texas-Gulf (7%), Pacific Northwest (5%), Great Basin (4%), Arkansas-White-Red (4%), and Colorado (4%).
Most strategies for increasing soil storage of carbon imply intensification of production from prime agricultural lands through widespread adoption of best management practices (BMP). BMPs are both soil and eco-region specific and therefore must be specified locally. However, BMPs typically involve conservation tillage in combination with:
Management of crop residues and other organic material;
Water management systems, including: Runoff management,Drought management through supplemental irrigation, and Drainage of seasonally-wet lands to improve aeration and infiltration capacity;
A conservation tillage (CT) system is defined by the USDA Conservation Tillage Information Center as: Any tillage and planting system that maintains at least 30 percent of the soil surface covered by residue after planting to reduce erosion and/or, where wind erosion is a primary concern, maintains (roughly) at least 1000 pounds per acre of flat, small grain residue (or equivalent) on the surface during the critical wind erosion period.
In fact, CT is a generic term that encompasses all tillage systems that reduce loss of soil and water from cropland, relative to conventional tillage. Conventional tillage includes plow-based methods, such as successive operations of plowing (soil turnover with a moldboard plow), mixing (as with a disk plow), and pulverization (as with a rotovator). CT eliminates one or more of these operations. CT often involves use of herbicides to control weeds.
Crop residue management is an integral part of most CT systems. It may include selecting crops that produce a sufficient amount of residue, as well as sowing cover crops to provide an effective ground cover. Rather than turning under plant materials or crop residues following harvest, they are left on the soil surface to protect the soil. Some important CT variations include no till, ridge till, minimum till, and sod till. About 37 percent of the cropland in the U.S. is now managed with a CT system.
Long-term use of a CT system leads to an increase in soil organic carbon content, as well as enhancement of soil quality and improvement in soil resilience.
Soil fertility status affects the amount of biomass produced. The soil organic carbon content of cropland is related directly to the quantity of crop residue returned to the land and, inversely, to the nitrogen deficit in the soil. Several experiments have shown that addition of fertilizers on a regular basis for many years often leads to an increase in soil organic carbon content.
Likewise, pasture experiments indicate that soil organic content in manured land is significantly higher than that of unmanured land. In addition to the high production of below- and above-ground biomass, erosion control is an important factor in high soil organic carbon content in manured land.
The conservation effectiveness of manuring and treatments using inorganic fertilizer under different management systems is revealed in topsoil thickness and the soil’s clay content.
Producing and transporting fertilizer requires energy, which typically results in the production of carbon dioxide due to the combustion of fossil fuel. In fact, about one third of the energy consumed by farming is in the production and transportion of fertilizer. As a consequence, improving the efficiency of fertilizer usage is a key element in any strategy for soil carbon storage. Efficient usage also reduces runoff and improves water quality.
Vast improvement in the efficiency of fertilizer use is possible in the U.S. and throughout the world
A "biofuel" is any type of solid, liquid or gaseous fuel that can be produced from biomass (including its substrates) and can be used as a substitute for fossil fuel. Biofuels are increasingly recognized as a feasible alternative to fossil fuel. Not only does a biofuel recycle (as opposed to creating) atmospheric carbon dioxide, it also stores carbon in the soil.
Sometimes biofuels can be burned directly. The forest products industry already generates a significant amount of its electricity by burning its wood waste. In rural areas many people heat their homes by burning wood. Examples of biofuels from substrates include ethanol from sugar crops, methanol from woody crops, and "biodiesel" from oil crops.
Several options can increase the agricultural production of biofuels. The land area for biofuel production can be increased by substituting it for other crops. One can grow biofuel crops on tree plantations as agro-forestry systems. Biofuel crops can also be grown as an integral component of soil and water conservation programs, such as in buffer strips.
Idle or marginal land can be converted to biofuel production. Idle cropland constitutes an estimated 15 percent of total cropland, or about 100 million acres.
