U.S. Senate Committee on Environment & Public Works
Director, Carbon Management and Sequestration Center
The Ohio State University
Agricultural Sequestration of Carbon Dioxide
Mr. Chairman, members of the Senate Committee on Environment and Public Works. I am Rattan Lal, Professor of Soil Science and Director of the Carbon Management and Sequestration Center at The Ohio State University. I am especially thankful to Senator Voinovich for the opportunity to offer testimony on “Soil Carbon Sequestration by Agriculture and Forestry Land Uses for Mitigating Climate Change.”
The basis of our shared commitment is the mutual concern about the quality of the nation’s soil and water resources and the environment. We realize how important and critical the quality of soil resources is for maintaining high economic agricultural production while moderating the quality of air and water. Soils constitute the third largest C pool (2,300 Gt or billion tons), after oceanic (38,000 Gt) and geologic (5,000 Gt) pools. The soil C pool is directly linked with the biotic (600 Gt) and atmospheric (770 Gt) pools. Change in soil C pool by 1 Gt is equivalent to change in atmospheric concentration of CO2 by 0.47 ppm. Therefore, increase in soil C pool by 1 Gt will reduce the rate of atmospheric enrichment of CO2 by 0.47 ppm.
The atmosphere C pool has progressively increased since the industrial revolution. With industrialization and expansion of agriculture, through deforestation and plowing, came soil degradation and emission of gases into the atmosphere. Indeed, the atmospheric concentration of three important greenhouse gases (carbon dioxide, methane and nitrous oxide) has been increasing due to anthropogenic perturbations of the global C and N cycles. For example, the pre-industrial concentration of CO2 at 280 parts per million (0.028% or 600 Gt) increased to almost 365 ppm (0.037% or 770 Gt) in 1998 and is increasing at the rate of 0.43%/yr or 3.2 Gt/y. The historic gaseous increase between 1850 and 1998 has occurred due to two activities: (1) fossil fuel burning and cement production which has contributed 270 (+30) Gt of carbon as CO2, and (2) deforestation and soil cultivation which has emitted 136 (+55) Gt. Of this, the contribution from world soils may have been 78 (+12) Gt of which 26 (+9) Gt may be due to erosion and related soil-degradative processes. In comparison with the global emissions, cropland soils of the United States have lost 3 to 5 Gt of C since conversion from natural to agricultural ecosystems.
The projected climate change caused by increase in atmospheric concentration of CO2 and other trace gases can be mitigated by reducing emissions and sequestering emissions. Strategies for emission reductions include enhancing energy production and use efficiency, and using biofuels. Emission sequestrations involve biotic and abiotic options. Important biotic options include C sequestration in soils, vegetation and wetlands. Together, biotic sequestration in soil and vegetation is called “terrestrial sequestration.”
Terrestrial C sequestration is a natural process with numerous ancillary environmental benefits. In contrast to geologic and oceanic sequestration, which may be expensive and have unknown ecological impacts, terrestrial sequestration is the most cost effective option. Natural C sinks (terrestrial and oceanic) are presently absorbing 4.7 Gt out of the total anthropogenic emissions of 8.0 Gt or about 60% of the total emission. It is prudent, therefore, to enhance the C storage capacity of natural sinks (such as soils and vegetation) through conversion to a judicious land use and adoption of recommended management practices for soil, water, and crop/vegetation. Agriculture has an important and positive role to play in enhancing the capacity of natural terrestrial sinks.
Greenhouse gases are released into the atmosphere when trees are cut down and burnt, soils plowed, and wetlands are drained and cultivated. In addition, excessive soil cultivation and inappropriate or inefficient use of nitrogenous fertilizers can result in emission of greenhouse gases from soil to the atmosphere. Finally, accelerated soil erosion can lead to a drastic reduction in soil organic carbon (SOC) content. Although the fate of the C that is transported by wind and water is not well understood, it is believed that a considerable portion of the eroded C may be mineralized and emitted into the atmosphere. It is estimated that soil erosion annually emits 1 Gt of C globally and 0.15 Gt from soils of the United States. Although agricultural processes are presently not the main source of gaseous emissions, they have clearly been a significant source. Yet, the emissions of C from soils are reversible through conversion to a restorative land use and adoption of recommended agricultural practices. These estimates of the amount of lost C, crude as these may be, provide a reference point about the sink capacity through land use conversion and adoption of recommended practices.
