True Value of Carbon in Agricultural Soils


J.L. Hatfield

USDA-ARS National Soil Tilth Laboratory

2150 Pammel Drive, Ames, Iowa 50011




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            Carbon (CO2) in the soil plays a critical role in the development of a stable soil aggregate and contributes to the formation of soil particles that are resistant to the destructive forces from wind and water.  The dynamics of carbon in the soil are complex because the amount of carbon is affected by the cycling of CO2 from the atmosphere into carbohydrates and ultimately into plant components of leaves, grain, stalks, and roots.  Over the course of a year the constant exchange of CO2 between the soil and atmosphere is dependent upon the dynamics of the cropping system.  There is a linkage between the CO2 uptake and water vapor release by the crop through the transpiration process. Carbon that is extracted from the atmosphere and incorporated into plant components is released through respiration.  Crop growth is dependent upon the soil water availability during the growing season and in the Midwest there is a direct correlation between available water during the grain-filling period and grain yield.  Crop residue on the surface mediates the water vapor and energy exchanges between the soil and atmosphere and provides an immediate impact on crop water use rates through a reduction in soil water evaporation.  In the longer term, the increase in soil organic matter content leads to an increase in soil water availability and increases the aggregate stability that allows for more effective gas and water exchange between the soil and the atmosphere.  The value of carbon in the soil has a positive effect on plant growth and yield through the effect on water availability, short–term water stress, and more effective gas exchange that benefits the root and biological systems in the soil volume.



            Carbon is a critical component of the soil in terms of the organic matter content.  Organic matter represents one of the most variable components within a given soil type, and over the course of time since the beginning of cultivation, there has been a decrease in the soil organic matter content. At the present time there is a steady state in organic matter content in most agricultural soils.  The potential increase in soil carbon has created the opportunity for carbon markets related to agricultural practices.  However, the value of carbon in soil is broader than the direct accounting of soil carbon.  In this paper we will explore the role of carbon in the soil to expand our understanding of the true value of carbon as a soil component.





Field Scale Yield Responses

            The role of organic matter can be seen in an examination of the variation that exists within production fields.  As part of the ongoing program to evaluate the spatial and temporal variation within corn and soybean production fields, a series of observations have been made throughout the growing season using remote sensing observations linked with yield monitor data collected at harvest.  An example of the variations is shown in Fig. 1 for a corn production field in Dallas County, Iowa. All fields exhibit a variation in reflectance from the different soils present within the fields.  Lighter colored soils reflect more solar radiation and appear brighter to the eye than soils with higher organic matter that appear darker to the eye.  In the visible wavelengths there is a large variation among soils within a field.  As the crop develops, this variation begins to diminish and at the maximum vegetative growth near the beginning of the gain-filling period, there is often no variation present in the field and the use of aerial photographs or satellite images confirms there is little variation in the amount of vegetative biomass produced across the field.   Observations made at the time of maximum vegetative growth showed little variation across the field but during the middle of the grain-fill period there was considerable variation in the red/green reflectance vegetative index.  This index is a measure of green leaf area of the crop canopy (Weigand and Hatfield, 1988).  A similar pattern of increased variation was observed in both the 70 and 170 lb N/A.  This pattern of increased variation during the grain-fill period has been observed in most of the fields and only those areas which maintain the leaf area produce yields that are the highest within the field.






















Figure 1.  Variation in reflectance index across two nitrogen application rates across a field in mid-July (maximum vegetative growth) and early September (mid-grain fill) for a corn field in Dallas County, Iowa, 2003. 


These patterns in reflectance are induced by the patterns in soil water availability across the field.  In other studies on the spatial and temporal variation in carbon and water exchanges across a number of production fields in central Iowa, we have found that the major source of variation is due to soil water availability and small differences on an given day can multiply into large differences in biomass or leaf area over the course of the growing season (Hatfield et al., 2007).  An example of this is shown in Fig. 2 for two soils typical of central Iowa.  There is a difference between these two soils through the amount of soil water available to the growing crop, especially during the grain-filling period.  The observations have shown the value of soil water availability in the production of crops and the role that temporal patterns have in determining the spatial variation in crop yield. 