Likewise, part of the above-ground crop residues produced on existing cropland could be used for biofuel, provided that this is consistent with the maintenance of adequate levels of groundcover for soil and water conservation.
Since soil carbon storage is driven by biological processes, the opportunities for bio-engineering are enormous. Obvious areas of interest include:
Increased plant carbon content
Increased root mass development
Increased crop residue mass or quality
Faster growing rates for crops and forage
More efficient microbial processes
Plants better adapted to degraded or salinized soils.
Moreover, soil carbon storage involves highly complex systems. Ecological systems and human technical systems, both of which vary by eco-region, soil type, and many other factors. Within this complex, the opportunities for subtle but large improvements through bio-engineering are likely to be numerous. Many, no doubt, have yet to be thought of and will depend on new research into the nature of the underlying biological mechanisms and processes.
Changes in land-use practices – and in land-use itself – may involve social changes.
Globally, among the most dramatic among such social changes would be abandonment of slash-and-burn agriculture. Deforestation, biomass burning, and similar methods account for an estimated 14 percent of anthropogenic carbon dioxide emissions. Other agricultural practices account for an additional five percent.
Deforestation is occurring mostly in tropical areas. The total area of tropical forests is about 4.7 billion acres. The annual deforestation rate is estimated to be 0.9 percent, or 42 million acres per year.
Tropical deforestation is occurring mostly in the Amazon Basin, Central America, the Congo Basin, and in Sumatra. The resulting greenhouse gas emissions are due to a combination of biomass burning, biomass decomposition, and mineralization of soil organic carbon content. In the U.S., deforestation is primarily confined to slash (the debris left behind) from timbering, land clearing, and woodcutting. The slash is either burned or left to decompose.
Improved, sustainable agricultural practices in the tropical Third World will involve significant social change [see discussion at A Third World Soil Carbon Storage Program].
In the U.S., the changes needed to increase soil carbon storage generally could not be characterized as "social change." Rather they represent shifts from one practice to another. Among the more dramatic would be a switch to conservation tillage, not cropping marginal lands, and restoration of wetlands. While these could be perceived as major changes by the individuals involved, they are likely to have already been accomplished locally by others.
People who think there is a low (but nonetheless real) risk of human-induced climate change often speak of a "no regrets" climate policy. The term is seldom defined, however. Rather, it is often vaguely alluded to as actions that are useful even if the climate change threat turns out to be illusory. In order to confine this study to genuine no regrets soils storage policies, it is necessary to offer a more precise definition for use. Hence, the following general derivation and definition.
Suppose there are two alternative policy options respectively labeled "A" and "B." Each enjoys positive net benefits apart from the issue of the threat posed by climate change. Suppose B, however, also enjoys positive benefits with regard to the threat of climate change.
Suppose, then, that (1) absent the threat of climate change we would choose option A. But, also suppose (2) that with the climate-change threat considered we choose option B. Taking action as described in (1) implies that apart from the threat of climate change, A is better than B. If we opt for taking the action described in (2) and the climate-change threat turns out to be false, we lose benefits, vis-à-vis Action (1). We would "regret" this loss. Therefore, to avoid the possibility of regret, the option chosen must have net benefits equal to the ones not chosen, absent the consideration of the threat of climate change. For purposes of this report, we will use the following definition of "no-regrets policy": The choice of one policy option, which has potential climate change net benefits from among a group of policies that have equal net benefits, absent the consideration of any purported climate-change threat.
Of course equality of benefits is itself a vague concept, probably necessarily so. The actual effects of any given policy are typically difficult to know in advance (or after the fact, for that matter). This merely means that the pool of equal options is likely to be rather large.
But the basic point remains – a no-regrets climate policy is a choice among policies that are equally beneficial, absent the consideration of climate change. Non-climate benefits are a necessary – but not a sufficient – condition for a no-regrets policy.
Soil carbon storage appears likely to be a legitimate no-regrets climate policy using this definition. Increasing carbon storage means improving agricultural land. Moreover, many of the storage techniques also improve air and water quality, perhaps even the quality of life itself. There are also numerous ecological benefits.
