1. Cropland

Responsible management of croplands should increase carbon sequestration and reduce greenhouse gas emissions. Carbon sequestration can be accomplished by growing additional biomass, particularly perennial plants, and by increasing the soil carbon levels of agricultural land.

We first describe four agricultural systems that offer a range of climate benefits: agroforestry, perennial agriculture, diversified farming, and organic agriculture. U.S. agricultural policy currently disfavors agroforestry and other forms of perennial agriculture, and these practices are unlikely to expand without significant changes in public law and policy. Nonetheless, this book highlights them due to their unrivaled capacity for long-term carbon sequestration. Diversified systems, which often incorporate trees and other perennial crops, have a number of climate advantages over conventional monocultures, including increased resilience and carbon sequestration. Although organic agriculture remains uncommon—less than 0.5% of agricultural land is devoted to organic production—it offers policymakers seeking to reduce emissions a number of important lessons.⁵

We next explain why policy should prioritize the production of crops that provide people with healthy food, rather than crops that become processed food, animal feed, or biofuels. The latter set of crops—namely corn, wheat, and soy—take up a huge amount of land and consume enormous amounts

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of energy,⁶ though, as explained below, they offer little nutritional benefit. If humans consumed these crops or other whole foods directly, the food system would produce more nutritious food with much less land and energy.

The discussion then moves to three methods to reduce net CO₂ emissions by increasing the organic matter content of soil—reducing tillage, increasing carbon inputs from crops, and adding soil amendments. Soil organic matter, which consists primarily of decomposing plants and animals, is rich in carbon. Thus, practices that increase the organic matter content of soil generally also increase soil carbon sequestration and, thereby, reduce net emissions. Increasing soil organic matter is a particularly important method of sequestering carbon in temperate parts of the world, such as the United States, where soils contain vastly more carbon than plants (both above- and belowground).⁷ Such healthier soils can also require less fertilizer, which decreases nitrous oxide emissions.

We next describe farming methods to reduce nitrous oxide emissions. And finally, we examine practices rice producers can adopt to reduce methane emissions.

Expand agroforestry. “Agroforestry” is a collective name for agricultural systems that integrate management with woody perennials and agricultural crops or animals on the same piece of land.⁸ While agroforestry is included in the broader category of perennial systems, discussed below, we focus on it here separately due to its unique potential to rapidly reduce emissions in the United States through already well-established crops and practices. The trees can be those producing food such as fruit or nut trees, those providing other services such as windbreaks, or those grown for wood products. Since trees substantially increase above- and below-ground biomass, agroforestry increases both the rate of sequestration and the total amount of carbon that a piece of agricultural land can store relative to annual cropping systems

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(although less than unconverted forest).⁹ As a result, agroforestry’s per-acre sequestration potential is far higher than that found in annual crop systems.

In the United States, existing agroforestry systems typically use trees and shrubs as windbreaks, buffers, and hedges on otherwise conventionally managed cropland; however, agroforestry can also include alley cropping—the side-by-side planting of annual crops with trees in adjacent rows—and silvopasture, discussed below, which incorporates trees in pastures. A 2012 literature review estimated that agroforestry systems, implemented nationwide, could sequester 530 million metric tons (MMT) of carbon a year— an amount equivalent to one-third of all fossil fuel emissions in the United States.¹¹ (Since a CO₂ molecule is about 3.7 times heavier than a carbon atom, increases in carbon sequestered will lead to about 3.7 times that amount of reduction of atmospheric CO₂.) Alley cropping and silvopasture alone could sequester more than 516 MMT of carbon annually.¹² In contrast to other practices focused exclusively on soil carbon, these large gains from agroforestry reflect both aboveground and belowground sequestration.

In addition to its enormous potential for carbon sequestration, agroforestry also reduces environmental harms, although its impacts depend heavily on the selection of tree species and their management.¹³ A 2019 review found that agroforestry systems reduce surface runoff, soil erosion, organic carbon losses, and related nutrient losses by an average of 58%, 65%, 9%, and 50%, respectively compared to conventional practices. Agroforestry practices also lower herbicide, pesticide, and other pollutant losses by 49% on average,

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decreasing overall nutrient and chemical runoff.¹⁴ Agroforestry practices can also increase yields.¹⁵ Windbreaks improve air quality and protect plants from wind-related damage, thereby enhancing wildlife and insect habitat, although they may reduce light infiltration very close to the trees, slightly reducing yields.¹⁶ Finally, riparian forest buffers are effective at protecting rivers and streams from bacterial contamination,¹⁷ surface runoff, and pesticide drift.¹⁸

Shift from annual crops to perennial crops.

