True Value of Carbon in Agricultural Soils
USDA-ARS National Soil Tilth Laboratory
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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
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
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
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
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
SOIL WATER AVAILABILITY
is one of the critical components of crop production and across the Corn Belt
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.
CROP RESIDUE REMOVAL
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.
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
CONCLUSIONS AND IMPLICATIONS
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
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.
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Carbon Fluxes in
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