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General Ecology (BIO 160)
The Nitrogen Cycle
Dr. Jim Baxter
Sacramento State
THE NITROGEN CYCLE
As a component of proteins, enzymes, nucleic acids and light-­‐harvesting pigments, such as chlorophyll, nitrogen (N) is a key element in biological organisms. In plants, total N ranges from between 1-­‐4% of dry mass. Plants take up N from the soil as either nitrate (NO3-­‐) or ammonium (NH4+); most plants take up N as nitrate. The nitrogen cycle (see figure below) is characterized by many transitions in the oxidation states of N, with the primary reservoir of N being in the atmosphere as N2 gas. This form of N is not available to plants, except via N-­‐fixing bacteria. Nitrogen gas is also fixed and can become available to plants by lightening conversion and industrial fertilizer manufacturing. The process of denitrification returns N to the atmosphere. Although N is typically thought of as a nutrient that is beneficial to biological organisms, it may also be a pollutant (e.g., derived from combustion reactions). Atmospheric N2 (~79% of air) is a large but relatively inert reservoir, accessible only to a few bacterial forms. Living and dead organic material (e.g., proteins, nucleic acids, humus) is also a fairly large reservoir, but smaller than that in the atmosphere. Inorganic N salts (e.g., NO3-­‐, NH4+) are very small, rapidly cycled reservoirs. These salts are present in soils and aquatic environments and are the forms on N that are available for uptake by biological organisms. N2
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The Nitrogen Cycle Steps in the nitrogen cycle Nitrogen fixation: This is the way N becomes available to organisms from its gaseous atmospheric source (N2). The process of N fixation compensates for losses of available N by denitrification. It is an energy-­‐demanding process (i.e., ~150 Kcal/mole N2). 1
General Ecology (BIO 160)
The Nitrogen Cycle
Dr. Jim Baxter
Sacramento State
Free-­‐living N fixers – Azotobacter, Azospirillum, Clostridium are all free-­‐living heterotrophic bacteria. Because of the high energy demand of the process, these bacteria do not fix a large amount of N (1-­‐3 kg N/ha/yr). Some Cyanobacteria (blue-­‐green algae) also fix nitrogen. Because they are photosynthetic, they can fix up to 50-­‐100 kg N/ha/yr. Symbiotic N fixers – Rhizobium bacteria occur in root nodules of leguminous plants (Fabaceae family). These bacteria may fix up to 150-­‐200 kg N/ha/yr. There are also many other non-­‐leguminous symbiotic N fixing associations (e.g., alder trees with actinomycetes). Assimilation: This step involves the uptake of N (either as NH4+ or NO3-­‐) into biological organisms. Ammonification: This is the liberation of N from dead organic matter. It is a bacterial process that occurs in both terrestrial and aquatic environments. R-­‐NH2 NH4+ Nitrification: This is a two-­‐step bacterial process whereby ammonium is first oxidized to nitrite and to nitrate. The bacteria that perform these reactions are called nitrifiers and they are chemosynthetic. Step I. NH4+ NO2-­‐ (Performed by Nitrosomonas, Nitrosocystis) Step II. NO2-­‐ NO3-­‐ (Performed by Nitrobacter, Nitrocystis) Most higher plants N in NO3-­‐ form. However, NO3-­‐ leaches out of soils rapidly, while NH4+ is held by ion exchange forces to clay particles in the soil. Denitrification: Denitrification occurs in anaerobic environments (e.g., flooded soil and anoxic sediments). It depletes available N and may threaten the ozone layer by production of N20 (nitrous oxide). NO2-­‐ NO N20 N2 2