Nitrogen Cycle

Planetary Boundary Tipping Points: Nitrogen Cycle


Diaz RJ, Rosenberg R. (2008): Spreading dead zones and consequences for marine ecosystems. Science 321, 629. Sanderson E. W, Jaiteh M, Levy M. A. (2002): The Human Footprint and the Last of the Wild. Bioscience 52, 891


Human Alteration of the Global Nitrogen Cycle: Causes and Consequences:

The Human Alteration of the Global Nitrogen Cycle [Vitousek et al (1997): Human Alteration of the Global Nitrogen Cycle: Causes and Consequences, Issues in Ecology, Number 1, Spring 1997; Ecological Society of America PDF] report presents the consensus reached by a panel of eight scientists[2] chosen to include a broad array of expertise in this area. The report underwent peer review and was approved by the Board of Editors of Issues in Ecology. It summarizes the findings of the panel, which were reported in full in the journal Ecological Applications (Volume 7, August 1997), and discusses and cites more than 140 references to the primary scientific literature on this subject.

Human activities are greatly increasing the amount of nitrogen cycling between the living world and the soil, water, and atmosphere. In fact, humans have already doubled the rate of nitrogen entering the land-based nitrogen cycle, and that rate is continuing to climb. This human-driven global change is having serious impacts on ecosystems around the world because nitrogen is essential to living organisms and its availability plays a crucial role in the organization and functioning of the world’s ecosystems. In many ecosystems on land and sea, the supply of nitrogen is a key factor controlling the nature and diversity of plant life, the population dynamics of both grazing animals and their predators, and vital ecological processes such as plant productivity and the cycling of carbon and soil minerals. This is true not only in wild or unmanaged systems but in most croplands and forestry plantations as well. Excessive nitrogen additions can pollute ecosystems and alter both their ecological functioning and the living communities they support.

Most of the human activities responsible for the increase in global nitrogen are local in scale, from the production and use of nitrogen fertilizers to the burning of fossil fuels in automobiles, power generation plants, and industries. However, human activities have not only increased the supply but enhanced the global movement of various forms of nitrogen through air and water. Because of this increased mobility, excess nitrogen from human activities has serious and long-term environmental consequences for large regions of the Earth.

The impacts of human domination of the nitrogen cycle that they identified with certainty include:

  • Increased global concentrations of nitrous oxide (N2O), a potent greenhouse gas, in the atmosphere as well as increased regional concentrations of other oxides of nitrogen (including nitric oxide, NO) that drive the formation of photochemical smog;
  • Losses of soil nutrients such as calcium and potassium that are essential for long-term soil fertility;
  • Substantial acidification of soils and of the waters of streams and lakes in several regions;
  • Greatly increased transport of nitrogen by rivers into estuaries and coastal waters where it is a major pollutant.

They are also confident that human alterations of the nitrogen cycle have:

  • Accelerated losses of biological diversity, especially among plants adapted to low-nitrogen soils, and subsequently, the animals and microbes that depend on these plants;
  • Caused changes in the plant and animal life and ecological processes of estuarine and nearshore ecosystems, and contributed to long-term declines in coastal marine fisheries.
Nitrogen Cycle: Causes

Dead Zones:

“The primary culprit in marine environments is nitrogen and, nowadays, the biggest contributor of nitrogen to marine systems is agriculture. It’s the same scenario all over the world.” (R. Diaz, Marine Biologist, The College of William and Mary)

‘Dead Zones’ are areas of the Ocean where the bottom water has become anoxic (i.e. has low or zero dissolved oxygen concentration), very few organisms are able to survive in such low oxygen conditions. Dead zones occur along large sections of the coastline of major continents and are continuing to spread over the sea floor, destroying the habitat of many organisms.

These dead zones are created when the organic matter produced by phytoplankton at the surface of the ocean (in the euphotic zone) sinks to the bottom (the benthic zone), where it is broken down by the action of bacteria, a process known as bacterial respiration. This is problematic because while phytoplankton use carbon dioxide and produce oxygen during photosynthesis, bacteria use oxygen and give off carbon dioxide during respiration. The bacteria use up the oxygen dissolved in the water which is essential to all of the other oxygen-respiring organisms on the bottom of the ocean, such as crabs, clams and shrimp, and also those swimming in the water, such as fish and zooplankton.  The overall impact is to make large parts of the ocean uninhabitable for the majority of organisms. For further explanation there is an excellent review of dead zones in the ‘Science Focus’ section of the NASA website, which provided much of the core information for this Nitrogen Cycle Human Impact post.

Nitrogen Cycle: How the Dead Zone Forms

An influential paper on dead zones is from Diaz and Rosenberg, published in Science in 2008, they state that oceanic dead zones have spread exponentially since the 1960s and that this formation of dead zones is exacerbated by anthropogenic influences on nitrogen entering the ocean due to riverine runoff of fertilizers and the burning of fossil fuels. This extra nutrient input fuels coastal eutrophication and the accumulation of particulate organic matter, which encourages microbial activity and the consumption of dissolved oxygen in bottom waters. The resulting lack of oxygen causes fish to migrate away from affected waters and the death of large numbers of less mobile organisms.

Spreading Dead Zones and Consequences for Marine Ecosystems:

Dead zones in the coastal oceans have spread exponentially since the 1960s and have serious consequences for ecosystem functioning. The formation of dead zones has been exacerbated by the increase in primary production and consequent worldwide coastal eutrophication fueled by riverine runoff of fertilizers and the burning of fossil fuels. Enhanced primary production results in an accumulation of particulate organic matter, which encourages microbial activity and the consumption of dissolved oxygen in bottom waters. Dead zones have now been reported from more than 400 systems, affecting a total area of more than 245,000 square kilometers, and are probably a key stressor on marine ecosystems. – Diaz Robert (15 Aug 2008): Spreading Dead Zones and Consequences for Marine Ecosystems; Science, Vol 321, No 5891 pp 926-929.