Critical Loads - Atmospheric Deposition

Fossil fuel burning emits air pollution in the form of sulfur dioxide (SO2) and nitrogen oxides (NOx), while agricultural activities are the primary source of ammonia (NH3) released to the atmosphere.  These emissions lead to the atmospheric deposition of sulfuric acids, nitric acids, and ammonium to ecosystems.

  1. In sensitive ecosystems, these acid compounds can acidify soil and surface waters, affecting nutrient cycling and impacting the ecosystem services provided by forests. Underlying geologic parent material can make some ecosystems resilient to the effects of acidification.
  2. In ecosystems resilient to the effects of acidification, nitrogen deposition can lead to chemical and biological changes through “nitrogen saturation,” which also results in impacts to forest ecosystem services.  The effects of N enrichment/eutrophication are generally more important than soil acidification in most areas of the Western US because of the higher amount of N deposition relative to S deposition.
  3. Toxic air contaminants like mercury, are emitted primarily by coal-fired utilities, and may be carried thousands of miles before entering lakes and streams as mercury deposition.  Sulfate affects conversion of mercury into biologically available methylmercury, which is one of the reasons mercury deposition is included in this discussion.

Sulfate is the primary pollutant of concern in the much of the eastern U.S. with the highest levels of emissions coming from the heavily industrialized Ohio River Valley.  In spite of recent reductions across the eastern U.S., sulfate deposition is still higher than the ecosystems of the Appalachian states can tolerate.  Nitrogen deposition is the primary concern in the mid and western United States.

Atmospheric deposition occurs as wet deposition (rain and snow), dry deposition (gases and particles), and cloud and fog deposition.  Wet deposition forms when NOx and SO2 are converted to nitric acid (HNO3) and sulfuric acid (H2SO4), or when ammonia is converted into ammonium.  Dry deposition can be converted into acids when deposited chemicals meet water (Figure 1). The amount of deposition received in a given area is affected both by the concentration of pollution in the atmosphere and the way in which it is deposited.  General factors such as climate, meteorology, and topography influence how much pollution reaches the area from both local and distant sources, as well as how much of that pollution actually impacts the earth's surface via the various wet and dry forms of deposition.

Figure 1: Nitrogen oxides and sulfur dioxide released into the atmosphere from a variety of sources fall to the ground as wet or dry acid deposition 1.

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Acidic inputs from the atmosphere, mainly sulfate and nitrate, can negatively impact terrestrial and aquatic ecosystems.  Their acidifying effects contribute to degradation of stream and lake water quality by lowering the acid neutralizing capacity (ANC) which represents the water's natural acid buffering system.  In areas such as the central and southern Appalachians, forest streams have acidified to the point where they are no longer capable of sustaining aquatic life such as fish or amphibians.  The sensitivity of lakes and streams to the negative effects of acidic deposition are often linked to natural watershed characteristics, most notably the type of bedrock geology.  Watersheds containing naturally occurring bedrock that weathers (breaks down easily) and that are made up of minerals containing high levels of base cations like calcium (nutrients that plants need) are less susceptible to negative impacts of acidic deposition.  Likewise, watersheds where the soils are derived from bedrock that is resistant to weathering like granite or that contains thin shallow soils are very susceptible to acidic deposition. These same susceptible areas may not only exhibit lake and stream water chemistry changes, but also changes in soil chemistry causing problems such as nutrient leaching.  Nutrient leaching can eventually lead to deficiencies in macro nutrients important for plant growth.

Acidic Deposition Impacts to Nutrient Cycling

Soils maintain baseline concentrations of essential nutrients including calcium (Ca2+), magnesium (Mg2+), and potassium (K+).  These base cations originate from bedrock weathering and the deposition of windblown dust.  Some soils can be inherently low in base cations due to the slow breakdown of parent rock material 2.  Base cations are withdrawn from the soil and used for vegetation growth; they are later returned to the soil during the decomposition of falling leaves and woody material.

