Reproduced, with permission, from:
Council for Agricultural Science and Technology (CAST). 1992. Preparing U.S. agriculture for global climate change. Task Force Report 119. Ames, IA: Council for Agricultural Science and Technology.
Preparing U.S. Agriculture for Global Climate Change
Task Force Report
No. 119 June 1992
Council for Agricultural Science and Technology
6 How Can Farming and Forestry Be Changed at Acceptable Cost to Emit Less or Stash Away More Greenhouse Gases?
Although the main concern of U.S. agriculture, faced with the threat of changing climate, is with how to adapt to change, agriculture is also a source and potential sink for greenhouse gases and may be expected to play a role in trying to limit the magnitude or rate of climate change. If limiting net emissions of greenhouse gases were to be added to the list of objectives of farmers, how much of a role might they play and at what cost to their other goals? We can also speculate how the multiple objectives of forest management might be altered if uptake and storage of CO2 were recognized as an important objective. We recognize that both farmers and foresters have, in recent years, increasingly adopted objectives related to minimizing their impact on the environment.
Carbon Dioxide (Emitting Less and Sequestering More)
The level of atmospheric CO2 can be controlled by lowering emissions; for example, by encouraging additional measures to reduce energy consumption in agriculture or by reducing rates of soil oxidation. Conversely, CO2 can be taken from the atmosphere during photosynthesis and thus additional C storage in soils or standing plants might be encouraged.
More energy efficient farm equipment, less mechanical crop drying, less use of agricultural chemicals, less irrigation, and minimum tillage would all save CO2 emissions but at some cost to productivity after optimal levels are passed. On the other hand, high productivity can yield returns in C storage because it implies minimum land area in cultivation and potentially more land available for protection or establishment of forest. Commitment of some agricultural land to energy crops creates the potential to circumvent burning fossil fuels but might raise land rents and food costs to consumers. Many of the strategies named in this section would help minimize net C emissions to the atmosphere and are also consistent with good farming practice. Other strategies might conflict with the maximum productivity or profitability of farms and for still others the optimal level of implementation simply is not yet clear.
Protection of old growth forests would certainly aid long-term C storage and environmental aesthetics but is widely debated for a variety of economic issues. As mentioned in Section 4.3, maintaining maximum C storage in forests means protecting old growth (especially in areas where the ratio between standing mass and productivity is low), prompt replanting after harvest, and efficient utilization of forest products. We do not yet know at what level of forest productivity and forest management the goal of C sequestering is best served by stashing C in trees as opposed to using efficient harvest to cycle C through energy systems and industrial products.
Strategies for use of trees to sequester C in the United States range from protecting existing forests, to planting trees where trees do not currently grow, to recycling C by burning energy crops instead of fossil fuels. If renewable crops could supply 8% of current U.S. energy demand (about 6 EJ, as suggested in Section 4.3) to displace an equivalent amount of fossil fuel burned for electric power generation, we would offset over 0.5 Gt of CO2 emissions, over 10% of the U.S. total (Table 4.1.1). This is nearly 4 times the total CO2 emissions from U.S. agriculture. Biomass fuels are now economically competitive under some circumstances (Committee on Science, Engineering and Public Policy, 1991b; Solar Energy Research Institute, 1990), but gaining higher levels of fossil fuel displacement will require more incentives than current markets provide. For example, if trees were planted on an area equivalent to that of currently idle cropland in the United States (about 27.5 million ha in 1987) to take up and store C and achieved an average net storage of 5.3 t/ha of C each year as suggested by Moulton and Richards (1990), the total C offset would similarly amount to over 0.5 Gt/yr of CO2.
Soils are important in the C cycle because they support plants and store much C below ground. Fixation of C by photosynthesis is the ultimate source of soil C and provides the energy that drives soil biological processes. Organic C in soils can be in either living organisms or their residues. An important character of soil C is its retention or turnover time. Soil retains C from a few to thousands of years. Longer times are related to cold, wet climates and greater depth. Organic C, deep within the soil profile, tends to be protected and less susceptible to decomposition than C on or near the surface.
In a recent workshop (U.S. Environmental Protection Agency, 1991a), strategies were identified to maintain and enlarge pools of C in the soil and to restore C to depleted soils. Generally practices summarized in Table 6.1 enhance cool (mulches, shade) or wet (mulches, irrigation) conditions, increase fertility (fertilizer), and reduce aeration (minimum tillage) of the soil. Raising the pH of acid subsoils is also identified to increase C sequestration.
