On the Air

Nitrogen Deposition Effects on the Terrestrial Environment

Introduction

Excess amounts of reactive nitrogen (N) in the atmosphere have been implicated in global climate change—as contributors to tropospheric ozone, as visibility-reducing fine particulate matter (PM_2.5), and as indirect and direct influences on aerosols and their radiative properties. Analysis of the effects on undisturbed ecosystems from long-term exposure resulted in a concept known as nitrogen saturation. Nitrogen saturation occurs when N in the environment exceeds the levels of other nutrients (especially calcium, magnesium, and potassium) required by organisms to function. This situation may cause changes in plant and tree productivity, alter competition among plant species, alter the structure of microbial communities, and may result in the loss of essential nutrients. The potential consequences of N saturation on terrestrial systems include loss of species biodiversity, changes in forest species composition, and increased incursion by more tolerant invasive species.

Nitrogen deposited from the atmosphere originates primarily from two kinds of human activities—the combustion of fossil fuels and agriculture. Combustion of fossil fuels such as petroleum and coal generates emissions that form nitrogen oxides (NOx) in the atmosphere and is the major contributor to N deposition. In 2002, the transportation sector contributed 56 percent of the total United States (U.S.) NOx emissions, while the electric utility sector contributed 37 percent with the balance coming from industrial and miscellaneous sources. A lesser, but important source, agricultural releases of N are primarily in the form of ammonia (NH₃) from fertilizer manufacturing and livestock production activities, but also as organic N from nitrogen-fixation processes in the cultivation of legumes and other crops. Unlike transportation and utility NOx emissions, agricultural NH₃ emissions are not regulated. NOx and NH₃ can be transported long distances and eventually are deposited on land and water as nitrate (NO₃^-) and ammonium (NH₄⁺) through precipitation (rain, snow, sleet, fog) in wet deposition or as gases and particles in dry deposition. This process is known as nitrogen deposition. The deposited N that is not utilized or absorbed by the ecosystem becomes available as NO₃- which may be leached to groundwater and streams. Detection of increased levels of NO₃- in streams is indicative of N saturation.

High levels of N deposition to terrestrial ecosystems do not necessarily result in N saturation. High-elevation forests show a greater sensitivity to high N deposition levels than do low-elevation sites. Many deciduous and coniferous forests in the eastern U.S. are limited by N availability. For example, many pine forests in the southeastern U.S. are N deficient so that their growth may be improved by additional N deposition. Factors such as climate, forest age, insect, and fire stressors also can make determination of the exact causes of N saturation difficult.

Evidence of Nitrogen Saturation

Nitrogen deposition in the northeastern U.S. is estimated to have increased 10 to 20 times above pre-industrial levels, with 60 to 80 percent of the deposition occurring as NO₃- and most of the remainder as NH₄⁺. As a consequence, approximately 36 percent of forests in the northeastern U.S. are potentially vulnerable to the early stages of N saturation. A strong N deposition gradient exists in the northeastern U.S. extending from moderately high rates of greater than 7 kilograms of N per hectare per year (>7 kg N/ha/yr) in southeastern New York and Pennsylvania northward to low rates (<4 kg N/ha/yr) in Maine (Figure 1). The increased levels of N deposition in the northeastern U.S. are thought to be at least partially explained by significant population growth, by significant changes in land-use (primarily farming in the pre-industrial era as compared to the urban environment of today), and by the large amount of reactive N the area receives.

Figure 1. Modeled annual nitrogen deposition gradient in the northeastern U.S.

Nitrogen deposition levels vary spatially across the southeastern U.S. as well (Figure 2). Evidence of N saturation has been documented in several high-elevation areas, including portions of the Appalachian Mountain range, where deposition may be substantially higher than that in the surrounding lowlands. High elevation sites receive greater amounts of dry deposition and cloud water deposition of N. Sites within the southeastern U.S. also receive a greater amount of precipitation than northeastern sites, a contributing factor to increased N cycling through an ecosystem.

