Perspective: Agriculture’s Role in Cutting Greenhouse Gas Emissions
Agriculture’s Role in Cutting Greenhouse Gas Emissions
Agriculture is responsible for 7% of total emissions of greenhouse gases into the atmosphere in the . Although agriculture is not the major source of greenhouse gas emissions—that title belongs to industrial plants that burn fossil fuel—it is nevertheless an important one and deserves increased attention. The good news is that useful remedies are at hand. Using various best management practices in a number of agricultural operations can reduce greenhouse gas emissions by nearly a third.
The federal government therefore should include in its next farm bill, scheduled to be debated in 2012, incentives to promote the use of these practices. The potential rewards are considerable. Cutting emissions of greenhouse gases by whatever means will help to minimize the risk of climate change. Cutting emissions from agricultural sources also will help to improve air and water quality in a number of important ways. For example, some of the practices will help reduce the runoff of nitrogen and phosphorus compounds that can contribute to the eutrophication of aquatic systems and subsequent harm to aquatic organisms. In an extreme case, large quantities of these compounds carried by rivers throughout the middle of the country have found their way into the Gulf of Mexico and created seasonal “dead zones” where commercial fish, oysters, and other organisms experience increased mortality.
The main source of greenhouse gases from agriculture is the emission of nitrous oxide (N2O) from soils treated with nitrogen-based fertilizers to aid in growing crops and grazing livestock. The next leading source is emission of methane (CH4) by ruminant livestock, especially cattle, through their burps and other digestive outgassings. Collectively, soils and livestock (at 40% and 25%, respectively) are responsible for nearly two-thirds of all agricultural greenhouse gas emissions. Other sources include carbon dioxide (CO2) from the operation of farm equipment (13%), CH4 and N2O from manure management operations (11%), and CO2 from cropped and grazed soils (5%). The rest (6%) results from a variety of minor sources.
N2O emitted from soils is particularly significant, because it has a heat-trapping greenhouse effect that is approximately 310 times greater than that of CO2. N2O emissions result from the biological processes of nitrification and denitrification. Put simply, nitrification occurs when a nitrogen-containing compound called ammonium, which is a main ingredient in many fertilizers, is transformed by microbes in the soil into another compound called nitrate. (To a lesser degree, manure applied to fields is also a source of ammonium.) Denitrification occurs when microorganisms metabolize the nitrate and convert it to N2O (and other byproducts). This process proceeds especially fast when soils are wet and nitrate levels are high. Ultimately, 1 to 5% of the nitrogen added to agricultural fields in fertilizer and manure is lost to the atmosphere via soil N2O emissions.
New tools can help
Recent advances in fertilizer technology could help to reduce N2O emissions. Timed-release fertilizers and fertilizers with nitrification inhibitors provide a gradual supply of nitrogen to the crop, synchronous with plant demand for nitrogen. When the supply of nitrogen in soils coincides with the demand by plants for nitrogen, less nitrogen is available to be converted to nitrogen gas or leached from the system. These stabilized fertilizers are an improvement over traditional fertilizers that provide a large, immediate supply of ammonium upon application.
Research conducted by the U.S. Department of Agriculture (USDA) has shown that stabilized fertilizers can produce similar yields with lower emissions of greenhouse gases. As a case in point, use of these fertilizers in western regions of the nation has resulted in a 60% reduction in N2O emissions from soils in irrigated cropping systems and a 30% reduction for nonirrigated crops. Recent results also suggest that combining no-tillage agriculture (in which soils are not turned over with farm implements) with the use of slow-release fertilizer can result in a reduction of 75% or more in soil N2O emissions from irrigated systems. However, the use of improved fertilizers in central and eastern regions of the country has demonstrated inconsistent results, with some studies showing little impact on emissions and others showing reductions of 30% or more.
The planting of winter cover crops also is beneficial. Cover crops absorb the nitrogen that remains in the soil after the previous crop has been harvested, making it unavailable for conversion into N2O that could be emitted into the air. In addition, the crops reduce the volume of water that flows over and into the soil during the winter months, thereby reducing the leaching of nitrates from the soil into waterways and slowing soil erosion, which is an important problem in some locations.