Managing soils to sequester more and emit less C requires more C entering the soil or less leaving, either through decomposition or erosion. Decomposition is a microbially mediated process that breaks down plant residues and produces CO2 as a waste product. Plant residues are the major substrate, or energy source, for decomposition. Poor substrate quality, high C/N ratio, or phenolic compounds slows residue decomposition. In addition, soil temperature and moisture affect decomposition. Decomposition is slow in cool wet soils and rapid in warm, moist, aerobic soils. In warm and arid climates, microbial decomposition is limited by drought. All practices shown in Table 6.1 and defined below are generally suitable for either autonomous or encouraged adaptation.
Maintain/Improve Soil Fertility
Because soil fertility is essential for plants to grow and sequester C in soils, fertility problems need to be corrected. Farming soils that have lost C or are naturally low in fertility could decrease sequestration of CO2. Fertilization with such nutrients as N, phosphorus, and potassium, and liming acid soils are important to maintaining or improving soil fertility.
Preserve Natural Wetlands
If undrained, the inundated soils in wetlands contain much C with a very long retention time. Draining them causes rapid oxidation and loss of C to the atmosphere.
Reforestation has the potential to sequester much new C both above and below ground, particularly on C-depleted soils as was discussed in Section 4.3.
Minimize Dryland Fallowing
Fallowing leaves semi-arid farm lands bare in alternate years to accumulate additional soil water for growing a crop in the other year. Fallowing depletes organic C because the soil is usually kept bare by tillage. The stirring and mixing of the soil for 'bare' fallow, combined with warmer soil temperatures and lack of cover, increases oxidation of organic C. Improved technology is showing that fallowing can be decreased without great cost to farmers.
Use Municipal/Animal/Other Wastes
Restoring or enlarging pools of C in soil may be limited by lack of nutrients for plant growth or the cost of fertilizer to replenish them. When they do not contain heavy metals or other toxic substances; municipal, animal, industrial, or food wastes can be excellent sources of nutrients and C. Using them in forests or in agriculture, particularly marginal lands, adds nutrients and water (sewage water or water used to carry waste materials through application machines), thereby promoting more growth and sequestration of C above and below ground.
Conservation tillage is an agricultural practice that includes either minimum tillage or no-tillage and farming along the contour of the land. It retains crop residues on-site while decreasing the number and severity of soil physical disturbances used to prepare seedbeds, control weeds, and apply fertilizer. Loss of soil C is diminished as compared to conventional tillage practices. If, as discussed in Section 5.4, increased use of conservation tillage in the United States resulted in a change from the net loss of C from soil organic matter of about 0.01 Gt/yr of CO2 to a net sequestration of about 0.04 Gt/yr of CO2 (U.S. Environmental Protection Agency, 1991b), current emission levels from all U.S. agriculture would decrease by one-third (Table 4.1.1).
Retain Forest Slash on Site
Retaining forest slash on site following forest harvesting helps retain nutrients and water to promote reestablishment of new forest vegetation as quickly as possible. If the cycle of burning or removing residues is interrupted, soil C stocks will not only be maintained but will potentially be enlarged.
Minimize Site Disturbances
Forest harvesting may result in extensive compaction or disruption of large portions of the soil in the harvested area. Likewise, yarding or skidding logs to landings can compact and expose the mineral soil. Productivity of the site can be decreased while loss of soil C is promoted by oxidation and soil erosion.
Leave Crop Residues
Crop residues are an important source of C and nutrients in agricultural systems when returned to the soil. Left on the soil surface, crop residues also serve as a mulch to decrease soil temperatures and maintain higher soil moisture.
Control Soil Erosion
The surface layer of soil, or topsoil, is generally more fertile and has better water holding characteristics than deeper soil horizons. Physical loss or degradation of the soil resource diminishes primary production and consequently, sequestration of C below ground. Because of its proximity to wind and rain, topsoil is the most vulnerable part of the soil to physical erosion.
The rate of soil organic matter decomposition is positively related to soil temperature. Mulching or using plant residues to cover the soil surface decreases extreme soil temperatures, thereby slowing decomposition, and resulting in retention of more C in the soil. Mulching also decreases evaporation and keeps the topsoil wetter.
Remove Marginal Lands from Intensive Agriculture Production
Marginal lands are usually steep and prone to soil erosion. They also tend to be less fertile and not well suited for agricultural production. The availability of low-cost fuel and fertilizer has allowed these lands to be brought into agricultural production. Their cultivation has resulted in depletion of soil-C stocks. Removing these lands from intensive agricultural production and their reforestation or revegetation will sequester C in above- and below-ground biomass.