Soil organic matter (SOM), of which 58% is carbon, is one of our most important national resources. It consists of a mixture of plant and animal residues at various stages of decomposition and by-products of microbial activity. The SOM is a minor component of the soil (1-3%), but plays a very important role in biological productivity and ecosystem functions. Enhancing SOM concentration is important to improving soil quality, reducing risks of pollution and contamination of natural waters, and decreasing net gaseous emissions to the atmosphere. The SOM pool can be enhanced through: (1) restoration of degraded soils and ecosystems, and (2) intensification of agriculture on prime soils.
Enhancing the SOM pool is an important aspect of restoration of soils degraded by severe erosion, salinization, compaction, and mineland disturbance. Degraded soils have been stripped of a large fraction of their original SOM pool. Globally, there are 1216 million hectares (Mha) (3 billion acres) of degraded lands of which 305 Mha (753 million acres) are strongly and extremely degraded soils. U.S. cropland prone to moderate and severe erosion is estimated at 19.4 Mha (48 million acres) by wind erosion and 26.2 Mha (65 million acres) by water erosion. An additional 0.3 Mha (0.7 million acres) are affected by salinization, 2.1 Mha (5.2 million acres) of land affected by all mining, and 0.6 Mha (1.5 million acres) of land strip-mined for coal is in need of restoration.
Land conversion and restoration transforms degraded lands into ecologically compatible land use systems. The Conservation Reserve Program (CRP) is designed to convert highly erodible land from active crop production to permanent vegetative cover for a 10-year period. In addition to erosion control, land under CRP can sequester carbon in soil at the rate of 0.5 to 1.0 t/ha/y (450 to 900 lbs C/acre/y). Erosion control also involves establishing conservation buffers and filter strips. These vegetated strips, ranging from 5 to 50 m wide (16.5 to 165 ft. wide) are installed along streams as riparian buffers and on agricultural lands to minimize soil erosion and risks of transport of non-point source pollutants into streams. The rate of C accumulation in soil under conservation buffers is similar to that of the land under CRP. The USDA has a voluntary program to develop 3.2 million km (2 million miles) of conservation buffers.
Wetlands are also an important component of the overall environment. Approximately 15% of the world’s wetlands occur in the United States (40 Mha or 100 million acres) of which 2 Mha (5 million acres) are in need of restoration. Natural wetlands have a potential to accumulate C (net of methane) at the rate of 0.2 to 0.3 t/ha/y (180 to 270 lbs/acre/y).
Surface mining of coal in the U.S. affected 2 Mha (5 million acres) between 1978 and 2002, of which 1 Mha (2.5 million acres) have been reclaimed. The land area affected by surface mining of coal was about 40,283 ha (100,000 acres) during 2002. Restoring minelands, through leveling and using amendments for establishment of pastures and trees, has a potential to sequester 0.5 to 1 t C/ha/y (450 to 900 lbs C/acre/y) for 50 years. Similar potential exists in restoring salt-affected soils.
The overall potential of restoration of degraded soils in the United States is 17 to 39 million metric tons (MMT) per year for the next 50 years or until the sink capacity is filled.
Intensification of agriculture involves cultivating the best soils using the best management practices to produce the optimum sustainable yield. Some recommended agricultural practices, along with the potential of SOC sequestration are listed in Table 1. Conversion from plowing to no till or any other form of a permanent conservation till has a large potential to sequester carbon and improve soil quality. There is a strong need to encourage the farming community to adopt conservation tillage systems.
Adoption of recommended practices on 155 Mha (380 million acres) of U.S. cropland has a potential to sequester 75 to 208 MMTC/y.
Grazing lands, rangeland and pastures together, occupy 212 Mha (524 million acres) of privately owned land and 124 Mha (300 million acres) of publicly owned land.
Total soil C sequestration potential of U.S. grazing land is 30 to 110 MMTC/y.
The potential of U.S. forest soils on 302 Mha (746 million acres) to sequester C is 49 to 186 MMTC/y.