Perennial crops include a wide range of plants that are capable of fulfilling important ecological and human needs, including woody crops (used in agroforestry systems), herbaceous oilseed crops, grains, and grasses. In the United States, farmers grow several common perennial crops, mostly in monocultures, including grapes, apples, blueberries, stone fruits, citrus, and almonds and other nuts. Perennials can also be a significant source of

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vegetables,²⁰ livestock forage,²¹ biofuel feedstock,²² and carbohydrate, protein, or oil constituents in food products that are now largely produced by annual crops.²³ Walnut and pistachio trees, for example, yield more oil per acre than either soy or corn.²⁴ Consumers, in response to dietary restrictions and health benefits,²⁵ now show a stronger demand for nut crops.²⁶ Since they can be processed into flour or meal, some nut flours are being used to replace wheat flour or corn meal on a commercial scale. Other tree crops, such as the leguminous pods produced by mesquite trees, also offer potential for commercial food production.²⁷

While current cultivars of perennial starch, oil, and protein crops have lower per-hectare caloric yields than soybean or corn at present, this is due at least in part to disparities in breeding and agronomic research.²⁸ Yields for five major crops in the United States—corn, soybean, wheat, cotton, and rice—quadrupled between 1950 and 2010 as public and private research dollars flowed to these crops.²⁹ Similar investments in perennial crops are likely to produce significant gains in yield.³⁰ While there are now no perennial grains ready for widespread commercial use in the United States, the Land

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Institute, a nonprofit research organization dedicated to developing perennial staple crops, has been making promising progress.³¹

Scientists at Iowa State University have also developed a system of row crop production integrated with strategically placed native perennial grasses, called prairie strips, modeled on agroforestry practices.

The ecosystem benefits of using perennial crops are well established.³⁴ Perennial crops generally have deeper rooting levels, reducing erosion risk and allowing them to conserve water more effectively.³⁵ Their extensive root systems also absorb nutrients more efficiently, reducing fertilizer runoff.³⁶ Additionally, perennial crops require less fertilizer and herbicide since the soil on which they sit is exposed and disturbed much less frequently than in annual systems.³⁷ Integrating livestock or additional crops into perennial systems can increase biodiversity, improve natural pest control, raise yields, and increase system resilience.³⁸

Implement diversified farming systems. Input-intensive crop monocultures and industrial feedlots dominate agricultural production in the United States, in large part due to government support in the form of research, subsidies, and lax regulations. These systems produce prodigious amounts of calories, but have a number of public health, environmental, and community

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externalities.³⁹ They also rely on emissions-intensive practices and inputs.⁴⁰ In response to the negative impacts of industrialized agriculture, many researchers and farmers have advanced an alternative approach known as agroecological intensification that uses ecological principles to minimize inputs and maximize production and environmental services.⁴¹ One of the key concepts of agroecological intensification is functional biodiversity, which seeks to diversify crop and animal production to minimize risk, fertilize soil, conserve resources, and intensify production.⁴² Many of the most promising carbon farming practices, such as alley cropping and crop-livestock systems, effectively utilize functional biodiversity.

Diversification on farms can reduce emissions from soil management and increase carbon sequestration in soil and biomass.⁴³ As discussed above, functional biodiversity is modeled on ecological principles and, thus, also has a number of environmental co-benefits, including improved resilience, soil health, wildlife habitat, natural pest control, and pollinator health, among others.⁴⁴

Employ organic farming and other more climate-friendly farming systems.⁴⁵ There are several agricultural systems, including organic agriculture, permaculture, agroecology (which includes practices such as crop rotations, integration, and diversification), and regenerative agriculture, built on the fundamental premise that soil health and natural ecological systems, such as the nutrient cycle between livestock and crops, are paramount to longterm

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productivity. This subsection focuses on organic agriculture, since it is well-studied and the U.S. Department of Agriculture (USDA) already has national organic standards,⁴⁶ making it easier to classify. However, certified organic operations are not necessarily more climate-friendly than noncertified operations implementing these other models—all can have significant climate benefits.

