Baltimore Ecosystem Study Institute of Ecosystem Studies

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Why is Nitrate important?

Nitrate, represented by NO3-, is a vital component in all living systems because it is the most readily assimilated form of nitrogen by plants. It is beneficial for plants to consume because:

  • Nitrate serves as an essential plant nutrient helping with tissue development and building immune systems, and
  • Nitrate helps plants develop and produce seeds. Most of the generic fertilizers have high concentration of nitrate, which are essential for plant growth and survival.
Nitrate is formed from the chemical combustion of nitrogen and oxygen. Although nitrogen constitutes almost 79% of earth's atmosphere, nitrogen is mostly inert. This inert / inactive form of nitrogen (N2) is unable to react with oxygen to form nitrate. Thus nitrogen in the form N2 is not available for uptake (assimilation) by plants. To be useful to plants, nitrogen must be converted into active forms. Natural events such as lightning and cosmic radiation create nitrates in the atmosphere, which are then brought out of the atmosphere and to the ground by precipitation (rain, snow, sleet, etc.). This process of nitrate formation is known as nitrogen fixation. There are two types of nitrogen fixation:
  1. Non-biological fixation: Also known as high-energy fixation is aided by lightning, cosmic radiation, meteorite trails. These naturally occurring events provide high energy needed to combine nitrogen with oxygen and hydrogen found in water. Research has shown that less than 0.4 kg N / ha (kilogram of Nitrogen per hectare of land) comes to Earth annually via this pathway.1
  2. Biological fixation: Studies have shown that this pathway produces approximately 10 kg N/ha, which is roughly about 90 percent of the fixed nitrogen contributed each year.1 As the name suggests, a biological component is involved in the fixation process. Bacteria living on roots of crops such as alfalfa and clover fix nitrogen in the soil. These bacteria are known as nitrogen fixing bacteria.

Un-natural Sources of Nitrates:

  • Discharge from wastewater treatment plants,
  • Runoff from fertilized lawns and cropland,
  • Failing septic systems,
  • Runoff from animal manure storage areas, and
  • Industrial discharges that contain corrosion inhibitors.

Relevance of nitrate study

Nitrate is the most common form of nitrogen exported from terrestrial ecosystems to freshwater systems. Excess levels of nitrates in water can create conditions that make it difficult for aquatic organisms to survive.2 The increase in magnitude of nitrate in aquatic ecosystems can be directly correlated with accelerated rates of eutrophication or massive algal blooms in water systems.3 These blooms can create oxygen deprived conditions for fish and other benthic (bottom-dwelling) macroinvertebrates.

Most nitrate studies have been focused around intermediate zones between streams and land, also known as riparian corridors or riparian zones. Forested riparian zones usually convert nitrate to gaseous nitrogen thus, reducing nitrogen loading to streams. This process is known as denitrification and is mainly carried out by denitrifying bacteria. This group of bacteria reduces NO 3- (nitrate) to N2O (nitrous oxide) and N2 (nitrogen gas) both of which are inactive forms of nitrogen. Riparian zones are known as nitrate sinks because of the abundance of these bacteria throughout the corridor. BES researchers wish to know whether this same process (denitrification) was at work in riparian zones throughout the Baltimore metropolitan area. Researchers found that urban riparian zones, which were assumed to be nitrate sinks, were not as large a sink as anticipated.3

According to the findings from the research conducted, three factors are believed to play crucial roles:

  1. Ground water recharge to water tables in riparian zones is reduced in urban areas due to impervious surfaces.
  2. Much of the drainage from impervious surfaces is channeled not through streams, but through a storm drain sewer system.
  3. Urban hydrology is extremely flashy, which means that during storms, floods rise quickly and flow at very large volumes for short periods. Such flash floods erode urban streams deep into the substrate, often to bedrock.4 This deepening of stream channels separates the water table from the floodplains where denitrification would otherwise occur. The riparian soils thus become aerobic, which inhibits the anaerobic process of denitrification.3

Figure 1: Urban and suburban watersheds consistently have nitrate concentrations that are higher than forested watersheds, but lower than agricultural watersheds.5

Detailed research findings can be found at the BES website.

Impacts on Ecosystem

  • Accelerate algal and phytoplankton blooms resulting in eutrophic conditions,
  • Toxicity in warm-blooded animals at higher concentrations, and
  • Create hypoxia resulting in low dissolved oxygen conditions in aquatic habitats


  1. Smith, R. L., & Thomas M. Smith., 2003. Elements of Ecology. Pearson Education, CA. p 438-439.
  2. EPA Volunteer Stream Monitoring: A Methods Manual Section 5.7 – Nitrates. Accessed on 8/8/2009.
  3. Groffman P. M., Daniel J. Bain, Lawrence E. Band, Kenneth T. Belt, Grace S. Brush, J. Morgan Grove, Richard V. Pouyat, Ian C. Yesilonis, Wayne C. Zipperer., 2003. Down by the Riverside: Urban Riparian Ecology. Frontiers in Ecology and the Environment, Vol. 1, No. 6, pp. 315-321
  4. Cadenasso M. L., Steward T.A. Pickett, Morgan J. Grove., 2006. Integrative approaches to investigating human-natural systems: the Baltimore Ecosystem Study.Natures Sciences Sociétés 14, 4-14.
  5. Baltimore Ecosystem Study Stream and Watershed Studies Overview

The National Science Foundation - The Long Term Ecological Network Website - The USDA Forest Service - United States Geological Survey - The Parks and People Foundation -

This material is based upon work supported by the National Science Foundation under Grant No. 0423476. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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