Tipping Points and Indicators Fact Sheet - Nitrogen

Department of Forestry and Natural Resources, Purdue University, West Lafayette, IN 47906


  • Humans have altered the nitrogen cycle, resulting in increased levels of reactive, or biologically available nitrogen, that is accumulating in various ecosystems and altering many ecological processes
  • Nitrogen is a key nutrient in eutrophication, especially in marine ecosystems
  • Within freshwater and terrestrial ecosystems, nitrogen has various negative impacts
  • Acidification is a large problem, because it has cascading effects on food web dynamics and heightens metals toxicity (i.e. increases mercury accumulation by fish)
  • Ingestion of groundwater high in nitrates has negative health effects in infants (such as methemoglobinema, or “blue baby syndrome), nursing mothers, women of reproductive capacity, and those with heart, lung, or enzyme conditions.  Long term ingestion of drinking water high in nitrates is also linked to cancer. 

      Inputs of reactive nitrogen to the Earth’s ecosystems increased by a factor of 20 since 1860 (Galloway & Cowling 2002).  The forms of reactive nitrogen that affect aquatic ecosystem include inorganic dissolved forms (i.e. nitrate, ammonium), a variety of dissolved organic compounds (amino acids, urea, and composite dissolved organic nitrogen (DON)), and particulate nitrogen. 

      The effects of nitrogen enrichment in the environment are numerous, including both detrimental and beneficial effects (Rabalais 2002).  Primary producers are the first to respond to increasing nutrient loads by increasing production.  This often leads to eutrophication, which is, according to Nixon (1995): “the increased accumulation of organic matter, usually as a result of increased nitrogen and phosphorus inputs, but could result from the supply of excessive decomposable organic carbon as well.”

     Despite the fact that nitrogen is generally considered the primary limiting nutrient for phytoplankton biomass accumulation within the marine ecosystem (Howarth 1988, Ryther & Dunstan 1971), studies have linked increased nitrogen inputs into marine ecosystems to the negative effects of eutrophication.  However, the limiting nutrient within these marine ecosystems also tends to change with season (i.e. phosphorus limitation may be more pronounced in the spring) and water body (estuarine vs. coastal systems) (Howarth 1988).  The ultimate symptom of eutrophication in marine ecosystems is a loss or degradation of habitat with negative consequences to marine biodiversity, shifts in community structure in both pelagic and benthic systems, and degraded habitats of coral reefs, seagrass beds, and productive continental shelves with important commercial fisheries. 

     Within freshwater ecosystems, the effects of atmospheric nitrogen and its contribution to acidification of fresh waters can be detrimental. While phosphorus is considered the limiting nutrient for phytoplankton production in freshwater systems (Hecky & Kilham 1988), evidence points to combinations of phosphorus and nitrogen as limiting for both algae and vascular plants in a variety of freshwater systems, including lakes, reservoirs, and streams (Smith et al. 1999). 

     Freshwater systems that are poorly buffered by surrounding soils can be acidified by increased deposition of nitrate and ammonium (Rabalais 2002).  The continuing acidification of Europe, northeaster and North America as well as parts of Asia is now increasingly a nitrogen-pollution rather than sulfur-pollution problem (Driscoll et al. 2001).  Acidic deposition has resulted in acidification of soil waters, shallow groundwater, streams, and lakes with cascading effects on the trophic structure of surface waters.  Besides direct mortality to acid-sensitive fish from acidic waters, inorganic monomeric aluminum (Al) is directly toxic to fish and increases with increasing acidity.  Surface water acidification enhances mercury accumulation in fish (Heath 1995).  Additionally, acidification of surface waters results in a decrease in the survival, size, and density of fish and in the loss of fish and other aquatic biota from lakes and streams (Rabalais 2002). 

      Lowered pH in surface water bodies cause a myriad of changes in freshwater systems.  Even small changes in pH can shift community composition in streams because pH alters the chemical state of many pollutants, such as copper and ammonia, changing their solubility, transport, or bioavailability.  This can increase exposure to and toxicity of metals and nutrients to aquatic plants and animals.  Acid rain, for example, tends to mobilize and leach metals such as aluminum into groundwater and to streams resulting in higher dissolved metal concentrations in streams combined with low pH.  Metals such as aluminum become increasingly bioavailable with decreasing pH (<6.0) due to increases in the free ionic form, which results in greater toxic effects at low pH than at neutral or high pH (Howells et al. 1983; Playle et al. 1989).  Depending on the concentrations of metal present and the pH level, pH may be the dominant direct cause of biological effects (with metals being contributors to the effect), or metals may be the dominant direct cause of effects (with pH being a contributor), or the combination of the two may be the cause (i.e., neither acting alone would have caused observed effects) (US EPA 2010).

     In addition to eutrophication, excess nitrogen can have other effects to terrestrial landscapes and freshwater systems.  Excess atmospheric deposition of nitrogen on temperate forests can lead to increased productivity but loss of biodiversity.  However, effects of nitrogen deposition on tropical rainforests are less certain (Matson et al. 2002).  Once nitrogen is transformed through microbial processes in soils to a biologically available form in ground and surface waters, excess nitrogen can become a groundwater contaminate and acidify surface-waters (Rabalais 2002). 

      Nitrate concentration in groundwater and surface water is normally low but can reach high levels as a result of leaching or runoff from agricultural land or contamination from human or animal wastes as a consequence of oxidation of ammonia and similar sources (WHO.org 2003).  Nitrate leaches into groundwater and surface waters from inorganic fertilizers, and nitrite forms and persists in groundwater as a result of anaerobic conditions.  Ingestion of water contaminated by nitrates and nitrites poses health risks, especially to infants and women of reproductive capacity (WHO.org 1998).

      All infants under 6 months old are at an elevated risk of nitrate poisoning.  Numerous studies have found that infants exposed to high nitrate concentrations through drinking water can develop methemoglobinema, also called “blue baby syndrome” because the skin appears blue-gray or lavender in color.  This color change is caused by a lack of oxygen in the blood (Bouwer 1989; WHO.org 1998) and requires immediate medical care because it can rapidly lead to coma and death (WHO.org 2003; dnr.state.wi.us 2003).  Therefore, feeding infants formula made with groundwater high in nitrates, or breastfeeding infants while drinking nitrate-contaminated drinking water poses health risks; because the amount of nitrate in breast milk may increase.  Although no confirmed cases of "blue-baby syndrome" have been associated with nitrate in breast milk, the World Health Organization (WHO) recommends nursing women to avoid drinking water that contains more than 50 milligrams per liter nitrate-nitrogen (WHO.org 1998; WHO.org 2003).

     Some scientific studies have found evidence suggesting that women who drink nitrate-contaminated water during pregnancy are more likely to have babies with birth defects.  Nitrate ingested by the mother may also lower the amount of oxygen available to the fetus (dnr.state.wi.us 2003). 
Ingestion of nitrate-contaminated drinking water has also been linked to aggravated health effects in those with cancer and heart, lung, or enzyme defects (dnr.state.wi.us 2003).  A high concentration of nitrate-nitrogen in drinking water leads to the production of nitrosamine, a potential carcinogenic, and some experts believe that long-term ingestion of water high in nitrate may increase the risk of certain types of cancer (Bouwer 1989; WHO.org 2003).