The potential for successful growth of winter cover crops is greatest in the middle and southern sections of the nation, due to the longer growing seasons and milder winters. Winter wheat and barley are both cover crops that are planted in the fall and harvested in the spring. is planted in the fall and then plowed into the soil or killed with herbicide before the planting of summer crops, such as corn and soybeans.
To address CO2 emissions, a variety of best management practices are available. For example, tractors and many other types of farm implements run on fossil fuels, so reducing the number of times that a tractor passes over a field or using the most efficient implements available will immediately reduce CO2 emissions. No-tillage agriculture and other forms of conservation agriculture, in which fewer tractor passes are needed because the previous year’s crop residue, such as corn stalks or wheat stubble, is left on fields before and after the next crop, are potentially useful in this regard.
Another strategy is to adopt management practices that promote soil’s role as a carbon sink. As a sink, soil currently offsets about 15% of agricultural greenhouse gas emissions. Among ways to boost this capacity, planting winter cover crops can add organic matter that locks in more carbon. Other options include planting marginal croplands with perennial grasses, planting hay or pasture in rotations with annual crops, and changing grazing practices on environmentally sensitive lands. Beneficial grazing practices may include keeping livestock from stream banks, properly resting pastures to restore degraded land, and determining the proper duration and season for grazing pastures.
Reducing tillage intensity also can help. Recent studies suggest that reduced tillage intensity is most effective in the more arid agricultural systems, as well as in warmer regions of the nation. Reduced tillage farming in the has the additional benefits of increased soil water storage and water use efficiency, because of the improved soil structure and reduced evaporation when mechanical soil mixing is reduced. In cold, humid agricultural lands, however, reduced tillage agriculture can lead to decreased plant production because of lower soil temperatures. In addition, the wetter soil conditions in these regions may promote denitrification and increase N2O emissions.
These various practices may offer environmental advantages beyond limiting CO2 and N2O emissions. Agriculture is a major source of many types of air and water pollutants. Soil conditions conducive to N2O emissions also make agricultural soils an important source of nitrogen dioxide and nitric oxide emissions. These gases, collectively called NOx, are health threats in their own right, as well as precursors for smog formation. In addition, inefficient plant nutrient management is considered to be the leading source of the nitrates and phosphates responsible for eutrophication in aquatic systems, as unused fertilizer nutrients are eroded or leached from soils. Excessive soil tillage degrades soil structure, increases erosion, and decreases water use efficiency. Most of the best management practices that address greenhouse gas emissions also address these other environmental concerns.
Barriers to adoption
Despite their potential benefits, the adoption of best management practices by agricultural producers is lagging. In general, the capacity of producers to adopt the practices is influenced by farm size, income, and available capital. Within this context, a number of barriers commonly stand in the way.
For example, stabilized fertilizers cost approximately 30% more than conventional fertilizers. In addition, fall applications of nitrogen fertilizer are common in some northern agricultural regions, leading to the potential for increased soil N2O emissions and leaching of nitrates during the winter and early spring before crops are planted. Fertilizer applied during the early growing season is more likely to be available when crop demand for nutrients is high. Even though spring fertilizer applications are more efficient from an environmental perspective, farmers sometimes apply fertilizer in fall because fertilizer prices and opportunity costs are often lower at this time.
Similarly, growing a winter rye cover crop involves energy and labor costs associated with fall planting, in addition to the extra costs incurred in the spring by plowing the rye, applying herbicide, or doing both. Often, the cover crop is not harvested and there is no benefit of substantially increased summer crop yields, so farmers do not recoup these additional costs.
Reduced tillage practices sometimes require farmers to purchase new equipment, which can be a significant investment. In the long run, reduced tillage can be expected to reduce fuel expenses and personnel time while maintaining or increasing crop yields, but the upfront cost of converting tillage equipment may prohibit farmers from adopting the practice.
Cultural barriers also impede the adoption of best management practices among some agricultural producers. A farmer’s age, education level, and access to information can influence the choices made. Even with adequate capacity and access to information, decisionmaking is influenced by individual perceptions, attitudes, and past experiences. The business of agriculture is inherently risky, and farmers tend to make decisions to minimize that risk.