Organic farming methods enhance production by supporting natural soil fertility and biological activity, and prohibiting the use of synthetic pesticides or fertilizers.⁴⁷ USDA, which sets standards for organic products in the United States, defines organic farming as a form of agriculture that uses methods designed to “support the cycling of on-farm resources, promote ecological balance, and conserve biodiversity.”⁴⁸ Organic agriculture encourages many of the practices mentioned here, such as cover cropping, crop rotation, and the incorporation of diverse elements on cropland, including forestry and livestock. Its primary climate benefits are reduced nitrous oxide emissions, lower energy requirements, and increased soil carbon sequestration.⁴⁹ Some studies suggest that organic farming can obtain equivalent yields to conventional farming,⁵⁰ or come close in certain contexts,⁵¹ while others suggest that the lower per-acre yields would reduce the climate benefits of the system by requiring more cropland.⁵² Most productivity research to date has focused on conventional systems; increased research into organic farming would likely narrow or close any productivity gap.

Organic agriculture offers a wide range of environmental and social benefits in addition to its potential to reduce net agricultural emissions. As the

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organic industry has grown, so too has the number of industrial-scale, capital-intensive organic operations, dampening these benefits.⁵³ Nonetheless, research consistently indicates that organic agriculture increases soil stability and fertility, on-farm biodiversity, and crop resilience to weather shocks, while reducing energy use (e.g., by reducing tractor usage) and the need for synthetic inputs.⁵⁴ Organic farms can also directly benefit people, especially in rural communities, who can enjoy better landscape preservation, less agricultural pollution,⁵⁵ reduced dietary exposure to pesticides,⁵⁶ and, according to some researchers, greater civic engagement.⁵⁷

Shift to more ecologically efficient crop use. Analyses of agricultural productivity generally focus on inputs, including labor, and crop yield. While these factors are important, they fail to provide an accurate account of whether a crop is a truly efficient use of land and energy from the perspective of meeting human needs. A crop with high yields and low labor requirements may be inefficient if it is integrated into an energy-intensive value chain, such as grain destined for a feedlot, or if it does not provide consumers with a nutritious end product, such as corn processed into high-fructose corn syrup. If the food system produced less animal feed, less feedstock for biofuels, and fewer processed foods—while producing more crops intended for human consumption as whole foods—it would be dramatically more efficient.

A 2013 study estimated that 67% of the calories and 80% of the protein in crops produced in the United States are diverted to animal feed.⁵⁸ This is an inefficient use of potential food. For example, approximately six pounds of grain are used for each pound of beef produced over the life-span of a cow.⁵⁹

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In the United States, approximately 70 million acres of cropland are used to produce corn and soybean for animal feed.⁶⁰ The same calories and protein now provided by animal products could be produced with a much smaller land footprint if crops were consumed directly by humans rather than fed to animals. Below, we discuss the concept of ecological leftovers, which would limit livestock production to grassland and food byproducts unsuited for human consumption, dramatically increasing the ecological efficiency of animal products.

The enactment of the renewable fuel standard (RFS) in 2005, and its subsequent strengthening in 2007 (RFS2), massively increased demand for biofuels in the United States.⁶¹ Roughly 37% of the corn sold in the United State has been used to make ethanol since RFS2 went into effect in 2008.⁶² While the biofuels mandate was justified (in part) as a way to reduce greenhouse gas emissions, studies examining the climate change impacts of corn ethanol show a wide range of results, reflecting the challenge of determining with precision or confidence the full life cycle impact. Most studies find a modest annual operational greenhouse gas benefit over gasoline, but they generally either ignore or minimize the impact of using land for fuel or the continuing impact of the prior conversion to cropland (referred to in Chapter III as the carbon opportunity cost).⁶³ However, other studies indicate that land use change impacts of current U.S. biofuels, and thus the carbon debt payback time from conversion of land to biomass feedstock production, are much more significant than conventional models indicate, resulting in a net harm to the climate.⁶⁴ Between March 2008 and May 2020, 75% of all corn sold was

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used as animal feed or biofuel, a tremendously inefficient use of resources.⁶⁵ Moreover, a U.S. Environmental Protection Agency (EPA) study found that between four and almost eight million acres of grasslands were converted to cropland since RFS2, with nearly 20 million more acres being devoted to corn and soybean for fuel, so that now about 40% of the corn crop is used for ethanol production.

