ecosystem part 2

The Nitrogen Cycle

• All life requires nitrogen-compounds, e.g., proteins and nucleic acids.
• Air, which is 79% nitrogen gas (N₂), is the major reservoir of nitrogen.
• But most organisms cannot use nitrogen in this form.
• Plants must secure their nitrogen in "fixed" form, i.e., incorporated in compounds such as:
  • nitrate ions (NO₃⁻)
  • ammonia (NH₃)
  • urea (NH₂)₂CO

Animals secure their nitrogen (and all other) compounds from plants (or animals that have fed on plants

Nitrogen Fixation

The nitrogen molecule (N₂) is quite inert. To break it apart so that its atoms can combine with other atoms requires the input of substantial amounts of energy.
Three processes are responsible for most of the nitrogen fixation in the biosphere:

• atmospheric fixation by lightning
• biological fixation by certain microbes — alone or in a symbiotic relationship with some plants and animals
• industrial fixation

Atmospheric Fixation

The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth.
Atmospheric nitrogen fixation probably contributes some 5–8% of the total nitrogen fixed.

Industrial Fixation

Under great pressure, at a temperature of 600°C, and with the use of a catalyst, atmospheric nitrogen and hydrogen (usually derived from natural gas or petroleum) can be combined to form ammonia (NH₃). Ammonia can be used directly as fertilizer, but most of its is further processed to urea and ammonium nitrate (NH₄NO₃).

Biological Fixation

The ability to fix nitrogen is found only in certain bacteria and archaea.
Some live in a symbiotic relationship with plants of the legume family (e.g., soybeans, alfalfa).

Link to a discussion of symbiotic nitrogen fixation in legumes.

Some establish symbiotic relationships with plants other than legumes (e.g., alders).
Some establish symbiotic relationships with animals, e.g., termites and "shipworms" (wood-eating bivalves).
Some nitrogen-fixing bacteria live free in the soil.
Nitrogen-fixing cyanobacteria are essential to maintaining the fertility of semi-aquatic environments like rice paddies
Biological nitrogen fixation requires a complex set of enzymes and a huge expenditure of ATP.
Although the first stable product of the process is ammonia, this is quickly incorporated into protein and other organic nitrogen compounds.

Decay

The proteins made by plants enter and pass through food webs just as carbohydrates do. At each trophic level, their metabolism produces organic nitrogen compounds that return to the environment, chiefly in excretions. The final beneficiaries of these materials are microorganisms of decay. They break down the molecules in excretions and dead organisms into ammonia.

Nitrification

Ammonia can be taken up directly by plants — usually through their roots. However, most of the ammonia produced by decay is converted into nitrates. This is accomplished in two steps:Bacteria of the genus Nitrosomonas oxidize NH₃ to nitrites (NO₂⁻).
Bacteria of the genus Nitrobacter oxidize the nitrites to nitrates (NO₃⁻).
These two groups of autotrophic bacteria are called nitrifying bacteria. Through their activities (which supply them with all their energy needs), nitrogen is made available to the roots of plants.
Both soil and the ocean contain archaeal microbes, assigned to the Crenarchaeota, that convert ammonia to nitrites. They are more abundant than the nitrifying bacteria and may turn out to play an important role in the nitrogen cycle.
Many legumes, in addition to fixing atmospheric nitrogen, also perform nitrification — converting some of their organic nitrogen to nitrites and nitrates. These reach the soil when they shed their leaves.

Denitrification

The three processes above remove nitrogen from the atmosphere and pass it through ecosystems.
Denitrification reduces nitrates to nitrogen gas, thus replenishing the atmosphere.
Once again, bacteria are the agents. They live deep in soil and in aquatic sediments where conditions are anaerobic. They use nitrates as an alternative to oxygen for the final electron acceptor in their respiration.
Thus they close the nitrogen cycle.

Are the denitrifiers keeping up?

Agriculture may now be responsible for one-half of the nitrogen fixation on earth through
the use of fertilizers produced by industrial fixation
the growing of legumes like soybeans and alfalfa.
This is a remarkable influence on a natural cycle.
Are the denitrifiers keeping up the nitrogen cycle in balance? Probably not. Certainly, there are examples of nitrogen enrichment in ecosystems. One troubling example: the "blooms" of algae in lakes and rivers as nitrogen fertilizers leach from the soil of adjacent farms (and lawns). The accumulation of dissolved nutrients in a body of water is called eutrophication
All living things are made of carbon. Carbon is also a part of the ocean, air, and even rocks. Because the Earth is a dynamic place, carbon does not stay still. It is on the move!
Carbon Cycle
In the atmosphere, carbon is attached to some oxygen in a gas called carbon dioxide.
Plants use carbon dioxide and sunlight to make their own food and grow. The carbon becomes part of the plant. Plants that die and are buried may turn into fossil fuels made of carbon like coal and oil over millions of years. When humans burn fossil fuels, most of the carbon quickly enters the atmosphere as carbon dioxide.
Carbon dioxide is a greenhouse gas and traps heat in the atmosphere. Without it and other greenhouse gases, Earth would be a frozen world. But humans have burned so much fuel that there is about 30% more carbon dioxide in the air today than there was about 150 years ago, and Earth is becoming a warmer place. In fact, ice cores show us that there is now more carbon dioxide in the atmosphere than there has been in the last 420,000 years
Water Cycle

Oxygen Cycle Just as water moves from the sky to the earth and back in the hydrologic cycle, oxygen is also cycled through the environment. Plants mark the beginning of the oxygen cycle. Plants are able to use the energy of sunlight to convert carbon dioxide and water into carbohydrates and oxygen in a process called photosynthesis.   This means that plants "breathe" in carbon dioxide and "breathe" out oxygen. Animals form the other half of the oxygen cycle. We breathe in oxygen which we use to break carbohydrates down into energy in a process called respiration. Carbon dioxide produced during respiration is breathed out by animals into the air. So oxygen is created in plants and used up by animals, as is shown in the picture above. But the oxygen cycle is not actually quite that simple. Plants must break carbohydrates down into energy just as animals do. During the day, plants hold onto a bit of the oxygen which they produced in photosynthesis and use that oxygen to break down carbohydrates. But in order to maintain their metabolism and continue respiration at night, the plants must absorb oxygen from the air and give off carbon dioxide just as animals do. Even though plants produce approximately ten times as much oxygen during the day as they consume at night, the night-time consumption of oxygen by plants can create low oxygen conditions in some water habitats Oxygen in water is known as dissolved oxygen or DO. In nature, oxygen enters water when water runs over rocks and creates tremendous amounts of surface area. The high surface area allows oxygen to transfer from the air into the water very quickly.

The earth has a limited amount of water. That water keeps going around and around and around and around and (well, you get the idea) in what we call the "Water Cycle". This cycle is made up of a few main parts: evaporation (and transpiration) condensation precipitation collection