Figure 2. Wet N deposition (from nitrate and ammonium) in the southeastern U.S. for 2002 (altered from www.nadp.sws.uiuc.edu).

Nutrient cycling research conducted prior to 1995 in the Great Smoky Mountains National Park (GSMNP) detected the loss of nitrogen to streams, a symptom of N saturation. Additional research showed that high-elevation forests in the GSMNP experienced the highest N loading found in the eastern U.S. It is estimated that of the 28 kg N/ha/yr deposited from the atmosphere at the high elevations of the GSMNP, approximately 10 to 20 kg N/ha/yr is lost in runoff. This same research indicated that most hardwood and pine forests in the eastern U.S. were not N saturated, and in fact these forests may exhibit higher productivity as a result of increased N deposition.

In contrast to eastern ecosystems, western U.S. ecosystems receive mostly dry N deposition. Spatial variability in the western U.S. is high. Chaparral watersheds in the San Gabriel Mountains near Los Angeles have reported NO₃- levels in streams that are about one thousand times higher than in chaparral regions outside the South Coast Air Basin in Southern California. In Colorado, the National Park Service reports evidence of nitrification in high-elevation alpine systems of the Rocky Mountains.

Current Nitrogen Regulations

Nitrogen dioxide (NO₂), ozone (O₃), and particulate matter (PM₁₀ and PM_2.5) are three of six "criteria" pollutants regulated under Title I of the Clean Air Act (CAA) as National Ambient Air Quality Standards (NAAQS, Table 1). The NAAQS were established by the U.S. Environmental Protection Agency (EPA) at levels deemed necessary to protect public health with an adequate margin of safety, and to protect against decreased visibility and damage to crops, materials, animals, and vegetation. As required by the CAA, states developed State Implementation Plans (SIPs) to achieve and maintain these and other NAAQS by limiting the amount of N emitted by sources within their jurisdictions. Substantial reductions in NOx emissions from power plants already have occurred and further reductions are expected under Title II of the CAA.

NAAQS Criteria Pollutant	Primary Standards (Averaging Times)
Nitrogen dioxide (NO2)	53 ppba (100 µg/m3)b (annual arithmetic mean)
Ozone (O3)	120 ppb (1 hour) and 80 ppb (8 hour)
Particulate Matter	PM10: 150 (24 hour) and 50 µg/m3 (annual arithmetic mean); : 65 (24 hour) and 15 µg/m3 (annual arithmetic mean)

a. parts per billion (ppb); b. micrograms/cubic meter of air (ug/m ³)

Table 1. National Ambient Air Quality Standards related to nitrogen emissions.

In 1998, the EPA determined that SIPs in 22 eastern states and the District of Columbia (D.C.) were not sufficient to prevent elevated ozone levels in downwind states. EPA required these states to establish additional N emission reduction requirements in a rulemaking commonly referred to as the NOx SIP Call. Phase I of the rule, as amended, required 19 states and D.C. to submit plans to reduce NOx emissions during the ozone season starting in May 2004. Phase II requires Georgia and Missouri to meet a compliance date of May 1, 2007. Reductions of approximately one million tons per year of NOx will be achieved under the NOx SIP Call Rule. Mobile sources, including cars and light trucks, account for approximately 56 percent of total NOx emissions and are regulated under Title II of the CAA. Nitrogen oxides emission standards implemented in 1994, known as "Tier 1" standards, range from 0.4 grams per mile (g/mi) for cars, 1.0 g/mi for diesel cars, and 1.1 g/mi for heavy light-duty trucks over 5,750 pounds. "Tier 2" NOx standards, finalized in 1999, require manufacturers of U.S. cars and light trucks to meet an average 0.07 g/mi beginning in 2004, whether they use gasoline, diesel, or alternative fuels.