Supporting the movement
If federal and state governments want to encourage the adoption of best management practices, they will need to take into consideration the risk aversion strategies and cultural barriers that limit their use by the farming community. One approach is to link use of these practices to farm subsidies included in the upcoming 2012 farm bill. The bill is the federal government’s primary agricultural and food policy tool and is updated every five years or so.
For example, USDA can expand promotion of and support for best management practices through its Conservation Reserve Program (CRP), which has proved quite popular and successful in many regions. Funded through the Commodity Credit Corporation and administered by the Farm Service Agency, CRP helps farmers to convert idle or highly erodible cropland or other environmentally sensitive acreage to grasses or other vegetative cover. Participants sign program contracts for 10 to 15 years. Cost sharing is provided to establish the vegetative cover practices, and participants receive an annual rental payment for the term of the contract.
The Environmental Quality Incentives Program, administered through USDA’s Natural Resources Conservation Service, also deserves to be expanded. The program provides technical and financial assistance to farmers and ranchers to address soil, water, and related natural resource concerns on their lands in an environmentally beneficial and cost-effective manner. It covers up to 75% of the costs of a variety of implementation practices and up to 90% for historically underserved producers, such as economically or socially disadvantaged producers or tribes. Among projects covered are nutrient management practices that can improve the use of nitrogen fertilizers and practices that increase levels of crop residues left in the field, thus sequestering carbon in the soil.
Partnerships among government and nongovernmental organizations can be promoted to encourage adoption of best management practices. For example, local and state governments in Maryland, Pennsylvania, and Virginia, along with the federal government, have joined with a number of private organizations in a program intended to decrease runoff of nitrates from agricultural lands into the Chesapeake Bay. The program, which incorporates a combination of regulations and incentives, aims specifically at optimizing the timing of fertilizer applications and reducing the rates of fertilization on winter cover crops. Although its main goal is decreasing nitrate leaching, the program has realized other benefits as well, such as increased carbon sequestration in soils. Such joint programs should be encouraged elsewhere, too.
In modifying the farm bill, one general need will be to increase incentives for practices that reduce greenhouse gas emissions, rather than using punitive measures. That is, agricultural producers should not be charged per unit of emissions, but rather should be paid per unit of reduction. For example, some observers advocate regulations to reduce fertilizer application as a mitigation option, but for most farmers, a significant reduction in fertilizer application rates would likely lead to reduced yields. A better approach would be to give farmers economic incentives to use stabilized nitrogen fertilizers with an optimal timing of application. In this way, farmers could maintain current yields while substantially reducing soil N2O emissions and nitrate leaching.
Programs supported under the farm bill also should be tailored to be region-specific, to encourage best management practices that make sense on the ground. One-size-fits-all approaches are ineffective and create skepticism among agriculturalists. The differences in where reduced tillage and stabilized fertilizers work best and where they do not illustrate the importance of such regionalization. Fortunately, considerable information is already available on where particular best management practices will be most effective, and policymakers and agricultural producers can draw on a number of recently developed support tools in matching practices to regions. For example, the COMET-VR decision-support tool provides farmers and ranchers an easy way to estimate expected greenhouse gas reductions achieved by different mitigation options and is freely available on the Web (http://www.comet2.colostate.edu/). Preliminary analysis using the COMET tool suggests that the universal use of improved fertilizers across the United States would reduce nitrous oxide emissions by about one-third.
The good news, then, is that many of the programmatic building blocks for promoting best management practices in agriculture are already in place. A focused expansion of these efforts in the 2012 farm bill could stimulate widespread adoption of the practices and reduce greenhouse gas emissions, without alienating the agricultural community. Indeed, with proper incentives, many agricultural producers are likely to be willing volunteers. As these efforts expand, the nation would achieve greater climate security and enjoy co-benefits such as improved soil, water, and air quality.
William J. Parton ([email protected]) is professor emeritus at Colorado State University (CSU) and senior research scientist at CSU’s Natural Resource Ecology Laboratory. Stephen J. Del Grosso ([email protected]) is a soil scientist for USDA’s Agricultural Research Service. Ernie Marx ([email protected]) and Amy L. Swan ([email protected]) are research associates at CSU’s Natural Resource Ecology Laboratory.