Observations of yield and crop growth patterns show the value of soil water availability.  Since water is critical to optimum plant growth and there is a relationship between soil organic matter and water holding capacity that will reveal a value of carbon in soil. 


Figure 2.  Seasonal water patterns for corn grown on a Clarion and Webster soil in central Iowa during 2000 using fall and spring applied N management practices.



            Soil water is one of the critical components of crop production and across the Corn Belt and the Great Plains  Timing and amount of rainfall is the difference between profit and loss.  Water stress causes a reduction in crop yield and although the years we remember the most are those that show large impacts on yield. However; within production fields there are areas of fields that show a yield reduction each year due to short-term water stress.  There are two aspects of no-till production that help alleviate some of the short-term water stresses.  First, is the reduction in the soil water evaporation rate by the presence of crop residue on the soil surface; and second, the long-term impact due to increased soil water holding capacity of the soil.

            The presence of crop residue on the soil surface reduces the soil water evaporation rate.  Sauer et al. (1996) showed that corn residue has a major impact on the surface energy balance.  The energy balance of the soil surface determines the rate at which water is evaporated from the soil and the temperature extremes of the upper soil profile. Crop residue acts as a barrier to the soil surface and solar radiation is absorbed at the upper boundary of the residue layer and because the crop residue is typically a fairly porous material with a large amount of air space the rate at which heat or water vapor is transferred through the residue layer is quite slow. Evidence of this is seen in how long it takes the soil surface to dry when there is a layer of residue on the soil. Sauer et al. (1996) showed that corn residue reduced the daily soil water evaporation by 0.016 in/day compared to a bare soil surface. They also found that the properties of the residue changed during the over-wintering period so that the residue acted more as a water-absorbing material in the spring because the waxy cuticle was no longer present.  This changes the dynamics of the residue layer on the soil microclimate. 

It is assumed that the presence of crop residue reduces soil temperature and places the crop at risk because of cool spring temperatures. Fresh crop residue has more of an impact on soil temperatures in the fall than in the spring for several reasons. In the fall the residue has a different reflectance and absorbs less energy while in the spring the residue has decayed and becomes denser because of the degradation process and settling over the winter due to the presence of snow on the residue. Soil temperatures are affected in the upper four inches of the soil profile (Hatfield and Prueger, 1996).   This decreases the maximum temperature extremes in this layer by as much as 10-15º F and the minimum temperatures by 5-10º F. Moderation of the soil temperature extremes and the increased soil water content in this layer promotes a more active soil microbial and megafuana system (earthworms, etc.).  This increase in biological activity helps to incorporate the residue material into the soil layer and maintain the soil organic matter content.  There is a linkage between the decline of microbial activity, soil organic matter, and the decrease in crop productivity.

To promote maximum crop production there needs to be an optimization of water use by the crop. Water use efficiency (WUE) relates yield or total biomass production to the amount of water used by the plant offers a concept to characterize field variation in yield.  Hatfield et al. (2001) described how WUE was affected by soil management practices and one of those practices was the adoption of residue management that decreased soil water evaporation and allowed more of the rainfall to be used directly by the plant through the transpiration process.  The large variations in the seasonal water use rates across a production field that we have observed almost 200 mm (8 inches) of difference between a Webster soil with 4-5% organic matter and a Clarion soil with 1-2% soil organic matter can be somewhat offset through the adoption of residue management to (Fig. 2).  This has been translated into differences among fields and Hatfield et al. (2007) showed that differences of CO2 and H2O vapor exchanges across different soils was related to their soil organic matter content and the variation across different fields and within fields could be directly related to differences in the soil organic matter content.

