Fig. 1a.
Effects of agricultural practices on the soil carbon balance. The thickness of the arrows represents the extent of each process.
Degradative practices associated with past agricultural
practices promoted soil C losses.
Cropland Management for Carbon Sequestration
Management practices that promote C sequestration improve soil quality and productivity.
There are about 400 million acres of cropland in the US and about 9 million here in Colorado. On most of this area, annual crops are grown and harvested each year -- thus, there is little C (as biomass) stored above ground. However, soils in general, including cropland soils, are huge repositories of organic C. In most ecosystems, the amount of C in the top 3 feet of soil is greater than that stored in all the vegetation, even in forests. Thus, C sequestration in croplands means increasing the storage of C in soil.
Most cropland soils contain much less C than they did in their original condition under prairie or forest vegetation -- soils brought under the plow usually lost 30 to 50% or more of their organic matter within a few decades. Frequent and intensive tillage combined with low productivity and minimal residue yields were typical in the past when most of our croplands were established. Such conditions tended to reduce C inputs to soil and accelerate C losses through organic matter decomposition and erosion, reducing soil C stocks (see Fig. 1a).
Fig. 1a. |
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Degradative practices associated with past agricultural
practices promoted soil C losses.
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Worldwide, it is estimated that conversion of land to agricultural uses resulted in the loss of 50-100 billion tons of C from soils, over the past 200 years. Even today, conversion of forests to agriculture in the tropics continues to be an important source of CO2 emissions, from biomass and soils, to the atmosphere. However, with improved management practices, the organic matter and C stocks of these soils can be restored, effectively removing CO2 from the atmosphere (Fig. 1b).
Fig. 1b. |
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Improved agricultural and conservation practices
can rebuild soil organic matter stocks.
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The amount and rate of C sequestration varies according to natural factors such as climate (temperature and rainfall) and soil physical characteristics (soil texture, clay mineralogy, soil depth), as well as agricultural management practices. In general, the amount of C stored in soils is determined by the balance between C inputs from plant (and animal) residues and C emissions from decomposition. Thus, increasing soil C stocks requires increasing C inputs and/or decreasing C decomposition. Hence, C sequestration will be favored under management systems that (1) minimize soil disturbance and erosion, (2) maximize the amount of crop-residue return, and (3) maximize water- and nutrient-use efficiency of crop production (Paustian et al., 2000). Although it may be impossible to optimize all these system attributes simultaneously, management practices that effectively sequester C share one or more of these traits.
Decreasing tillage intensity, especially by using no-till, has been found to promote C sequestration. In long-term field experiments comparing no-till to conventionally tilled annual cropping systems, adoption of no-till typically resulted in increases in soil C of 100 to 1000 lbs C/acre/year over periods of 20-30 years (Paustian et al., 1997). Sequestration rates tend to be higher in moist climates with high levels of crop residue inputs and lower in semi-arid regions supporting lower levels of primary production. In semi-arid regions, no-till also provides increased water storage, enabling more continuous crop rotations and a reduction in summer fallow frequency (Peterson et al., 1998). The effects of no-till systems under these conditions are synergistic, in that, no-till enables higher crop inputs through more intensified rotations, reduced decomposition rates with less summer-fallowing, greater water use efficiency, and less soil disturbance. No-till by itself, without decreasing or eliminating summer fallow, will have much less of a positive impact on soil C sequestration.
Increasing the amount of residue returned to soil can be accomplished through a variety of practices, including growing high-residue yielding crops, using hay in rotation with annual crops, application of manure and biosolids, and improved management of fertilizer, water, and pests. Most cropland soils show a clear response to increasing amounts of C return, such that, SOC levels, over time, are often directly proportional to the amount of C added to soil under different management treatments (Paustian et al., 1998). Planting annual cropland to perennial grasses, such as in the Conservation Reserve Program (CRP) or in field buffer strips, grass waterways, shelterbelts, or other conservation plantings, tends to promote high rates of C sequestration and can also greatly reduce emissions of another soil-borne greenhouse gas, nitrous oxide (N2O). With productive grass or grass-legume cover, the amount of C returned to the soil is often high, and the lack of tillage disturbance promotes stabilization of SOM. Rates of soil C increase as high as 1000-1500 lbs C/acre/year have been reported for CRP land in the Corn Belt region; however, lower rates of soil C increase would be expected in semi-arid regions such as eastern Colorado (see Follett article - The Conservation Reserve Program and Carbon Sequestration).
Regardless of how management or land use changes, C sequestration does not go on indefinitely. Eventually, under a new management regime, soil C levels tend toward an equilibrium, where the amount of C in soil remains roughly constant. In addition, energy costs associated with manufacture and distribution of agricultural inputs such as fertilizer, energy for machinery and irrigation pumping, as well as emissions of other greenhouse gases (nitrous oxide and methane) must be considered in choosing the best management practices to sequester C. In general, practices that promote efficient use of resources, including water, nutrients and energy, will have the greatest benefits in terms of sequestering C and reducing other greenhouse gases.
Whereas, C sequestration through improved agricultural practices can help to reduce the buildup of greenhouse gases, the benefits to the health and productivity of the soil are of equal or greater importance. Soil organic matter is widely recognized as one of the key attributes affecting soil quality. Soil organic matter performs many important functions controlling water and nutrient availability to crops. Increasing the amount of organic matter in agricultural soils is almost always beneficial and carries along with it increased water infiltration, reduced runoff (and erosion), increased soil buffering capacity, and increased storage of essential plant nutrients. Thus, management practices that promote C sequestration can provide a host of resource and environmental benefits that improve the health and sustainability of the soil and ultimately the farmer's bottom line.
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