Agriculture's Role in Greenhouse Gas Mitigation
Farmers’ decisions about whether to adopt new management practices and whether to grow energy crops will ultimately determine the level of success of any agricultural sector GHG mitigation strategy. Farmers’ decisions are motivated first and foremost by what they perceive to be most profitable. Thus, mitigation practices must be economically attractive to farmers. If farmers can be persuaded to adopt desired practices, the impacts on GHG emissions could be significant. It is technically feasible that 70 to 220 million metric tons (MMT) of carbon could be added to U.S. agricultural soils annually over two to three decades. This would remove 260 to 810 MMT of carbon dioxide (CO2) from the atmosphere annually, offsetting 4 to 11 percent of current U.S. GHG emissions. Economic potential to store carbon varies substantially by region, and current studies suggest that at prices of $50 per tonne of carbon ($13 per tonne CO2), soil carbon increases would be limited to 70 MMT per year. If an aggressive research and development (R&D) program succeeds in substantially improving per-acre yields of energy crops and reducing costs of conversion technologies, biomass from agricultural sources could supply up to 19 percent of total current U.S. energy consumption. This would yield GHG savings on the order of 180 to 470 MMT of carbon, which is equivalent to reducing CO2 emissions by 670 to 1,710 MMT CO2 per year (by substituting for fossil fuels) or 9 to 24 percent of total U.S. year-2004 GHG emissions.
Overall, studies so far indicate that agriculture is likely to be a competitive supplier of emission reductions if and when farmers are offered suitable payments. Among agricultural mitigation options, soil carbon sequestration will likely be most significant for lower carbon prices (less than $50 per tonne of carbon or $13 per tonne CO2). At higher prices, afforestation and biofuel options become increasingly more competitive.
Agricultural activities have a broad and multi-faceted impact on all three of the main GHGs—carbon dioxide, methane, and nitrous oxide—and policies designed to mitigate GHGs must consider impacts on all three GHGs. Globally, land use (including agriculture) accounts for about one-third of all GHG emissions due to human activities. In the United States the proportional contribution is smaller, about 8 percent of net U.S. GHG emissions. A variety of agricultural sources contribute to these emissions, including fossil fuel consumption in agricultural production; oxidation of soil organic matter and attendant CO2 releases; nitrous oxide emissions from nitrogen fertilizer, manure, and plant residues; and methane emissions from ruminant animals, animal wastes, and flooded rice.
However, agriculture as a sector is unique in that it can function as a sink for both CO2 and methane, helping to reduce their concentrations in the atmosphere. In addition, agricultural production of biofuels can provide a substitute for some of the fossil fuel currently used for energy. Thus, agricultural mitigation of GHGs includes utilization of agriculture’s sink capacity, reduction of agricultural emissions, and bioenergy production. Utilization of agriculture’s sink capacity is primarily accomplished through increasing soil carbon stocks. Soil carbon increases, which are typically in the 0.1 to 1 tonnes per hectare per year range, could be achieved through adoption of practices such as:
• Reducing the frequency and intensity of soil tillage;
• Including more hay crops in annual rotations;
• Production of high-residue-yielding crops and reduced fallow periods;
• Improved pasture and rangeland management; and
• Conservation set-asides and restoration of degraded lands.
Although soil emissions of nitrous oxide constitute the largest GHG emissions from U.S. agriculture in terms of global warming potential, both measuring emissions and achieving large reductions will be challenging. On average, nitrous oxide emissions are roughly proportional to the amount of nitrogen added to soils, through nitrogen fertilizer, manure, and nitrogen-fixing legume crops. Since nitrogen fertilizer use is an important component of modern, high-yield agriculture, more efficient use of nitrogen inputs is the key to reduction of nitrous oxide emissions through:
• Use of soil testing to determine fertilizer requirements;
• Better timing and placement of fertilizer; and
• Use of nitrification inhibitors and controlled-release fertilizer.
Agricultural methane emissions in the United States occur largely from livestock production through enteric fermentation and during manure storage. Methane capture and use to produce energy is an almost ideal way to address emissions from manure, as it reduces methane emissions, reduces GHG emissions from fossil fuels by providing a substitute energy source, and also provides air and water quality benefits. Strategies to address emissions from enteric fermentation include: improving animal health and genetics, feed additives, and more productive grazing systems.