In addition, consumers now eat substantial amounts of processed and “ultra-processed” foods;⁶⁶ an estimated 75% of the average person’s calories comes from such food.⁶⁷ In most wealthy countries, like the United States, most people have “commodity-based diets,” where they eat heavily processed foods made largely from corn, wheat, and soy as well as some animal products.⁶⁸ These diets are deficient in nutrients and other beneficial compounds found in whole or minimally processed foods⁶⁹ and are associated with a higher risk of cancer.⁷⁰ It may not be feasible for farms to produce an adequate supply of nutritious foods if they do not reduce production

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of commodities used in processed foods. Research suggests that higher atmospheric levels of CO₂ will decrease the protein and mineral content of common staples such as wheat, rice, and soybeans, further increasing the need for a more ecologically efficient and nutritious food supply chain.⁷¹ Shifting away from such high reliance on heavily processed foods would reduce inefficiencies in the food system, produce healthier food, and help mitigate climate change.⁷²

Reduce or eliminate tillage. To prepare for planting, farmers routinely till their land by plowing or otherwise breaking up the soil, and eliminating unwanted material. This process accelerates the breakdown of organic matter in the soil, increasing emissions of CO₂. Thus, researchers and farmers who want to reduce emissions are examining ways to prepare soil for planting with no, or reduced, tillage. No-till agriculture, which completely eliminates tillage, uses herbicides or other methods to control weeds instead of tillage, and leaves the soil physically undisturbed, protecting organic matter from soil microbes that could otherwise accelerate the carbon cycle by returning soil carbon to the atmosphere as CO₂.⁷³ Reduced tillage practices that integrate some amount of plant residue into soils can also reduce nitrous oxide emissions and further increase carbon sequestration in some circumstances.⁷⁴

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No-till agriculture began to grow steadily in the United States after inexpensive herbicides and specialized equipment became widely available in the 1970s.⁷⁵ A 1972 survey of USDA conservationists found that there were an estimated 3.3 million acres of no-till cropland.⁷⁶ By 2017, farmers reported practicing no-till on 104 million acres and reduced tillage on another 98 million acres.⁷⁷ In contrast, intensive tillage was practiced on 80 million acres— down to 28% of the 282 million acres suitable for tilling according to the 2017 Census of Agriculture.⁷⁸ While no-till’s impact on crop yields varies according to a number of factors, including soil conditions, management techniques, weather, and crop type, a 2016 meta-analysis found that no-till generally results in similar yields to conventional tillage after a transition period of five or more years.⁷⁹ Even with yield reductions during the transition phase, however, no-till may remain more profitable for farmers than conventional tillage due to its potential to reduce expenditures on labor, fuel, and, in some cases, fertilizer.⁸⁰ Although farmers initially adopted no-till to reduce their heavy reliance on tractors—and thus reduce costs—and to limit soil erosion by reducing the amount of bare soil, USDA,⁸¹ industry groups,⁸² and some scientists now promote no-till as a way to sequester carbon. Indeed, conservation tillage, which includes no-till farming and some methods of reduced tillage, is among the most widely studied agricultural practices with respect to climate change.

Despite this attention, however, there are questions about the potential of no-till practices to mitigate greenhouse gas emissions.⁸³ A 2007 review

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noted flaws in how some of the more favorable studies measured soil organic carbon,⁸⁴ while a 2015 study found that increased earthworm activity can negate any carbon sequestration effects from no-till, at least in the short term.⁸⁵ Nonetheless, the evidence suggests that no-till agriculture can increase soil carbon stocks in many regions, although its effect varies considerably by soil type and location.⁸⁶ A 2013 meta-analysis also found that notill significantly decreases nitrous oxide emissions after five years, especially in dry climates.⁸⁷

Researchers have also expressed concerns that no-till farming as practiced by commercial farmers often differs considerably from how it is implemented on research fields.⁸⁸ The available data suggest that many farmers who consider their methods “no-till” actually till their fields periodically.⁸⁹ Since even a single tillage event can release carbon built up over years of no-tillage, these periodic tillings means many “no-till” farms sequester far less carbon than a naïve analysis suggests.⁹⁰ One expert estimates that less than a third of no-till farms in the United States are truly no-till, and that the number

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of these continuous no-till farms is likely decreasing.⁹¹ Since estimates of carbon sequestration from no-tillage often assume continuous no-till,⁹² the aggregate climate benefits of no-till agriculture as currently practiced are often overestimated.