Title IV provisions of the CAA focus on coal-fired electric utility sources of N under the NOx reduction strategies of the Acid Rain Program. Unlike the sulfur dioxide (SO₂) reductions mandated under Title IV, there was no cap set on NOx emissions nor were any allowance trading programs established (although utilities are allowed to average NOx emission rates across multiple units). Phase I of the program reduced annual NOx emissions by over 0.4 million tons per year between 1996 and 1999. Phase II of the program began in 2000 and set lower emissions limits. The overall annual reduction per year in Phase II is expected to reduce NOx emissions by an additional 1.7 million tons.

Additional, more stringent, NOx emissions regulations are being proposed by the EPA under the Clean Air Interstate Rule (CAIR). This regulation focuses on reducing NOx and sulfur emissions in 29 eastern states in two phases beginning in January 2010. These new limits are anticipated to be reached through the enforcement of a cap-and-trade program in the utility sector. If these regulations are adopted, the 29 states would be required to respond to the EPA through a SIP by mid-2006. The CAIR potentially would result in a 65 percent reduction in utility NOx emissions below current levels.

Modeled Nitrogen Reduction Scenarios

Mathematical models incorporating NOx reduction strategies for current and proposed regulations have been tested to estimate the effect on N-saturated ecosystems in the northeastern and southeastern U.S. and for various locations across Europe. Models applied to two northeastern U.S. experimental forested watersheds showed that a 90 percent reduction beyond Tier 2 standards would be needed to reduce N deposition levels to a targeted 8 kg N/ha/yr in these watersheds. This target was based upon the deposition level where stream water begins to show increases in NO₃^- concentration. In one of the modeled watersheds, additional large decreases in N emissions from agricultural operations and electric utilities still were not predicted to mitigate acid rain effects on streams by 2050.

The Southern Appalachian Mountains Initiative (SAMI) has modeled the environmental effects of several emission control strategies. Results indicate that both stream water N levels and forest health indicators at the highest elevations would change only slightly by the year 2010, regardless of the particular control strategy adopted. One particular scenario assumed implementation of emission reductions already established under the CAA and its amendments, and under several recently promulgated regulations (the NOx SIP Call and Tier 2 vehicular emission rules). Modeling of this scenario predicted a small decrease (11 percent) in N deposition levels in high elevation forests and 30 percent fewer strongly acidic streams than did a scenario that assumed no change at all in control measures from 1995 levels. These strongly acidic high-elevation streams only represent approximately 2 percent of the stream length for the southern Appalachian region. Under all scenarios modeled, changes in total N deposition were smaller than expected because N emissions reductions from the utility and transportation sectors were offset by increases in NH₃ emissions from livestock and agriculture.

Conclusions

Research on N saturation has demonstrated that too much N in an ecosystem may result in NO₃- leaching into streams, which in turn causes other essential nutrients to leave the soil. High-elevation forests show a greater sensitivity to high N deposition levels than do low-elevation sites. Many confounding factors exist—such as climate, forest age, insect and fire stressors—making the determination of the exact causes of N saturation difficult.

Multiple sources contribute to the levels of N deposition that have direct and indirect impacts on terrestrial and aquatic environments. Sources of N emissions include the transportation sector, fossil fuel combustion from electric utilities, and agricultural sources. While agricultural sources of NH₃ are not currently regulated, Titles I (NAAQS and SIPs), II (motor vehicles) and IV (acid rain) provisions of the CAA require reductions in NOx emissions. Uncertainty remains, however, as to whether the resulting reductions will lower N emissions sufficiently to reverse symptoms of N saturation in some locales. Results from a variety of model projections suggest that further large reductions in NOx emissions would reduce N deposition levels by only a small amount and that many years will be required for N saturation recovery.

Contacts

For more information on this and other air quality issues, please contact:

L. Suzanne Fisher, 865-632-6935
William J. Parkhurst, 256-386-2793
Frances P. Weatherford, 256-386-2344

If you would like additional information on important air quality topics, please contact Jeanie Ashe by telephone (256-386-2033), E-mail (jbashe@tva.gov), facsimile (256-386-2499), or TVA mail at CTR 1K-M, Muscle Shoals, Alabama 35662.