Thus, the total potential of U.S. agricultural and forest soils (Table 2) is 171 to 546 MMTC/y or an average of 360 MMTC/y.
In addition to crop residue, there are other biosolids produced that can be composted and used on agricultural lands. The potential of using manure and compost on agricultural lands need to be assessed.
Of the total national emission of about 1,892 MMTCE/y for 2001, agricultural practices contribute 143 MMTCE/y. Therefore the potential carbon sequestration in U.S. soils represents 19% of total U.S. emissions, and 2.5 times the emissions from agricultural activities. Thus, soil C sequestration alone can reduce the net U.S. emissions from 1,892 MMTCE to 1,532 MMTCE/y.
If the full potential of soil C sequestration is realized, the total sink capacity can be 609 MMTC/y (Table 3). These statistics indicate the need for a serious consideration of determining what fraction of the total potential is realizable, at what cost and by what policy instruments.
There is a widespread perception that agricultural practices cause environmental problems, especially those related to water contamination and the greenhouse effect. Our research has shown that scientific agriculture and conversion of degraded soils to a restorative land use can also be a solution to environmental issues in general and to reducing the net gaseous emissions in particular. Thus, soil carbon sequestration has a potential to reduce the net U.S. emissions by 360 MMTC/y. This potential is realizable through promotion of CRP, WRP, erosion control and restoration of degraded soils, conservation tillage, growing cover crops, improving judicious fertilizer use and precision farming, and composting.
Actions that improve soil and water quality, enhance agronomic productivity and reduce net emissions of greenhouse gases are truly a win-win situation. It is true that soil C sequestration is a short-term solution to the problem of gaseous emissions. In the long term, reducing emissions from the burning of fossil fuels by developing alternative energy sources is the only solution. For the next 50 years, however, soil C sequestration is a very cost-effective option, a “bridge to the future” that buys us time in which to develop those alternative energy options.
1. Birdsey, R. 2001. Potential carbon storage in forest soils of the U.S. Unpublished, USDA-FS.
2. Lal, R., J. Kimble, E. Levine and B.A. Stewart (eds). 1995. Soils and Global Change. Advances in Soil Science, Lewis Publishers, Chelsea, MI, 440 pp.
3. Lal, R., J. Kimble, E. Levine and B.A. Stewart (eds). 1995. Soil Management and Greenhouse Effect. Advances in Soil Science, Lewis Publishers, Chelsea, MI, 385 pp.
4. Lal, R., J.M. Kimble, R.F. Follett and B.A. Stewart (eds). 1998. Soil Processes and the Carbon Cycle. CRC. Boca Raton, FL, 609 pp.
5. Lal, R., J.M. Kimble, R.F. Follett and B.A. Stewart (eds). 1998. Management of Carbon Sequestration in Soils. CRC, Boca Raton, FL, 457 pp.
6. Lal, R., J.M. Kimble, R.F. Follett and C.V. Cole. 1998. The Potential of U.S. Cropland to Sequester C and Mitigate the Greenhouse Effect. Ann Arbor Press, Chelsea, MI, 128 pp.
7. Lal, R., J.M. Kimble and B.A. Stewart (eds). 2000. Global Climate Change and Pedogenic Carbonates. Lewis/CRC Publishers, Boca Raton, FL, 378 pp.
8. Lal, R., J.M. Kimble and B.A. Stewart. 2000. Global Climate Change and Tropical Ecosystems. Lewis/CRC Publishers, Boca Raton, FL, 438 pp.
9. Lal, R., J.M. Kimble and B.A. Stewart 2000. Global Climate Change and Cold Regions Ecosystems. CRC/Lewis Publishers, Boca Raton, FL.
10. Lal, R., J.M. Kimble and R.F. Follett (eds). 2001. Assessment Methods for Soil Carbon. CRC/Lewis Publishers, Boca Raton, FL, 676 pp.
11. Follett, R.F., J.M. Kimble and R. Lal (eds). 2000. The Potential of U.S. Grazing Lands to Sequester Carbon and Mitigate the Greenhouse Effect. CRC/Lewis Publishers, Boca Raton, FL, 442 pp.
12. Lal, R. and J.M. Kimble. 1997. Conservation tillage for carbon sequestration. Nutrient Cycling in Agroecosystems, 49, 243-253.