While most no-till systems rely on herbicides to eliminate weeds, organic no-till systems could offer significantly higher levels of carbon sequestration. The Rodale Institute has developed a mechanical mounted roller that knocks down and kills cover crops, suppressing weed growth without herbicides.⁹³ Short-term studies of organic no-till systems indicate that they likely sequester more carbon than conventional no-till farming.⁹⁴ Although the Rodale Institute’s field results are promising, it is still conducting trials, and commercial farmers have yet to adopt organic no-till.⁹⁵

Given the uncertainties of the climate benefits of no-till as currently practiced, it may not deserve the attention it is getting as a strategy to fight climate change. Yet its many other well-documented environmental benefits suggest that researchers should continue to study, refine, and integrate it with other climate-friendly practices to optimize its climate impact. By leaving more plant residue and organic matter in and on the soil, it can improve soil quality, reduce erosion, provide food and cover for wildlife, and reduce dust and diesel pollution from tillage.⁹⁶ However, conservation tillage as commonly practiced in the United States often requires higher levels of herbicides than conventional tillage systems.⁹⁷

Increase carbon inputs from plants through cover crops and crop rotations. Farmers can also foster soil carbon by increasing carbon inputs from plants. Cover cropping and conservation crop rotations are among the most common practices designed to do this in annual crop systems. Cover crops are plants grown to enhance soil conditions rather than to produce an agricultural

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product. Farmers generally grow them during the late fall and winter when common commodity crops such as corn, wheat, and soy are not in season. In addition to increasing soil organic carbon by increasing carbon inputs, cover crops have also been shown to significantly reduce nitrate loss, thereby indirectly reducing nitrous oxide emissions.⁹⁸ Cover cropping with legumes also increases biological nitrogen fixation, reducing the need for nitrogen fertilizers.⁹⁹

Conservation crop rotations refer to planting systems designed to decrease the frequency at which fields are left fallow and to rotate between a diverse set of crops, thereby increasing carbon inputs.¹⁰⁰ Crop rotations that include perennial plants, such as alfalfa or grass hay, can be especially effective at sequestering carbon.¹⁰¹ Iowa State University researchers have shown that three- and four-year rotations that include alfalfa increase yields and require less fertilizer and herbicides.¹⁰² While farmers rotate most crops on a seasonal basis, producers with perennial crops in their rotation do not need to return to annual crops for one to three years.¹⁰³

Although neither of these methods offers transformative climate benefits when practiced in isolation, both can play an important role in reducing net agricultural emissions when integrated into climate-friendly systems. Diversified crop rotations, for example, are even more effective at increasing soil carbon when combined with cover cropping,¹⁰⁴ although likely sequestration rates have not been established.¹⁰⁵ Cover cropping has also been shown to sequester carbon more quickly when used in conjunction with no-till agriculture and it likely complements other environmentally friendly practices as well.¹⁰⁶ As cover crops also use water, farmers who grow them may need

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more water for the cash crop. However, since cover crops reduce evaporation, they may also conserve water—the best practices for cover cropping will depend on the region. Cover crops that can be grazed or be marketed can also increase operational profitability.

Practices that increase carbon inputs from plants also offer a range of ecosystem benefits. Both cover cropping and diversified crop rotations have been shown to improve soil health, nutrient cycling, pest regulation, and crop productivity,¹⁰⁷ while reducing herbicide and fertilizer use.¹⁰⁸

Add soil amendments. Farmers who apply amendments such as manure or other organic fertilizers to their soil can lower emissions by decreasing manure waste, reducing emissions from the production of synthetic fertilizers,¹⁰⁹ and increasing soil carbon stocks.¹¹⁰ While livestock manure remains the dominant source of organic fertilizer for agriculture, the United States has large amounts of compostable solid waste and solid residues from sewage treatment plants, called biosolids, which also can be, and often already are, used as soil amendments.¹¹¹ According to the Guardian, 60% of sewage sludge produced by treatment plants was sold as fertilizer in 2019.¹¹² Nonetheless, these biosolids are insufficiently regulated and pose serious health risks to farmworkers, local residents, and consumers.¹¹³ In 2018, the EPA Office of Inspector General released a report on biosolids, warning that it had identified more than 350 pollutants, including 61 acutely hazardous, hazardous, or priority pollutants in biosolids samples. The report concluded that EPA “lacked the