Prescribed Burns, if Needed
Burning has been used for centuries as an effective method for removing slash following a forest harvest or in shifting agriculture. Hot slash burns (i.e., hot-dry windy weather with dry slash) may severely damage topsoil and affect its productivity. If burning is required, lighter burns (i.e., in cool-wet weather) will damage the entire system less and ease the growth of a new forest.
The use of small forests and individual trees in urban areas helps capture and store C in above- and below-ground biomass and as soil C. Soil C that has been lost because of urban land use can be restored. Additional societal benefits include: recreation, lower urban temperatures, aesthetics, and air purification. Lower urban temperatures during the summer months will conserve the C that would otherwise be used for energy generation for running urban air conditioners.
Methane (Emitting Less and Sequestering More)
Following CO2, CH4 is the most abundant atmospheric C species. Soils represent an important global sink for methane, mostly not on agricultural lands (World Resources Institute, 1987). Arable- and crop-land area of the world is only about 13% of the terrestrial land area. If soil CH4 oxidation is dependent on concentration of CH4, then CH4 uptake by soils may be increasing with the rise in CH4 concentration in and near the soil surface due to the more than doubling of atmospheric CH4 since 1850 (Ojima et al., 1992). However, the effect of land use change and N additions on CH4 uptake may be a factor contributing to a decreased CH4 sink (Melillo et al., 1989; Mosier et al., 1991). Exact mechanisms by which CH4 oxidation is decreased following conversion to cropland or change in N dynamics is not clear; however factors such as water availability, fertilizer application, atmospheric N deposition, soil structural changes, and cropping management all seem to have an effect (Ojima et al., 1992). Thus, changes resulting from human activities on any of the major terrestrial ecosystems of the world may alter their capacity to serve as sinks for atmospheric CH4.
Soils also represent the most important CH4 source, which is produced predominantly by microbial degradation of organic C in rice paddies, natural wetlands, and landfills. However, our discussion is limited to agricultural emissions and will deal only with rice paddy soils. Following rice paddies, ruminant animals provide the next most important global CH4 source from agriculture.
Rice cultivation occurs on approximately 10% of the world's arable and permanent cropland area (U.S. Department of Agriculture, Agricultural Statistics, 1990; Food and Agriculture Organization of the United Nations, 1989), an area somewhat greater than all U.S. cropland in crops in 1987. However, U.S. land area used to grow rice occupied only about 0.7% of the world's rice production area and about 0.9% of U.S. cropland area in crops (U.S. Department of Agriculture, Agric. Stat., 1990). Therefore, adaptation and/or mitigation of CH4 emissions for rice production in the United States will have minimal impact, except for technology that is developed and transferred to other countries. Adaptations might include improved crop residue, organic waste and fertilizer management, and improved biological efficiency. Mitigation might range from increased to total substitution of other cereals (including upland rice) for paddy rice.
Crop Residue and Organic Waste Management
Incorporating organic materials (crop residues and other wastes) as an N source or for other purposes generally increases CH4 emissions. If crop residues are instead burned, the benefits of return and sequestration of some of the C contained in these materials into soil-C pools are lost. Adapted cultural practices allowing the return of most of these residues without incorporation would diminish CH4 emission, including rotation of the land used for paddy rice into other crops.
Although experimental results differ, deep placement of mineral-N fertilizers likely decreases CH4 emissions. Fertilizer N is extremely important for obtaining high yields. It also decreases the ratio of CH4 produced per kg of rice produced. However, much more needs to be known to define and improve fertilizer and other cultural management practices that minimize this ratio while still providing required amounts of rice to feed the world's populations.
Improved Biological Efficiency
As the genetic potential for increased rice yield per ha of cropland area increases, the ratio of CH4 produced per kg of rice should also decrease. Such improvement might also include a lower straw/grain ratio to decrease the relative amount of straw to be returned to paddy fields or burned. Increased grain yield might also help lower the land area of paddy rice required to satisfy demand, thus further decreasing CH4 emissions.
Substitution for Paddy Rice
Enhancing supplies of substitutes for paddy rice include improvement of non-paddy (upland) rice production as well as improvements in production of other cereal grains. Technological improvements for paddy rice have been achieved much more rapidly than for upland rice. In addition, for a staple commodity such as rice, it is doubtful that consumption will decrease without government intervention, which may be untenable in many countries.
Of the total number of cattle and buffalo reported for countries of the world (U.S. Department of Agriculture, Agric. Stat., 1990), about 100 million, nearly 10%, are in the United States. Autonomous adaptations by farmers to the changing pressures of agriculture could include improved biological efficiency, improved feed quality or composition, and improved use of biotechnology. An additional strategy to specifically address the risk of climate change might include decreased consumption of animal products.