After being deposited from the atmosphere into the soil, nitric and sulfuric acid separate into hydrogen ions (H+) and negatively charged sulfate and nitrate particles.  In order to maintain an ionic balance, an equivalent amount of positively charged base cations adhere to the negatively charged sulfates and nitrates and move into the soil water solution, acidifying the remaining soil and fundamentally altering soil processes.  In addition, aluminum begins to dissolve from previously insoluble minerals; Al3+ enters the soil water solution when the soil pH falls below 4.5.

Once in soil water solution the H+, Al3+, and base cations travel until they reach a surface water body.  This can result in acidification of streams and lakes.  Surface waters surrounded by limestone-rich soil and bedrock can buffer the acidity in the soil solution, minimizing surface water acidification (with its increasing Al3+ content) (Figure 2).

Figure 2: Surface water acidification begins with acid deposition to adjacent terrestrial areas 1.

Certain ecosystems are more susceptible to the effects of acid deposition; parent material geology, elevation, and forest overstory can all be used to identify potentially sensitive ecosystems.  Soils developed from sandstone rocks are inherently low in base cations, leaving these areas vulnerable to acidic deposition.  High elevation areas are vulnerable due to increased acidic cloud deposition and decreased soil depth to buffer acid inputs.  Finally, with respect to forest overstory, the breakdown of needles naturally acidifies forest soils, increasing the vulnerability of conifer forests.  These characteristics have been used by USDA Forest Service staff to identify areas vulnerable to the effects of acidic deposition.

Acid Deposition Impacts to Soil Health

When base cations are removed from the soil by acidic deposition at a rate faster than they are replenished by slow mineral weathering or deposition from windblown dust, this results in the reduced availability of Ca2+, Mg2+, and K+ in the soils of acid-sensitive forest ecosystems.  This reduced availability hinders the capacity for sensitive soils to recover from acidic deposition and compromises the health and continued growth of the plants dependent on these nutrients.

Soil base saturation is highly correlated with base cation availability, and can therefore be used to identify sensitive terrestrial ecosystems (ecosystems likely to experience the effects of acid deposition).  Base cation availability is generally not limited when soil base saturation is above 20%, and availability is severely limited and forest mortality effects are likely when soil base saturation is below 5%.  In between these saturation rates, base cation availability is limited and forest growth reductions and mortality risks from various stressors increase 3.

Acid deposition can also lead to an increase in H+ ions in the soil, resulting in decreased soil pH and increased mobilization of aluminum in soils, affecting the soil water solution.  Because of the strong positive charge, Al3+ enters plant roots more easily than other bases, thus displacing other nutrients during uptake, resulting in a nutrient deficiency.  This deficiency is compounded by the toxic effect of Al3+ on fine roots, further reducing the potential uptake of nutrients and water by plants.

Acid Deposition Impacts to Stream Health

Stream acidification is accompanied by decreasing pH levels (increased H+ ions), increasing aluminum concentrations, and decreasing acid-neutralizing capacity (ANC).  ANC is a measure of a water body’s ability to neutralize acidic inputs.  ANC is calculated as the difference in concentrations (µeq/L) between the sum of the base cations (Ca, Mg, Na, K) and the sum of the acid anions (SO4, NO3, and Cl). A reduction in stream ANC further reduces the stream’s ability to buffer against additional acids entering the system.  Decreases in pH and increases in Al3+ result in reduced diversity and abundance of aquatic species (fish, plankton, and invertebrates).  High acidity and Al3+ disrupt the salt and water balance in fish blood, rupturing red blood cells and increasing blood viscosity, resulting in heart attack and suffocation.