Residue management offers some potential to modify crop production through increased soil water availability.  These are temporary changes that can be realized immediately as more residue is maintained on the soil surface.  Soil water and soil temperature are linked as we begin to modify crop residue on the soil surface.  These components need to be considered as we develop strategies for managing soil carbon.



Wilhelm et al. (2004) reviewed the literature on corn residue removal and concluded that there was no standard answer to question about residue removal.  Optimum removal rates would depend upon regional corn yields, climatic conditions, and cultural practices.  Their review of the current literature suggested that removal of corn residue could lead to the reduction in the amount of carbon placed into the soil from crop residue and would have an effect on the soil microclimate and nutrient cycling in the upper soil profile. 

            Hudson (1994) reviewed the current literature on crop residue removal and the potential impact on soil organic matter content and soil water availability. He concluded that soil organic matter content is a strong determinant of available soil water and should be given priority in terms of developing and implementing crop residue management strategies that enhance organic matter levels in soil.  An illustration of the effect of changing soil organic matter on available water content is shown in Fig. 3.  He found that the field capacity of soils increased 5-6 times faster than the permanent wilting point per unit of organic matter and this increase affected the available water capacity of soils.  The effects are not constant across all soils as shown in Fig. 3 but illustrate the importance of maintaining or increasing the soil organic matter content in agricultural soils.  The linear relationship between available water content and soil organic matter content demonstrates the value of increasing organic matter in the soil.  Crop residue added to the soil surface will not immediately increase organic matter content but the process of reducing the evaporation rate, maintaining an environment for optimum biological activity, and allowing these processes to operate in a more stable environment will provide the key components needed to affect soil water availability.



Figure 3. Available soil water content in a sand, silt loam, and silty clay loam soils relative to organic matter content. (Adapted from Hudson, 1994).


            The increase in soil water availability will have a positive impact on crop growth because of the increased availability of soil water to the growing plant.  Reduction of tillage operations required to achieve this increase will require some adoption of new crop management strategies that reduce the disturbance of the soil surface.  Adoption of no-till or strip-tillage systems that have a minimum disturbance will benefit the soil water content and reduce the extreme soil microclimatic conditions that the soil microbes and fauna are exposed to during the course of a growing season.  The value of carbon when incorporated into organic matter to increase water holding capacity and decrease soil water evaporation rates will be of benefit as we enter into a period of more variable rainfall events during the growing season.



            The formation of a soil crust on the surface of a soil presents a barrier to water vapor and gas exchange from the soil into the atmosphere.  Gupta et al. (1992) found that curst formation or a surface seal altered the infiltration of water into the soil.  This physical barrier to water movement into the soil would also affect the exchange of gas into and out of the soil volume with the atmosphere.  The movement of gases and water vapor into and out of the soil volume is reduced by the formation of a surface crust.  The surface crust can be an effective barrier to water vapor exchange from a soil.

            Maintenance of soil organic material on the surface helps to prevent crust development and maintains the rate of water vapor and gas exchange between the soil and the atmosphere.   Gupta et al. (1992) showed that infiltration rates on soils decreased rapidly with cumulative rainfall at all sizes of aggregates until the aggregate size was greater than 25 mm.  Prevention of surface crusting will enhance gas and water vapor exchange between the soil and atmosphere and have a positive impact on crop growth.  Maintenance of the soil organic matter content and prevention of surface crusting will have a positive impact on plant growth. The presence of a crop residue layer on the soil surface will further delay the formation of a surface crust because of the prevention of the degradation of the soil surface into small particles that rapidly form a crust under small rainfall events.  As the aggregate stability decreases the potential for a soil to form a surface crust rapidly increases.  Bradford and Huang (1992) outlined six factors that affect soil crusting as soil texture, antecedent moisture, aggregate stability, slope steepness, surface roughness, and climatic variables. The positive or negative expression of these factors on surface crusting are related to the amount of organic matter in the soil.

