Opportunities for U.S. Agriculture
Opportunities for U.S. Agriculture
Storage of Carbon in Plants and Agricultural Soils. Photosynthesis removes carbon dioxide (CO2) from the atmosphere and stores the carbon in plant materials and soils. U.S. cropland soils currently sequester 20 million metric tons of carbon per year (MMTC/yr), and have an estimated biophysical potential to sequester 60-150 MMTC/yr more; grazing lands could sequester up to another 50 MMTC/yr.3 To put this in context, 60-200 MMTC is about 12–40 percent of the reduction that would be needed to return expected 2010 U.S. GHG emissions to their 1990 level.
Carbon sequestration can be accomplished through the following measures:
Soils have natural carbon-carrying capacities, and it may be difficult or impossible to increase their carbon content beyond these limits. Most soil carbon gains from conservation tillage are achieved within approximately 20 years, and the carbon stored can be released later—for example, if farmers revert to traditional farming practices. Reversion to traditional practices will result in most of the carbon being released back into the atmosphere within a few years. However, temporary storage of carbon may offer significant benefits by reducing the rate of increase of atmospheric CO2 until more permanent solutions are found.
Production of Fuels and Electricity. Fossil fuel combustion is the major source of U.S. GHG emissions. The agricultural sector can help reduce reliance on fossil fuels in several ways. Agricultural lands can be used as sites for generation of electricity via wind power, reducing the need to generate electricity from fossil fuels. In addition, use of plant materials and animal waste as an energy source can help reduce reliance on fossil fuels. Plant materials can be used either to generate electricity or to produce transportation fuels. Unlike the release of CO2 from fossil fuel combustion, CO2 released during combustion of plant materials and animal wastes is counterbalanced by the CO2 that plants remove from the atmosphere during photosynthesis. However, the overall net GHG benefits of ethanol are uncertain due to GHG emissions from the farming, transportation, and conversion methods currently used in the U.S.
Where large amounts of animal wastes are available in a concentrated location, as in large confined animal feeding operations (CAFOs), CH4 can be captured and used to generate electricity. The most significant constraints to utilization of animal wastes for power generation are: the rates offered by utilities to medium-scale independent power producers; lack of access to capital; lack of appropriate farm-scale technologies; lack of standardized connection requirements; and lack of “net metering” requirements.4
Options for Biofuels and Bioenergy — i.e., use of plant materials and animals wastes to produce energy — include:
Reducing CH4 and N2O Emissions from Agricultural Lands and Livestock Operations. As shown in Figure 2, N2O from agriculture soils constitutes the bulk of agricultural GHG emissions. Agricultural lands contribute to N2O emissions through the breakdown of nitrogen fertilizers, manure decomposition in soils, and releases from legumes. Emissions can be reduced by increasing efficiency of fertilizer use, including more precise fertilizer placement and timing, immediate incorporation of fertilizers into soils, and improved matching of manure application rates to crop utilization rates. Efficient fertilizer management will also improve water quality by reducing nutrient runoff into waters.
Figure 2 Source: U.S. EPA. Inventory of Greenhouse Gas Emissions and Sinks: 1990-1999. |
Whereas most N2O emissions come from cropland, over 95 percent of CH4 emissions are due to livestock,5 both from the digestion process and from manure. Digestive processes of beef cattle account for 40 percent of these emissions. Further reduction of these emissions through more efficient feed rations is somewhat limited given the large feed efficiency gains over the last 20 years. However, digestive process CH4 emissions can be further reduced through improvements in grazing-plant quality. Improved herd management — particularly improved nutrition and increasing the percent of cows producing calves — can reduce CH4 emissions per unit of beef produced. It is estimated that widespread adoption of these measures could reduce CH4 emissions from beef cattle by 20 percent.6
Manure management options to reduce CH4 emissions include:
Tradeoffs and Complementarities. Agricultural practices may affect more than one greenhouse gas as well as other environmental goods and services. Consequently, optimizing the net GHG or environmental effects of an agricultural practice requires a comprehensive evaluation of a complex set of environmental interactions. For example, while irrigation can increase soil carbon, the increased CO2 emissions due to energy used in pumping and the increased N2O emissions due to increased fertilizer use may negate much of the gain.
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