Agricultural scientists and farmers have achieved improvements in animal efficiency. The result has been improved animals that convert grains and roughage into meat, milk, and other products more efficiently and the potential for further improvement is considerable. Increases in the biological efficiency of ruminant animals decreases the production of CH4 per unit of livestock product by decreasing the numbers of animals required to satisfy demand.
Improved feed formulations and pharmaceuticals can alter fermentation in the rumen and improve efficiencies of protein production. These measures can be applied to intensively managed animals such as dairy cattle and beef cattle in feedlots to provide some benefits. However, animals that are not intensively managed, such as cattle on forages, pasture, or range, will continue to yield more CH4 per unit of available feed energy and grow more slowly than those on grain or other improved feed formulation. Rates of improvement in the efficiency of livestock production have averaged 1% per yr over the past 40 years. These advances are attributable to genetic selection and improvements in feeding and management strategies. These rates of improvement may decline somewhat in the future in some sectors. A conservative estimate of an increase of 10% in efficiency can clearly be defended based upon current knowledge and the management practices of our best producers (Section 5.4). Such improvements would lead to a 10% decline in CH4 emissions from livestock agriculture.
Advances in biotechnology provide products such as somatotropin to increase feed efficiency and animal gains. Pharmaceutical or other products and technologies alter fermentation in the rumen and increase efficiency of protein production. All of these technologies serve to decrease CH4 produced per unit of animal product and have the potential to alleviate CH4 emissions from livestock in the order of 10%.
Decreased Consumption of Ruminant Livestock Products
Consumption of ruminant products, especially meat, in the United States has been decreasing steadily and could be decreased further either by enhancing the supply of substitute products or by making ruminant products more costly to consumers. Methods might include development of cereal substitutes for meats; through taxes and other price-increasing practices, the price of ruminant commodities could be made to rise relative to prices of substitute commodities. On the other hand, we must realize that 30% of the energy and 55% of the protein consumed by the U.S. population are from animal products. The argument that ruminant animals are not efficient does not apply in this regard since returns on human edible inputs of energy and protein into animal agriculture run from 85 to 100 and 110 to 150%, respectively. In other sections, it has been implied that reductions in consumer demand for livestock products in the United States would further reduce methane emissions. For example, from a peak sale of retail cuts of red meat of 42.8 kg per capita in 1976, sales decreased to 31.2 kg in 1989. However, sales have been stable at 30.4 to 30.8 kg per capita for the past three years. Thus it is difficult to project a reduction in CH4 emissions attributable to reduced demands for red meat at this
Nitrous Oxide (Emitting Less)
Adaptations to emit less N2O from agriculture could include strategies of fertilizer management, nitrification inhibitors, irrigation water management, and efficient fossil fuel use. Mitigation might include additional fertilizer management requirements. In general, agricultural N2O emissions can be decreased through N-management practices that (1) optimize the crop's natural ability to compete with processes whereby plant available N is lost from the soil-plant system (i.e., denitrification and leaching), and (2) direct lowering of rate and duration of these loss processes; however, these simple principles may not be easily accomplished by adaptations alone.
Use of fertilizer N is extremely important for obtaining high yields. However, much more effort and information is required to define and improve N fertilizer and other cultural management practices to minimize atmospheric losses of N2O, such as: (1) use of soil testing; (2) dispense with the "maintenance" concept, which fails to recognize the amount of residual N in the soil and the soil's nitrification potential; (3) adjust the rate of N to a reasonable yield goal for the specific crop and field or soil; (4) place N deep enough in the soil to lower the N2O/N2 ratio when denitrification does occur; (5) take into account soil N mineralization and the N from legumes, manures, organic wastes, irrigation water and other potential sources; and (6) time N application to when it is needed by the crop. Timing and amount of fertilizer N application should have a goal of leaving as little residual N in the soil during the noncropped periods of the year as possible. In large measure, these are all strategies that both reduce N2O emission and improve the efficiency of N fertilizer use.
Change in agriculture, in addition to being a technological process, is also sociological even when economic benefits result. This makes accomplishing improved N-fertilizer use efficiency a long-term endeavor as illustrated by Iowa's large-scale agricultural demonstration and education programs (Iowa Department of Natural Resources, 1991), which are showing significant results. Statewide N-fertilizer use data indicate that even with declining fertilizer prices, Iowa producers have decreased N use on corn since 1985 and diverged from regional trends for the midwestern United States. From 1985 to 1990, N-fertilization rates in the Midwest region increased slightly, possibly in response to declining fertilizer prices. However, during this same period, Iowa departed from parallel regional trends of the prior 20 yr; N-fertilization rates declined approximately 12%, yet no relative reductions in crop yields have occurred.
