Estimated U.S. anthropogenic nitrous oxide emissions totaled 1.224.2 thousand metric tons in 1999, 0.1 percent more than in 1998 and 5 percent above 1990 levels (Table 24). Nearly all the increase from 1990 levels can be attributed to emissions from the nitrogen fertilization of agricultural soils and emissions from mobile combustion, which grew by 23.3 and 61.5 thousand metric tons, respectively, between 1990 and 1999.

The largest component of U.S. anthropogenic nitrous oxide emissions is emissions from agricultural activities. almost three-quarters of which result from nitrogen fertilization of agricultural soils. Most of the remainder is from the handling of animal waste in managed systems. Smail quantities of nitrous oxide are also released from the burning of crop residues. Estimated emissions of nitrous oxide from agricultural sources were 870.1 thousand metric tons in 1999, less than 1 percent below 1998 levels but 3 percent above 1990 levels (Figure 6).

There are large uncertainties connected with the emissions consequences of adding nitrogen to agricultural soils. Models used for estimation are based on limited sources of experimental data.46 The uncertainty Increases when moving from emissions associated with animal manure to soil mineralization and atmospheric

deposition, where both estimating emissions and partitioning emissions between anthropogenic and biogenic sources become increasingly difficult.

The second-largest source of anthropogenic nitrous uxide emissions is energy consumption, which includes mobile source combustion from passenger cars, buses, motorcycles, and trucks and stationary source combus. tion from commercial, residential, industrial, and electric utility energy use. Energy use was responsible for the release of 278.9 thousand metric tons of nitrous oxide in 1999, 3 percent higher than in 1998 and 32 percent higher than in 1990.

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Nitrous Oxide Emissions

Nitrous oxide emissions from motor vehicles are caused primarily by the conversion of pollutant nitrogen oxides (NO) into nitrous oxide (NO) by vehicle catalytic converters. The normal operating temperature of catalytic converters is high enough to cause the thermal decomposition of nitrous oxide. Consequently, it is probable that nitrous oxide emissions result primarily from "cold starts" of motor vehicles and from catalytic converters that are defective or operating under abnormal conditions. This implies that the primary determinant of the level of emissions is motor vehicle operating conditions. however, different types of catalytic converters appear to differ systematically in their emissions, and emissions probably vary with engine size. Thus, emissions also depend on the "mix" of vehicle age and type on the road.

Stationary Combustion

During combustion, nitrous oxide is produced as a result of chemical interactions between nitrogen oxides (mostly NO2) and other combustion products. With most conventional stationary combustion systems, high temperatures destroy almost all nitrous oxide, limiting the quantity that escapes; therefore, emissions from these systems are typically low. In 1999, estimated nitrous oxide emissions from stationary combustion sources were 51.1 thousand metric tons, 4 percent higher than in 1998 and 15 percent higher than in 1990 (Table 26). Nearly two-thirds (63 percent) of the emissions increase from this source between 1990 and 1999 can be attributed to coal-fired electricity generation, which grew in response to the growing demand for electricity and lower costs and improved availability at coal-fired power plants. Coal-fired combustion systems produced 59 percent of the 1999 emissions of nitrous oxide from stationary combustion, and electric utilities accounted for 55 percent of all nitrous oxide emissions from stationary combustion. www

On a global scale, agricultural practices contribute approximately 70 percent of anthropogenic nitrous axide emissions.47 Similarly, in the United States, agricultural activities were responsible for 71 percent of 1999 nitrous oxide emissions. Seventy-three percent of agricultural emissions are associated with nitrogen fertilization of agricultural soils (Table 24). Nearly all the remaining agricultural emissions can be traced to the management of the solid waste of domesticated animals. The disposal of crop residues by burning also produces nitrous oxide that is released into the atmosphere; however, the amount is relatively minor, at 1.6 thousand metric tons or 0.2 percent of total U.S. emissions of

nitrous oxide from agricultural sources in 1999. Nitrous oxide emissions from agricultural activities grew by 3 percent between 1990 and 1999.

Nitrogen Fertilization of Agricultural Soils

Nitrogen uptake and nitrous oxide emissions occur naturally as a result of nitrification and denitrification processes in soil and crops, generally through bacterial action. When nitrogen compounds are added to the soil, bacterial action is stimulated, and emissions generally increase, unless the application precisely matches plant uptake and soil capture. 18 Nitrogen may be added to the soil by synthetic or organic fertilizers, nitrogen-fixing crops, and crop residues. Nitrogen-rich soils, called "histosols," may also stimulate emissions.

Adding excess nitrogen to the soil also enriches ground and surface waters, such as rivers and streams, which generate indirect emissions of nitrous oxide. Additional indirect emissions occur from atmospheric deposi

tion," in which soils emit other nitrogen compounds that react to form nitrous oxide in the atmosphere. EIA estimates that a total of 636.8 thousand metric tons of nitrous oxide was released into the atmosphere as a result of direct and indirect emissions associated with fertilization practices in 1999 (Table 27). Estimated emissions increased by 4 percent compared with 1990 and decreased by less than 1 percent compared with 1998. Nitrous oxide emissions from the application of nitrogen-based fertilizers and biological fixation in crops accounted for 61.5 percent of total nitrous oxide emissions from this source during 1999.

Nitrous Oxide Emissions

3). In 1999, estimated emissions of nitrous oxide from crop residue burning were 1.6 thousand metric tons, down by 3 percent from 1998 levels (Table 24). The small decrease is mainly attributable to reduced corn, soybean, and wheat production. Emissions from this source remain very small, at 0.1 percent of all U.S. nitrous oxide emissions.

Solid Waste of Domesticated Animals

Estimated 1999 nitrous oxide emissions from aniinal waste management were about 231.7 thousand metric tons, down by 1 percent from 1998 levels and 1 percent higher than 1990 levels (Table 28), making animal waste the second-largest U.S. agricultural source of nitrous oxide emissions, after nitrogen fertilization of soils. Nitrous oxide emissions from animal waste are dominated by emissions from cattle waste, which account for 94 percent of emissions from the solid waste of domesti cated animals. Thus, changes in estimated emissions result primarily from changes in cattle populations. Cattle populations grew during the first half of the decade, leading to higher emissions through 1995, but have since declined slowly, bringing emissions close to 1990 levels. Nitrous oxide is released as part of the microbial denitrification of animal manure. The total volume of nitrous oxide emissions is a function of animal size and manure production, the amount of nitrogen in the animal waste, and the method of managing the animal waste. Waste managed by a solid storage or pasture range method may emit 20 times the nitrous oxide per unit of nitrogen content than does waste managed in anaerobic lagoon and liquid systems. Generally, solid waste from feedlot beef cattle is managed with the solid storage or pasture range method, accounting for the majority of nitrous oxide emissions. Solid waste from swine is generally managed in anaerobic lagoons and other liquid systems. Anaerobic digestion yields methane emissions but only negligible amounts of nitrous oxide.

Waste Management

Nitrous oxide emissions from waste management are estimated at about 18.5 thousand metric tons for 1999, 2 percent of all U.S. anthropogenic nitrous oxide emissions (Table 24). During 1999, emissions from human sewage in wastewater were responsible for 96 percent of the estimated emissions from this source, and the remainder was associated with waste combustion. Estimated emissions from waste management grew by 12 percent between 1990 and 1999 and by 1 percent between 1998 and 1999. Because of the lack of reliable