Nitrogen Cycle APES Ch. 4 Miller 17 th ed.

Fixation Atmospheric Nitrogen (N 2 ) must go through a process— nitrogen fixation This is the first step of the nitrogen cycle where specialized bacteria convert gaseous nitrogen to ammonia that can be used by plants N 2 is converted to ammonium (NH 4 +) or nitrate (NO 3 -) BEFORE it can be used by most living organisms Two processes can fix N 2 : High energy fixation & living organisms Sources of high energy: solar radiation, lightning strikes, meteorites done by cyanobacteria or specialized living organisms live symbiotically within the roots of legumes Legumes: peanuts, clover, peas, etc.

Assimilation In the form of ammonia (NH3), ammonium (NH4+) or nitrate (NO3-), nitrogen is readily taken up by plant roots for use in making molecules such as DNA, amino acids and proteins. This is an energy consuming process When animals eat plants, they acquire nitrogen in the form of amino acids

Ammonification After nitrogen has served its purpose in living organisms, decomposing bacteria convert the nitrogen-rich compounds, wastes, and dead bodies into simpler compounds such as ammonia. Bacteria breakdown proteins in organic matter and discard excess nitrogen as ammonia (NH 3 ) or ammonium (NH 4 + ) Ammonia is toxic to animals especially aquatic organisms in ponds or lakes

Nitrification Several species of bacteria can use ammonia (NH3) as an energy source Nitrosomas oxidize ammonia to produce energy, creating nitrite (NO 2 -). Nitrite is toxic to plants (but rarely a problem) Nitrobacter uses nitrite as a food source and releases nitrate (NO 3 -) Both process are aerobic Plants LOVE nitrates!! FYI: ALL nitrates compounds are SOLUBLE in water.

Denitrification Bacteria take useable nitrate and convert it back to nitrogen (N 2 ) gas. Denitrifying bacteria and anaerobic, metabolizing nitrate (NO 3 1- ) without oxygen Nitrogen (N 2 ) is returned to atmosphere until it goes through the fixation process again

Human Impact on Nitrogen Cycle One by-product of burning fossil fuels is Nox Fertilizers contain high levels of nitrogen to increase productivity of soil Plants do not use all the fertilizer Excess nitrogen gets washed into soil and then surrounding waterways

Eutrophication Excess nitrogen can cause algal blooms and increase the growth rates of many aquatic plants Excess algal bloom chokes the waterways and reduce dissolved oxygen levels Influx of excess nutrients is eutrophication It greatly disturb the ecosystem