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data or risk assessment tools” needed to regulate biosolids effectively.¹¹⁴ More recent reports have found that biosolids can be contaminated by per- and polyfluoroalkyl substances (PFAS) (a class of chemicals widely used in non-stick and other industrial uses that are extremely persistent), which can then be taken up by plants and grazing animals.¹¹⁵ Until government agencies can ensure the safety of biosolids, they should not be used as agricultural soil amendments regardless of their potential to reduce net emissions.

A type of charcoal called biochar may be able to store even more carbon than traditional organic amendments.¹¹⁶ Biochar is produced by pyrolysis: the thermal decomposition of organic material at high temperatures in the absence of oxygen. This process results in a carbon-rich char that is more stable than uncharred plant material, although local environmental conditions, such as climate and soil type, play an important role in determining how long it persists in soils.¹¹⁷ Biochar primarily reduces emissions by stabilizing and adding to carbon stores in the soil;¹¹⁸ however, it may also reduce nitrous oxide emissions and fertilizer requirements.¹¹⁹

Both organic fertilizer and biochar can increase agricultural productivity, particularly in degraded soils, and reduce irrigation and fertilizer requirements.¹²⁰ Organic soil amendments also have some potentially negative environmental impacts. If not managed well, they can result in odor and particulate pollution, nitrate runoff, and phosphorus loading.¹²¹ As with

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synthetic fertilizers, farmers must manage application timing, methods, and rates with care to minimize nitrous oxide emissions.¹²²

Improve management practices for synthetic fertilizers. Because plants utilize nitrogen from the soil and crops carry it away from the field after harvest, farmers must replenish nitrogen in their fields to maintain yields. This is typically accomplished through the application of synthetic or organic (such as manure) nitrogen fertilizer. However, farmers routinely apply fertilizer at higher rates than crops require for a variety of reasons: as a form of insurance or risk avoidance, hope for a great year, over-focus on yield over return, habit, and misinformation.¹²³ On average, only 50% of the nitrogen applied as fertilizer to annual grains is removed at harvest.¹²⁴ Similarly, a 2011 study found that farmers applied at least 40% more nitrogen than the prior harvest removed on nearly one-third of acres planted with key commodity crops.¹²⁵ In addition, because farmers have applied excess fertilizer for so long there is often an excess in soil, so they can now apply fertilizer less frequently— and, when necessary, apply less fertilizer per acre—without reducing yield. When they do this, they will also reduce the amount by which the supply of nitrogen in the soil exceeds the demand for nitrogen by crops. This will limit excess nitrogen that is released into the environment, including as nitrous oxide.¹²⁶

In general, best practices for fertilization include reducing the rate of application so that the amount of nitrogen is closer to what crops need; timing the application so that nitrogen is available when crops can best utilize it; and varying the placement of nitrogen within fields to account for spatial variability in utilization by crops. Fertilizer companies, industrial farmers, and many extension programs call these practices the “4Rs”: apply the right fertilizer product at the right rate, right place, and right time.¹²⁷ These practices

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are not mutually exclusive, and they will likely be most effective when combined in broader nutrient management plans.¹²⁸

Even if the rate of fertilizer application matches crop needs, improper timing and placement can increase greenhouse gas emissions. One of the most important things a farm can do is apply fertilizer no earlier than the planting season.¹²⁹ Nonetheless, due to ease of application, soil and water conditions, the lower cost of fertilizer in the fall, availability of machinery, and other reasons, farmers now fertilize a significant portion of the nation’s cropland each fall, even though those fertilized fields will not be seeded until the following spring.¹³⁰ Fertilizer left unutilized in the soil over winter is vulnerable to environmental loss, including as nitrous oxide.¹³¹