Storing carbon in soils, reducing nitrous oxide and methane emissions, and producing energy from animal wastes all are potential sources of income or cost reductions for farmers. Relatively few studies of the economic feasibility of agricultural soil carbon sequestration have been done to date, and studies of the economics of nitrous oxide and methane reductions are even more limited. Initial conclusions from studies of the profitability of practices that sequester carbon include:
• Geographic differences in the technical potential and cost of carbon sequestration are substantial;
• Cost considerations are likely to limit agricultural mitigation to levels well below those suggested by technical potential; and
• Strategies based on contracts that pay per tonne of carbon stored or that take into account geographic variation in environmental and economic conditions are more economically efficient (less costly) than contracts based on average conditions.
In addition to profitability strictly defined, several other factors are likely to affect farmers’ willingness to participate in mitigation programs:
• Risk, particularly given the likelihood of long-term contracts for carbon sequestration and the high likelihood of changes in economic and technological conditions that can result in unforeseen costs;
• Financial constraints and access to credit when adopting new practices;
• Uncertainty about the long-term effects on crop productivity of adopting carbon sequestering practices;
• Program implementation costs, including contract and transaction costs; and
• Sociological factors, such as age and education level of farmers, farm size, and access to information.
Production of biomass energy could provide a significant opportunity for agriculture to contribute to GHG mitigation. The overall impact of agricultural biomass on GHG mitigation depends on (1) how much energy can be produced from biomass, and (2) the net (life cycle) GHG impact of biomass use for energy. Biomass is particularly well-suited to providing liquid fuel substitutes for petroleum. However, further development of advanced technologies for conversion of biomass into transportation fuels is needed to make biomass more cost-competitive with petroleum.
Current U.S. agricultural bioenegy products for transportation fuels include ethanol made from corn grain, and biodiesel. Although the efficiency of grain-based ethanol production has improved over time, fossil energy use in its production is still high (three units of fossil energy required to produce four units of ethanol energy), limiting its value as a GHG offset. Moreover, there is likely to be an upper limit on the amount of corn-grain ethanol that can be produced economically, currently estimated at 10 billion gallons per year, less than one percent of current energy demand. Biodiesel, made from oil seed crops (e.g., soybean, sunflower) is more energy efficient—about 1 unit of fossil energy to produce 3 units of biodiesel energy, but biodiesel from oil seed crops is currently 50 to 90 percent more expensive than conventional diesel.
Responsible use of agricultural residues such as corn stover or wheat straw for biofuel production could supply 2 to 6 percent of current total U.S. energy demand or 7 to 24 percent of total U.S. petroleum energy demand in the on-road transportation sector. Addressing sustainability issues (soil conservation) is important in determining the amount of residues that could be utilized. Production of energy crops such as switchgrass at current yield rates could displace perhaps an additional 3 percent of current energy supply while utilizing about 10 percent of the total U.S. agricultural area. Improvements in grass genetics could potentially boost this amount to 6 to 12 percent of current energy supply, using up to 15 percent of prime cropland. Potential bioenergy supply from corn, animal manure, CRP lands, agricultural residues, and energy crops grown on prime agricultural land could represent almost one-fifth of total year-2004 U.S. energy demand and more than 80 percent of current U.S. petroleum energy demand in the on-road transportation sector.
Designing and implementing effective agricultural mitigation strategies depends on cost-effective and reliable methods to estimate GHG fluxes and carbon stock changes. Collecting information on management activities such as tillage practices, fertilizer use, and grazing practices at some or all of the NRI locations would improve GHG inventories and assessments. Establishment of a national soil-monitoring network along with additional long-term experiments that include measurements of nitrous oxide and methane fluxes are also needed to improve GHG estimation methods and reduce uncertainty.
A single “magic bullet” solution to the problem of reducing GHG emissions from fossil energy is unlikely, and biomass can play a useful role within a diverse portfolio of GHG reduction strategies. Practices that sequester carbon can maintain and increase soil organic matter, thereby improving soil quality and fertility, increasing water-holding capacity, and reducing erosion. More efficient use of nitrogen and other farm inputs is key to reducing GHG emissions and nutrient runoff, as well as to improving water quality in both surface and ground waters. Using digesters to capture methane from animal wastes can improve air quality and reduce undesirable odors. Consequently, policies should consider not only the GHG benefits but also associated co-benefits to arrive at the most effective solutions in a comprehensive framework. Further R&D is needed to improve the assessment of agriculture’s GHG contributions, to find better ways to manage lands to improve environmental quality, to design efficient policies to implement mitigation options, and to strengthen agriculture’s potential to contribute to producing renewable energy. Although challenges remain, agriculture has much to offer in helping to reduce GHGs in the atmosphere while at the same time improving the environment and the sustainability of agricultural resources.