Acid Deposition Impacts to Forest and Aquatic Ecosystems


Acid Deposition

The atmosphere is a complex mixture of gases and other compounds and some are considered air pollutants because they can decrease visibility and have an adverse impact to people's health or to forest and aquatic ecosystems.  As the atmosphere moves across the landscape the air pollution can be deposited on the forest vegetation and soils.  Scientists refer to this as dry deposition, but this is not the only means that pollutants are deposited from the atmosphere.  Air pollutants can also travel though the atmosphere in the clouds and are deposited when it rains, or snows -- also called acid rain.  The third method of deposition is when fog or clouds intercept the landscape, especially the tops of mountains.  The amount of acid compounds deposited from clouds can be far greater than from dry deposition or rainfall and snow.

The primary compounds in the atmosphere that contribute to acidification of forested ecosystems are:

  • Sulfur compounds – Sulfur dioxide (SO2) is converted in the atmosphere and forms sulfates and sulfuric acid.  Sulfur dioxide is released primarily from coal-fired power plants.
  • Nitrogen compounds – Nitrogen oxides and ammonia (NH4) can increase nitrogen deposition.  Most forests types respond favorably to nitrogen, which is usually limiting, except old growth spruce-fir ecosystems.  Automobiles and utilities are the major sources of nitrogen oxides.  Animal feed operations are the major source of ammonia.

In the southern Appalachian Mountains it is the deposition of sulfur compounds that are of greatest concern.  The deposition of acid compounds in high concentrations or for a long time periods can impact forest nutrient cycling of base cations.   Excessive removal of base cations from forest soils can lead unhealthy vegetation, and poor water quality for aquatic biota. Forest Service Researchers have utilized the Nutrient Cycling Model (NuCM) to evaluate the impacts of sulfur depostion at 3 federally mandated Class I areas -- Joyce Kilmer/Slickrock, Linville Gorge and Shining Rock Wilderness. Click here to download a copy of the publication.

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Recently, the USDA Forest Service has gathered water chemistry and soils data in the states of North Carolina, South Carolina, and Tennessee to be used (along with other information) in an atmospheric deposition effects modeled called Model of Acidification of Groundwater (MAGIC). The principal objectives of the study were to:

  1. 1. Use MAGIC to estimate future trends for stream chemistry and soil base saturation under a range of future atmospheric deposition scenarios.

  1. 2. Use MAGIC to estimate changes in stream chemistry and soil base saturation since pre-industrial time.

  1. 3. Develop an approach to predict critical load or future stream ANC for a new stream, based on the known current stream water chemistry, geology, elevation, and forest cover of the catchment.

  1. 4. Estimate the critical loads of sulfur deposition needed to protect those streams that are not yet acidic, and to restore streams that are already acidic to specific chemical criteria values.

  1. 5. Evaluate model uncertainty and regional representativeness of the modeled streams.

  1. 6. Assess, based on available data, episodic variability in stream chemistry and biological dose-response relationships within the study area.

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Select the first two links to download a copy of the MAGIC report:

1.  Report

2.  Appendices


Download Field Sample Protocol and Data Sheets:

1. Water chemistry and soil sampling protocols (revised 01/26/12)

2. Chain of custody form and instructions

3. Stream data form instructions

4. Stream data forms


Also, you can download a presentation describing how sulfur deposition impacts nutrient cycling and some of the findings from the reports above:

1. Presentation (Power Point, 34 mb)

updated: 3/11/13

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