Some experts argue that farmers can increase efficiency by practicing “split application”—that is, applying small amounts of fertilizer early in the planting season and, again, when nitrogen demand is highest, typically after plants emerge from the ground.¹³² Studies have found that split application may reduce emissions by a significant amount. In one study on potatoes, an especially nitrogen-intensive crop, split application resulted in a 30% reduction in cumulative emissions compared to a single application.¹³³ Slow-release fertilizer formulations can also improve efficiency. For instance, polymer-coated urea fertilizes crops continuously as soil temperature, moisture, and other factors break down its coating over the course of the growing season.¹³⁴

Nitrogen availability can vary within fields, as factors like prior yields (and thus nitrogen removal at harvest) affect its distribution. Precision agriculture, also called satellite or soil-specific farming, allows farmers to optimize placement via the Global Positioning System (GPS) and other forms of technology

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that use spatial and temporal data about fields.¹³⁵ Precise harvesting machines can track the yield in each small section of each row; improved satellite imagery can accurately estimate plant nitrogen and soil moisture levels in each area; and soil and plant samples can determine soil type and plant needs. These data then inform how and when fields are fertilized as well as irrigated, sprayed with pesticides, and harvested, leading to productivity gains and reduced pollution. The market for precision agriculture technology is developing rapidly, but nonetheless such tools may still be prohibitively expensive for smaller operations.¹³⁶

Farmers can also improve nitrogen placement by applying fertilizer in irrigation water via subsurface drip irrigation (SDI) systems. These systems deliver nitrogen precisely and in proximity to plant roots, increasing plant uptake and limiting excess nitrogen in the soil.¹³⁷ SDI is also less likely to fill soil pore space with water, avoiding the anaerobic conditions that are especially conducive to the generation of nitrous oxide.¹³⁸ At present, SDI systems have been studied only on fruit and vegetable crops.¹³⁹ However, some evidence indicates that SDI systems would be cost effective for corn in the Great Plains.¹⁴⁰ Because corn cultivation uses almost half of the nitrogen fertilizer applied in the United States,¹⁴¹ SDI systems could substantially reduce nitrous oxide emissions.

Some studies have suggested that nitrification inhibitors, chemicals that delay the conversion of ammonium to nitrate, may reduce nitrous oxide emissions by allowing plants to absorb a larger share of nitrogen.¹⁴² However,

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reductions may be modest compared to split application.¹⁴³ Moreover, nitrification inhibitors are antimicrobial pesticides that kill or inhibit the soil microbes involved in nitrification. The broader impact of these inhibitors on soil microbial communities, and soil health and fertility, requires further study.¹⁴⁴ Growers can also reduce net emissions by replacing synthetic nutrients with manure or other organic soil amendments, discussed above.

In addition to climate benefits, reducing excess fertilizer and improving fertilizer management would reduce surface and subsurface runoff of nitrogen, a major source of contamination of rivers, lakes, and drinking water supplies.¹⁴⁵ It can also save farmers money, as fertilizer purchase and application are often a significant expense.

Optimize flood irrigation and drainage in rice cultivation. Flood irrigation of rice fields, a standard part of rice cultivation, causes methane emissions because it creates anaerobic conditions in which methane-producing bacteria thrive.¹⁴⁶ While rice cultivation is a relatively small source of national greenhouse gas emissions, accounting for about 0.2% of all emissions and 2% of agricultural emissions in 2016,¹⁴⁷ the concentration of rice production in two regions, the lower Mississippi River basin and California, makes it an important consideration for policymakers in these areas.¹⁴⁸ Furthermore, increased atmospheric CO₂ concentrations, temperatures, and natural flood risks may increase methane emissions from rice cultivation over time—one study estimated that emissions per ton of rice may double by 2100.¹⁴⁹

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Rice farmers can lower methane emissions by reducing the continuous flooding during the growing season and instead using alternate wetting and drying. Periodic drainage temporarily restores aerobic conditions, which rapidly diminishes the amount of methane-producing bacteria and stimulates other bacteria that metabolize methane for energy.¹⁵⁰ The Intergovernmental Panel on Climate Change estimated that, on average, draining once per season reduces emissions by 40% while draining multiple times reduces emissions by 48%.¹⁵¹ In 2016, California approved a protocol for rice farmers to quantify reductions at the farm level as the basis for generating credits under the state’s cap-and-trade program; this system may incentivize rice growers to adopt mitigation practices.¹⁵² Periodic drainage, which requires farmers to suspend irrigation, uses less water and can help farmers and communities in areas that experience water shortages.¹⁵³