ANC is highly correlated with pH, and is the most commonly used indicator of stream health for the protection of streams from acidification.  The protection of aquatic biota is generally based on maintaining surface water ANC at an acceptable level (0, 20, 50, or 100 µeq/L in various European and North American applications) 4, 5. Fish species and aquatic macroinvertebrate community diversity and richness are likely unaffected when average ANC is > 100 µeq/L, while lethal effects on brook trout populations and complete extirpation of fish populations and macroinvertebrate communities are expected when average ANC is < 0 µeq/L.  In between these ANC levels, macroinvertebrate communities begin to decline, followed by fish species richness reductions at ANC levels between 50-100 µeq/L, and eventually lethal and sub-lethal effects on brook trout populations and marked declines in aquatic insect families begin at ANC levels between 0-50 µeq/L 4. The ANC level selected to protect aquatic biota is a policy decision based on balancing these effects with what is known about natural background conditions, the current status of the resource, and competing management concerns.

For more information:

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Nitrogen Saturation/Eutrophication

In addition to nitrogen’s role as an acidifying compound, there is concern about excess input of nitrogen in the ecosystem and N saturation.  This excess nitrogen initially will accumulate in soil and subsequently be lost via leaching.  While increased nitrogen may increase productivity in many terrestrial ecosystems (which are typically N limited) this is not necessarily desirable in protected ecosystems, where natural ecosystem function is desired.  Excess nitrogen can lead to nutrient imbalances, changes in species composition (trees, understory species, nonvascular plants (lichens), or mycorrhizal fungi), and ultimately declines in forest health.

Effects of Excess Nitrogen on Ecosystems 6

The nitrogen gas that makes up most of the Earth's atmosphere is inert, with little impact on ecosystems.  Nitrogen converted to its reactive forms such as NH3 and NOx, however, can cause profound biological changes.  Activities such as fertilizer manufacturing, intensive livestock production, and the burning of fossil fuels convert nitrogen to these reactive forms which can then enter and potentially over-fertilize ecosystems.  Nitrogen deposition rates are likely to be highest downwind of large urban areas and agricultural sources.  Over-fertilization of nitrogen can lead to problems such as algal overgrowth in lakes, reduced water quality, declines in forest health, and decreases in aquatic and terrestrial biodiversity by favoring “nitrogen loving” species at the expense of other species with low nitrogen preferences.  For example, most estuaries and bays in the Northeast U.S. and Mid-Atlantic regions experience some degree of eutrophication (where excess nutrients promote a proliferation of plant life, which can deplete oxygen in the deeper waters), as a result of nutrients from atmospheric deposition and agricultural, urban and industrial runoff.  Excess nitrogen can also change species composition.  In Waquoit Bay, Massachusetts elevated nitrogen allows tall cord grass to thrive but not eelgrass, which decreases critical fish habitat.  Sensitive ecosystem components (e.g., lichen species, diatoms, and streamwater nitrate levels) can be substantially influenced, in some instances, by N deposition levels as low as 3 to 8 kg/ha/yr.

Adding nitrogen to forests whose growth is typically limited by its availability may appear desirable, possibly increasing forest growth and timber production, but it can also have adverse effects such as increased soil acidification, biodiversity impacts, predisposition to insect infestations, and effects on beneficial root fungi called mycorrhizae.  As atmospheric nitrogen deposition onto forests and other ecosystems increases, the enhanced availability of nitrogen can lead to chemical and biological changes collectively called “nitrogen saturation".  As nitrogen deposition from air pollution accumulates in an ecosystem, a progression of effects can occur as levels of biologically available nitrogen increase (Figure 3).  Because of the multiple potential effects of nitrogen deposition in terrestrial and aquatic ecosystems, the ecosystem services affected vary depending on the sensitive receptors found within a given ecosystem and the level of atmospheric deposition.  Prominent examples of affected ecosystem services  in forests include timber production, climate regulation, recreational use, and biodiversity loss. In freshwaters, affected ecosystem services include recreational fishing and provision of high quality drinking water.