13. Lal, R., R.F. Follett, J.M. Kimble and C.V. Cole. 1999. Managing U.S. cropland to sequester carbon in soil. J. Soil Water Conserv. 54: 374-381.
14. Lal, R. (ed) 2001. Soil Carbon Sequestration and the Greenhouse Effect. Special Publication, Soil Science Society of America, Madison, WI.
15. Kimble, J., R. Lal and R.F. Follett (eds) 2002. Agricultural Policies and Practices for Carbon Sequestration in Soils. CRC Press, Boca Raton, FL, 512pp.
16. Kimble, J., R. Birdsey, L. Heath and R. Lal (eds) 2002. The Potential of U.S. Forest Soils to Sequester Carbon and Mitigate the Greenhouse Effect. CRC Press, Boca Raton, FL, 429pp.
17. Lal, R. 1999. Soil management and restoration for C sequestration to mitigate the greenhouse effect. Prog. Env. Sci. 1: 307-326.
18. Lal, R. and J.P. Bruce. 1999. The potential of world cropland to sequester carbon and mitigate the greenhouse effect. Env. Sci. & Policy 2: 177-185.
19. Lal, R. 2000. Carbon sequestration in drylands. Annals Arid Zone 38: 1-11.
20. Izaurralde, R.C., N.J. Rosenberg and R. Lal. 2001. Mitigation of climate change by soil carbon sequestration. Adv. Agron. 70: 1-75.
21. Lal, R. 2001. World cropland soils as a source or sink for atmospheric carbon. Adv. Agron. 71: 145-191.
22. Lal, R. 2000. We can control greenhouse gases and feed the world…with proper soil management. J. Soil Water Conserv. 55: 429-432.
23. Lal, R. 2001. Potential of desertification control to sequester carbon and mitigate the greenhouse effect. Climate Change 15: 35-72.
24. USEPA 2001. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2001 (draft). EPA 23RR-00-001.
25. Akala, V.A. and R. Lal. 2001. Soil organic carbon pools and sequestration rates in reclaimed minesoils in Ohio. J. Env. Qual. 30: 2098-2104.
26. Starr, G.C., R. Lal, R. Malone, L. Owens, D. Hothem and J.M. Kimble. 2000. Modeling erosional impacts on soil carbon. Land Degrad. & Dev. 11: 83-91
27. Lal, R. 2002. The potential of soils of the tropics to sequester carbon and mitigate the greenhouse effect. Adv. Agron. 74: 155-192.
28. Lal, R. 2002. Soil carbon dynamics in cropland and rangeland. Env. Pollution 116: 353-362.
29. Lal, R. 2002. Carbon sequestration in dryland ecosystems of West Asia and North Africa. Land Degrad. & Dev. 13: 45-59.
30. Lal, R. 2002. Soil C sequestration in China through agricultural intensification and restoration of degraded and desertified soils. Land Degrad. & Dev. 13: 469-478.
31. Lal, R. 2003. Global potential of soil C sequestration to mitigate the greenhouse effect. Crit. Rev. Plant Sci. 22: 151-184.
Table 1. Recommended practices for soil C sequestration.
Potential rate of soil carbon sequestration (t/ha/yr)
Conservation tillage & mulch farming
Compost and manuring
Elimination of summer fallow
Growing winter cover crops
Integrated nutrient_management/precision farming
Improved varieties and cropping systems Water conservation and water table management
Improved pasture management Afforestation/reforestation
Fertilizer use in forest soils
Restoration of eroded mineland and otherwise degraded soils
Source: Lal et al. (1998); Follett et al. (2000); Birdsey (2000)
Table 2. Total potential of U.S. agricultural soils for C sequestration.
Potential of soil C sequestration (MMT C/yr)
Land conversion and restoration Intensification of cropland
Improved management of grazing land Improved management of forest soils
Source: Lal et al. (1998); Follett et al. (2000); Birdsey (2000); Kimble et al. (2002)
Table 3. Potential sink capacity of terrestrial ecosystems.
Sink capacity (MMTC/yr)
*The soil sink potential can be realized through policy intervention, and needs to be adjusted for hidden C costs of input used.
Table 4. Potential of soil carbon sequestration.