The NH4+ ion is sorbed to the exchange capacity of the soil; whereas, NO3- ion is not and can be readily denitrified or leached. Both forms can be readily utilized by crops. Nitrification inhibitors include chemicals added to soils to stabilize fertilizer applied as NH3 or in the NH4+ form by inhibiting activity of the Nitrosomonas bacteria in the first step of the nitrification process. Nitrogen losses can be minimized if applied N remains in the NH4+ form for several weeks after application, especially when fall applied or when there may be heavy rainfall periods during the spring. An inhibitor, such as nitrapyrin, can be effective in many field crop situations. Denitrification (and NH3 volatilization) especially leads to low fertilizer use efficiency in rice, (Buresh and DeDatta, 1990; Fillery et al., 1986). An apparent highly-effective nitrification inhibitor for rice paddy conditions is encapsulated calcium carbide (ECC) (Banerjee and Mosier, 1989; Bronson and Mosier, 1991).
Irrigation Water Management
Generally fluxes of denitrification gases occur immediately following each irrigation. Because N2O may be further changed to N2 during transport to the soil surface, especially when O2 is more limiting, there is greater opportunity to decrease the N2O/N2 ratio of the resulting gases when the mineral N that is being denitrified is deeper in the soil. An effect of infrequent, compared to more frequent irrigation, is to not only decrease the number of resulting denitrification cycles, but to also help move soluble N deeper into the soil profile where supplies of O2 are more limited and therefore there is increased opportunity to reduce any N2O that may form to N2 (Figure 5.4.3). However, with the same total amount of applied irrigation water, very frequent irrigations tend to result in the largest amount of denitrification, whereas infrequent irrigation may increase leaching losses. Therefore, a balance would need to be drawn between the two types of N-losses.
Efficient Fossil Fuel Use
This option is likely an autonomous adaptation for two reasons. First, combined gasoline and diesel fuel use has fallen dramatically in recent years as a result of energy-saving farm production technologies and shifts from gasoline to more fuel-efficient diesel-powered units (Section 4.2). Secondly, decreasing requirements for land are projected to meet domestic and export demand scenarios to the year 2030 (Figure 5.4.1) along with increasing agricultural technology development. These combined should serve to decrease on-farm and likely off-farm energy requirements for agriculture.
Additional Fertilizer Management Considerations
Nitrous oxide emissions from microbially mediated nitrification and denitrification may need to be decreased beyond what can be accomplished through autonomous adaptations. For example, urea fertilizer is reported to emit more N20 per unit of applied N than do nitrate or anhydrous fertilizers (Keller et al., 1988). However, energy required to manufacture urea is much less than that required to manufacture, for example, ammonium nitrate. Thus, there is a trade-off between N20 emissions for the on-farm use of urea versus a higher fossil fuel use to manufacture ammonium nitrate. Deeper placement of some N-fertilizer materials would require more energy and would be less easily adopted with present farming practices.
The goals of less expense and more yield per unit of land area used for production have often induced less emission of the three greenhouse gases per unit of food or fiber produced. That is, during the past decade at least, less fuel has been burned and so less CO2 emitted per unit of production. Higher yields per unit area of land and conservation tillage have lessened emission of CO2 from soil organic matter. Depending upon their use in the United States, tree crops not only sequester C, but also serve to recycle C as an energy crop. Improvements in animal husbandry and decreased consumption of ruminant animal products have lessened the emission of CH4. Emission from rice production has always been inconsequential because less than 1% of U.S. cropland grows rice. Careful application of nitrogenous fertilizer has lessened emission of N20. Additional decreases in emissions of all three greenhouse gases are possible through autonomous adaptations.
So long as the rules for controlling emission of greenhouse gases from farming and stashing C in soil organic matter and forests reinforce the goals of higher yields per unit of land and lower costs, as the past proves is often sensible, the controls should not hobble and may help U.S. farming. Likely, increased amounts of C will be stashed by farmers and foresters in the future, but incentives beyond those found in current markets will be required. Strong evidence exists that mitigation of about one-third of current U.S. agriculture emissions of C is possible by increased stashing in soil organic matter through increased use of conservation tillage. Possible future sequestration of even larger amounts of C is possible using trees if large areas of land were planted and managed and if the wood products were effectively used. When wood is burned as fuel, it displaces fossil fuel. Displacing enough fossil fuel to reduce U.S. emissions of CO2 from all sources by 10% and thus making U.S. agriculture effectively a net storer of carbon is a plausible future.