Nitrogen and Ecosystem Nutrient Cycling Nicole and Sarah Biogeochemistry of Northern Ecosystems March 2005

 Interactions between plants, animals, and soil microbes link the internal biogeochemistry of terrestrial ecosystems.  Essential life elements: C, N, P, S, P, Ca, Mg, F -often limiting and their supply may control NPP  Biosphere exerts strong control on major life elements- intrasystem cycle is the annual cycle of nutrients in vegetation and is responsible for long-term retention of elements Cycling Essential Life Elements

Annual Nutrient Requirements for Growth (Schlesinger, 1997 p.168)

 Out of those elements required to sustain life, Nitrogen has the greatest total abundance in the atmosphere, biosphere, and hydrosphere but it is the least readily available to sustain life.  In the form of molecular nitrogen, two nitrogen atoms are held together by a triple bond. Only a small number of N-fixing species have the energy required to break apart this bond.  There are two categories of Nitrogen in nature, N which is the non-reactive molecular form of nitrogen and Nr which is all the reactive nitrogen in the earths atmosphere and biosphere.  Reactive nitrogen was originally formed by one of two ways: lightning or biological fixation. General Nitrogen Information

Note: Nitrous oxide is N 2 0. The Nitrogen Cycle

Another View of the Nitrogen Cycle

 Denitrification is the loss of Nr from the biosphere because of its conversion back to its molecular form, with a triple bond.  Three conditions required: 1) presence of NO3- or nitrate, 2) availability of labile organic matter, and 3) absence or low concentration oxygen.  The residence time of water is an important factor in controlling how much denitrification goes on.  Denitrification is an effective sink for N  Denitrification can control phytoplankton blooms and degree of eutrophication Denitrification

Land Rivers Denitrification Reduces the Downstream N Transport (Galloway et al, 2003) 30-70% entering rivers denitrified Cont. Shelves Estuaries 10-80% entering estuaries denitrified >80% entering shelves denitrified ?

How Humans Have Impacted the Nitrogen Cycle Three main causes of the global increase in N r : 1.Cultivation of plants associated with n-fixers (e.g. legumes) increasing BNF 2.Industrial N Fixation by the Haber-Bosch process- (fertilizer and bombs) 1. Generate Hydrogen CH 4 +H 2 O Ni catalyst CO+3H 2 React H 2 + N 2 to produce NH 3 N 2 + 3H 2 2NH 3 3. Fossil Fuel burning- converts non reactive N into NO x

World population Haber-Bosch Crop Fixation Fossil Fuel x10 12 g yr -1 billion

Discussion Questions Hypothetical Situation 1)A farmer lives in close proximity to the watershed of a major river which feeds into the ocean. He uses nitrogen fertilizer to increase his crop yield. In an effort to become more eco-friendly he wants to know what effect his farm will have on the surrounding ecosystem and its nitrogen cycle? Suggest possible ways he can improve his farming practices and maintain economic stability. 2)Since the industrial revolution there has been a dependency on the burning of fossil fuels for energy. One of the effects of this process is the release of NO x to the atmosphere. How is NO x created and what are the implications of its presense in the atmosphere?

AgricultureFossil Fuel Over saturation of N leads to high N loss from system to soil, atmosphere and water Inputs of N r exceeds the systems ability to remove N through denitrification transported N can alter species composition and trophic structure in surrounding ecosystems High levels of N can degrade habitats (e.g. eutrophication, areas of anoxia and hypoxia) Initially release of NO x in troposphere produces ozone (health implications) Increases accumulation of small particles in air- decreases atmospheric visibility Acid precipitation and N deposition N 2 O emitted back to atmosphere can first contribute to greenhouse warming N 2 O in the stratosphere can deplete ozone layer Discussion Questions- Potential Ecosystem Effects

Atmospheric Deposition of N r (mg N m -2 yr -1 )

Nitrogen Saturation and Ecosystem Degradation  Inputs of N from the atmosphere in the northeastern U.S. are 5 to 20 times higher than those under pristine conditions (10-50kg N ha -1 yr -1 )  Added N may be accumulated in organic matter however, forest decline is observed as the ecosystem becomes saturated with N

Discussion Questions- Potential Solutions AgricultureFossil Fuel ?ideas