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Background

Soil Carbon and Organic Matter
 
The USA prairies and Great Plains are blessed with an abundance of deep, fertile soils. Before European settlement, these soils contained large amounts of stable organic matter that had formed over the millenia from decomposition and retention of plant residues and roots. Carbon atoms form the backbone of this organic matter, and this carbon is derived from photosynthesis that converts atmospheric carbon dioxide into plant biomass via photosynthesis. Hence, the native prairie soils had stored (also called sequestered) large amounts of carbon before they were converted for to agriculture for crop production. Today these soils still contain considerable amounts of carbon (Fig. 1), but the amounts are much smaller than before cultivation.

Figure 1. Surface (0 to 20 cm) soil carbon in the continental US. Data sources include NRCS carbon
(STATSGO base), Fenneman and Johnson's (1946) physiographic provinces and
US Census state boundaries (US Census Bureau, 1995).
Nebraska (orange) and the Great Plains (red) are also shown.

Effect of Agriculture on Soil Carbon Stocks
 
When broken out for crop production using traditional farming practices that the settlers brought from Europe, the carbon stocks in these soils began a steady decline (Fig. 2). Conventional tillage with a moldboard plow and other tillage operations to establish a seedbed and for weed control caused increased rates of soil organic matter decomposition such that they exceeded the amount of organic matter returned to soil in crop residues and roots. Over time, soil organic matter levels stabilized at new, much lower levels.

More recently, new farming methods have been developed and adopted that reduce the need for soil tillage operations (called no-till or reduced tillage farming). In addition, crop yields have steadily increased due to the use of improved varieties, fertilizers, and other modern practices. Therefore, there is now evidence that soil carbon levels are increasing in some USA cropping systems although the rate of increase and the potential amount of stored carbon has not been well documented under field conditions that are representative of modern farming practices. Likewise, the plant and soil processes and environmental conditions that govern rates of soil carbon sequestration are not understood well enough to allow accurate prediction of trends in soil carbon storage as influenced by adoption of improved management practices.

Figure 2. Soil organic carbon (SOC) trends in the US corn belt. From Donigian, A.S., Jr. et al. 1994

More recently new farming methods have been developed that reduce the need for soil tillage operations (no-till or reduced tillage systems), and crop yields have steadily increased due to improved crop varieties, fertilizers, and other management practices. As a result, there is evidence that soil carbon levels are increasing in some USA cropping systems. However, the rate of increase and the potential amount of stored carbon has not been well documented under field conditions representative of modern farming practices. Likewise, the plant and soil processes responsible for soil carbon sequestration in these systems are not well enough understood to allow accurate prediction of the rate of carbon storage that can be expected from adoption of improved management practices.

How Much Carbon Can Be Stored and Will It Reduce Global Warming?
 
Carbon dioxide is released during the process of soil organic matter decomposition, and it is removed from the atmosphere when plants convert carbon dioxide to biomass via photosynthesis (Fig. 3). Because atmospheric carbon dioxide levels are increasing (Fig. 4), mostly due to combustion of fossil fuels, and because increased atmospheric levels of carbon dioxide are thought to contribute to global warming and climate change, there is interest in investigating whether agricultural systems could reduce carbon dioxide in the atmosphere through carbon sequestration in soil. Carbon sequestration occurs when the amount of carbon retained in soil organic matter from inputs of plant and root residues exceed the amount of carbon lost from organic matter decomposition. However, carbon sequestration in annual cropping systems of the USA Corn Belt has not generally been accepted as a viable means of storing carbon due to concerns about the longevity of such storage compared with carbon storage in forest systems. Therefore, the goals of the UNL Carbon Sequestration Program is to quantify the carbon and energy balances in irrigated and dryland maize-based cropping systems, which are the predominant land use in the north-central USA, and to better understand the effects of crop management and climate. Any gain in soil organic matter levels, however, can be offset by the release of carbon dioxide in from fossil fuels used to produce the crops (for example, fossil fuels are combusted by farm equipment used for tillage, planting, and harvest; to produce nitrogen fertilizer; to pump irrigation water; and for grain drying and transportation). Therefore, both the soil carbon balance and the energy balance must be considered to determine if the cropping system contributes to a net reduction in atmospheric carbon dioxide levels, and thus a reduction in greenhouse warming potential.

Figure 3. Carbon partitioning in an agricultural system.

Figure 4. Concentration of CO2 in the atmosphere over time. From: Sarmiento and Gruber, 2002.

Cropping systems also release or sequester two other radiatively active gases--nitrous oxide (N2O) and methane (CH4), and these gases are even more potent in terms of global warming potential than carbon dioxide (Table 1). Therefore, research to quantify and understand the impact of a cropping system on global warming potential must consider the net release or absorption of these greenhouse gases in addition to the carbon and energy balances. The research we conduct under the CASMGS project, in tandem with the CSP program at UNL, considers all of these sources of greenhouse gases in order to determine whether the cropping systems under investigation are net contributors to reducing global warming potential from greenhouse gases.

Global Warming Potential

  Gas   Radiative "Forcing"   Mean Lifetime Total   Global Warming Potential  
      (relative to CO2)   (years)   (relative to CO2)  
  CO2   1   125   1  
  CH4   58   12   21  
  N2O   206   120   310  

Table 1. Greenhouse gases behave uniquely with respect to radiative forcing, lifetime and global warming potential.

CSP Research Groups
 
The UNL Carbon Sequestration Program includes a large number of faculty, postdocs, and graduate students from a wide range of disciplines who work on a different components of carbon ecology in the major cropping systems of the north-central USA. The following links provide information and results from these different components. Some of them are components contribute directly to our UNL efforts within the CASMGS project, and those components are shown in green.

Descriptions of each of these topics, the research currently being conducted, some preliminary results, and the names and contact information for the scientists involved are provided at each of the above links. We invite you to visit them.

References
 
Carbon data (personal communication; Sharon Waltman)

Donigian, A.S., Jr. et al. 1994. Assessment of Alternative Management Practices and Policies Affecting Soil Carbon in Agroecosystems of the Central United States. EPA/600/R-94- 067. U.S. EPA, Environmental Research Laboratory, Athens, GA 193 pp.

Fenneman and Johnson, 1946, Physical Divisions of the United States USGS map, scale 1:7,000,000.

Sarmiento, J. L., and N. Gruber, Sinks for anthropogenic carbon, Phys. Today, 55(8), 30 36, 2002.

US Census Bureau, 1995. Tiger Counties for Nebraska (1:100,000). Washington DC




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