Figure 3:  Continuum of nitrogen deposition impacts demonstrated from past observations and potential future effects in Rocky Mountain National Park. As ecosystem nitrogen accumulation continues, additional acidification or eutrophication impacts occur to various ecosystem receptors. The trajectory line is conceptual even though the effects below the current nitrogen deposition level have been documented. Similar trajectories of additional ecosystem effects as nitrogen accumulates in the ecosystem occur in other ecological regions. (Figure courtesy of Ellen Porter, National Park Service).

For more information:

  • Download the briefing paper about the use of empirical CLs for nutrient nitrogen for lichens for an explanation of how to interpret this information for Line Officers.
  • Download the briefing paper about the use of lichen monitoring to identify air pollution impacts on National Forests and use this information to negotiate with point source polluters to lower emissions.
  • Download Porter and Johnson (2007) that describes the process used by the National Park Service to select a nitrogen critical load protective of multiple resources in Rocky Mountain National Park.

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Toxic Methylmercury

Toxic air contaminants like mercury, are emitted primarily by coal-fired utilities, and may be carried thousands of miles before entering lakes and streams as mercury deposition.  Mercury can bioaccumulate and greatly biomagnify through the food chain in fish, humans and other animals.  Non-organic forms of mercury are converted to methylmercury by sulfur reducing bacteria in aquatic sediments.  Methylmercury is a potent neurotoxin, and has been shown to have detrimental health effects in human populations as well as behavioral and reproductive impacts to wildlife.  Almost every state has consumption advisories for certain lakes and streams, warning of mercury-contaminated fish and shellfish.  High concentrations of mercury are measured in sediments and fish tissue, even in remote areas of the Arctic.  Recently, elevated methylmercury loads have been monitored in upland bird species, calling into question the traditional wisdom that methylmercury contamination is directly linked solely to aquatic systems.  The link between sulfur-reducing bacteria and biotic mercury concentrations has led researchers to establish that reductions in sulfur dioxide emissions and a resulting reduction in sulfate deposition will abate mercury concentrations in wildlife.  Consequently, as the level of sulfates is reduced in aquatic systems, sulfur reducing bacteria will reduce less sulfur, and this will in turn lead to less inorganic mercury being methylated.

For more information:


1 Pidwirny, M. (2006). "Acid Precipitation". Fundamentals of Physical Geography, 2nd Edition.

2 Elwood, JW, MJ Sale, PR Kaufmann, GF Cada. 1991. The Southern Blue Ridge Province. In: Charles, DF (Ed.), Acidic deposition and aquatic ecosystems: regional case studies. Springer-Verlag, New York, pp. 319-364.

3 Elliott, KJ, JM Vose, JD Knoepp, DW Johnson, WT Swank, W Jackson. 2008. Simulated Effects of Sulfur Deposition on Nutrient Cycling in Class I Wilderness Areas. Technical Reports: Atmospheric Pollutants and Trace Gases. Journal of Environmental Quality 37: 1419-1431.

4 Cosby, BJ, JR Webb, JN Galloway, FA Deviney. 2006. Acidic deposition impacts on natural resources in Shenandoah National Park. Technical Report NPS/NER/NRTR-2006/066. National Park Service, Philadelphia, PA.

5 Sullivan, TJ, BJ Cosby, W Jackson, KU Snyder, AT Herlihy. 2011.  Acidification and prognosis for future recovery of acid-sensitive streams in the Southern Blue Ridge Province.  Water Air Soil Pollution: 219: 11-26.

6 Fenn, ME, KF Lambert, T Blett, DA Burns, LH Pardo, GM Lovett, R Haeuber, DC Evers, CT Driscoll, DS Jeffries. 2011. Setting limits: Using air pollution thresholds to protect and restore U.S. ecosystems. Issues in Ecology 14: 1-21.

7 Porter, E and S Johnson.  2007.  Translating science into policy: Using ecosystem thresholds to protect resources in Rocky Mountain National Park.  Environmental Pollution 149: 268-280.