            One of the agronomic impacts from crusting is an uneven emergence of crops through the soil surface.  The more variable the soil surface the more variable plant emergence and ultimately crop yield.  An example of this is shown in Fig. 4 where the observations of individual plants were collected throughout the growing season from emergence through final yield. The variation in crop yield observed in the field is due to uneven stand emergence induced by variable conditions in initial stand density.  This is often due to problems with soil crust and an uneven soil at planting.

            It is difficult to place a value on carbon as part of the soil crusting problem.  The effect on crop yield is difficult to quantify because of the long period of time from emergence to final grain yield of the crop.  However, the more even the initial stand the less variation in crop yield within the field. 



















Figure 4.  Crop yield and leaf chlorophyll readings collected with a SPAD meter at stage R1 for a Pioneer 33P67 hybrid under fall strip tillage at Ames, Iowa in 2005.



            The value of carbon in the soil encompasses more than a simple sequestration of carbon into the soil.  The value of increased carbon impacts the agronomic performance of a crop and this value may exceed other benefits.  Increased crop yield as a result of increased soil water derived from reduced soil water evaporation from the soil surface or increased soil water holding capacity developed from higher organic matter contents provide benefits in rainfed cropping systems.  The benefit of the increased soil carbon in the upper soil surface that prevents crusting and surface compaction allows for optimum gas and energy exchange that directly benefits crop growth and yield.  These benefits allow for more optimal grain or forage yield from our production systems and can enhance the overall return derived from carbon sequestration.  A common aspect of all of these changes derived from an effort to sequester carbon in the soil as increased production efficiency and more stable crop yields among the variable growing seasons typical of the Great Plains and the Corn Belt.

            As we further refine our cropping systems to minimize the surface disturbance to reduce the loss of carbon into the atmosphere the benefits on the agronomic performance will begin to reveal the true value of carbon.  The reclaiming of our soil value by enhancing carbon and ultimately organic matter will allow for more efficient and profitable cropping systems.












Bradford, J.M. and. C. Huang. 1992. Mechanisms of crust formation: Physical components. In Sumner, M.E, and B.A. Stewart. (eds.) Soil Crusting: Chemical and Physical Processes. Lewis Publishers, Boca Raton, FL. pp. 55-72.


Gupta, S.C., J.F. Moncrief, and R.P. Ewing. 1992. Soil crusting in the Midwestern United States. In Sumner, M.E, and B.A. Stewart. (eds.) Soil Crusting: Chemical and Physical Processes. Lewis Publishers, Boca Raton, FL. pp. 205-231.


Hatfield, J.L. and J.H. Prueger. 1996. Microclimate effects of crop residues on biological processes. Theor. Appl. Climatol. 54:47-59.


Hatfield, J.L., T.J. Sauer, and J.H. Prueger. 2001. Managing soils for greater water use efficiency: A Review. Agron. J. 93:271-280.


Hatfield, J.L., J.H. Prueger, and W.P. Kustas. 2007.  Spatial and Temporal Variation of Energy and Carbon Fluxes in Central Iowa. Agron. J. 99:285-296.


Hudson, B.D. 1994. Soil organic matter and available water capacity. J. Soil and Water Conserv. 49:189-194.


Sauer, T.J., J.L. Hatfield, and J.H. Prueger. 1996. Crop residue age and placement effects on evaporation and soil thermal regime. Soil Sci. Soc. Am. J. 60:1558-1564.


Wiegand, C.L. and Hatfield, J.L.  1988. The spectral-agronomic multisite-multicrop analyses (SAMMA) project.  Int. Archives Photogramm. and Remote Sensing.  27:696-706. 


Wilhelm, W.W., J.M.F. Johnson, J.L. Hatfield, W.B.Voorhees, and D.R. Linden. 2004. Crop and soil productivity response to corn residue removal: A literature review. Agron. J. 96:1-17.