Further research and careful analysis will be necessary to confirm the benefit potential of soil carbon storage. But the policy certainly appears to be promising.
Soil storage potential and the practices needed to increase storage depend on the type of land use. The primary non-urban uses are cropland, grazing land, woodland, and wetland. Cropland area in the U.S is around 450 million acres. Grazing land is about 600 million acres. Forest land is about another 700 million acres. Wetlands represent about 100 million acres, nearly 30 percent of which are coastal.
The U.S. already has several policy initiatives that have the secondary benefit of soil carbon storage. These include:
The Conservation Reserve Program to take highly-erodable and other marginal farm lands out of production;
The Wetlands Reserve Program to restore and protect wetlands that have been converted to agricultural use; and
A voluntary program to establish conservation buffers.
Each of these programs can be expanded on a no-regrets basis. Principally what is lacking is:
A program to promote use of conservation tillage systems, as described earlier in this report (at Intensification of Prime Agricultural Land);
A program to promote restoration and intensification of degraded grazing lands; and
Research and development to support soil carbon storage initiatives.
The greatest potential for soil carbon storage lies in restoring degraded soils in tropical regions. Earl Kulp, a farm system scientist with experience in these regions, has designed a model program to achieve this goal. The program, called "Greenled Growth," is interesting in three respects.
First, Greenled Growth is projected to store from five to seven times as much carbon, annually, as the U.S. would have to eliminate under terms of the Kyoto Protocol (an amendment to the UN Framework Convention on Climate Change or "Rio Treaty" that, if ratified by the U.S. Senate would require the United States to return its greenhouse gas emissions to seven percent of what they were in 1990).
Second, the program is projected to significantly improve the living standards of three billion of the world's poorest people.
And, third, the Greenled Growth business plan shows the program paying for itself and earning a reasonable return-on-investment. In other words, the standard of living increases are such that the affected people can afford to pay back the investment with interest – at a rate of 14 percent, as estimated by Kulp.
Kulp's approach is to analyze the basic eco-regions, with a farm system model for each. Assuming presently-available technology, he optimizes each farm model for maximum soil carbon storage. This has the effect of greatly enhancing agricultural production.
Kulp calls the resulting farm model "Hi-Biomass Farming." It uses the initial high yields on basic food crops to diversify land and labor, first into livestock, and then into high-biomass lumber, forage, and mulch. The mulch is used with manure to build up the soil and its nutrients.
Kulp's Greenled Growth model offers compelling evidence that soil carbon storage is the ultimate no-regrets climate policy.
Soil Carbon Storage Research and Development Needs.
There are four critical aspects to be considered in planning a research and development program to address carbon storage in soil.
Basic Science
What is the potential for a given strategy to actually work? What are the scientific principles that govern carbon storage in soil? How much carbon can we actually store?
There has been a great deal of research on soil science. But because carbon storage, per se, is a new concept, there are significant unknowns in the basic science.
Measurement and Sensing
How can we measure or otherwise detect actual carbon storage rates and amounts? There are well-established laboratory techniques for measuring the carbon content of soil. But these are too laborious and expensive to be used for verification purposes. What is lacking are large-scale methods suitable for program verification. These might be direct measurements, indirect measurements, remote sensing of land use changes, or a some combination.
As with the science, because soil storage, per se, is a new concept, the basic verification tools do not exist. Moreover, the science itself needs improved large-scale measurement techniques.
Implementation Methods
If a strategy appears feasible, how should it actually be pursued? Here many engineering issues arise, likewise issues of cost and benefit estimation methodologies (as with any large-scale land use program).
Assessment
Where are the best opportunities to implement various strategies? What are the possible (or likely) consequences of implementation – not only with regard to soil carbon storage, but to the local, regional or global ecosystems? While we have rough answers to these and related questions, there is nothing approaching a well-formulated decision support system.
Buscar
Imprimir 
Almacenaje de carbono en los suelos. Greening Earth Society, noviembre 1999 (en inglés)