The Nitrogen Cycle

Nitrogen Cycle• Nitrogen in

Atmosphere = 79% • Problem is getting N2

into a form that plants can use.

• Most N in soil used by plants for growth and for Agriculture.

Nitrogen Fixation- Conversion of N2 into ammonium (NH4

+) or nitrate (NO3

-)A . Non-Biological Fixation -Air Pollution - Nitrate is primarily the result of

gases from motor vehicle exhaust and stationary fuel combustion sources like electric utilities and industrial boilers

- rainfall additions from electrical discharge (lightning) 2-5 lbs....../acre/year

N2 -----> NO3-

B. Biological Fixation 1. Non-Symbiotic Bacteria - An

independent organism that lives in the soil - They fix about 5-50 lbs/acre /year of nitrogen.

2. Symbiotic Bacteria - mutually beneficial for host organism and bacteria. It is a complex plant - bacteria interaction

Symbiotic N- Fixation Bacteria = Rhizobia Forms the relationship

with a Legume. Eg. peas, clover, alfalfa,

peanuts, beans, soybeans

Alfalfa - 200 lbs....../acre/year

Soybeans - 100 lbs......./acre/year

Beans - 40 lbs...../acre/year

Symbiotic N Fixation•Bacteria invades the root of a

host plant.•Response of host plant root is to

grow a nodule for the bacteria to live in.

•Bacteria takes N2 from the air and converts it some of it into NH4+ which the plant is able to use.

• Fate of N Fixed by Rhizobium: 1) used by host plant, 2) leaks out of root to become available to surrounding plants,

3) as roots and nodules are sloughed-off, heterotrophic organisms immobilize (take in) the N and it eventually becomes part of the Soil Organic Matter.

Symbiotic N Fixation

The Nitrogen Cycle• After nitrogen from the atmosphere has been

fixed into the soil, plants can now use it to make proteins.

• This is how nitrogen enters the food chain. Consumers must get their nitrogen from consuming plants or other organisms. They are unable to utilize any of the nitrogen from the atmosphere.

• The route N takes into the food chain forms a sub cycle within the larger N cyle.

Nitrification• Like other nutrients, nitrogen is returned to

the ecosystem through animal wastes and dead organisms.

• Decomposers produce ammonia which is used by some plants.

• Ammonia can be converted back to nitrates by nitrifying bacteria, in a process called nitrification.

• Nitrates are the source of most of the nitrogen for plants.

Denitrification• To complete the nitrogen cycle, the

nitrates are then converted back to nitrogen gas by denitrifying bacteria.

• This process is called denitrification.

The Nitrogen Cycle

Human Impacts on the N Cycle

• Most wild plants are adapted to thrive in low levels of N that are found in the soil and water.

• The development of industrial fertilizer and the burning of fossil fuels has disrupted the natural N balance in many ecosystems.

• Commercial fertilizers are now commonly used to increase the growth and yield of agricultural crops.

Human Impacts on the N Cycle

• Modern farming and the combustion of fossil fuels now move about 140 million tonnes of extra nitrogen through the environment each year.

• More nitrogen is being added to the environment than what plants can absorb and use.

Effects on the Soil• With a surplus of nitrogen, plant growth soon becomes

limited by the scarcity of other resources such as phosphorous, calcium and water.

• The plants are not able to use any more N so it washes from the soil into streams and groundwater without being absorbed by organisms.

• This extra N can harm forests by stunting growth, turning trees yellow or by killing mycorrhizal fungi which is essential for some tree roots to absorb nutrients.

• This occurs because the acidity of the soil changes with extra nitrogen.

Effects on the Atmosphere• Industry and automobiles are huge contributors of

nitrogen containing gases in the atmosphere.• When these compounds dissolve into water

droplets in the atmosphere, it forms acid precipitation.

• This precipitation has affected lakes and forests in Eastern Canada by killing many species of fish, amphibians and trees.

• The only cure right now is to dump powdered limestone into the acidified lakes to rebalance the pH.

Effects on Freshwater Ecosystems• Much of the excess N from the land ends up in

freshwater ecosystems due to runoff.• These nutrients build up in these ecosystems

and create a process called eutrophication.What is Eutrophication?1. Runoff carries nitrates from farms and cities

into water systems.2. An increased growth of plants on the surface of

the water blocks sunlight from penetrating to deeper waters.

What is Eutrophication?3. Plants below the surface are unable to carry out

photosynthesis. They begin to die and therefore stop producing oxygen.

4. As the plants die, the population of decomposers explodes, feeding on the extra decaying matter. The decomposers cause a further decline in oxygen levels through their cellular respiration.

5. The altered conditions kill fish and other animals that require high oxygen levels

Nitrogen Pollution and Eutrophication

• Nitrates can also be a problem with human drinking water sources, especially in rural areas.

• The excess nitrogen leaches into ground aquifers and wells where many rural communities and residences get their drinking water.

• When excess nitrates are ingested, they are converted to nitrites which reduce the blood’s ability to carry oxygen and produce a form of anemia.

• This is especially a concern with infants and young children.

Effects on Marine Ecosystems• Excess nitrates in a marine ecosystem that has

warm surface water can create a population explosion of algae called an algal bloom.

• As the algae die they sink into cooler waters and are broken down by bacteria.

• Colder water holds less oxygen, and the use of that oxygen by the bacteria can cause death of many organisms that need higher levels of oxygen such as fish and shellfish.

Baltic Sea between Russia and Sweden

Effects on Biodiversity• Biodiversity is a measure of the variety of species

on Earth.• Biodiversity is not the same everywhere on Earth.• As a rule, biodiversity is reduced in extreme

conditions.• This means that only a few species are able to

thrive in an environment that have very high levels of nutrients.

• Only these few organisms will survive and become dominant, the others will die off because they cannot compete for the other scarce nutrients.

Assignment

• Read pages 52 – 69• Answer questions 1-6 pg. 65 & 1-5 pg. 68

• Use the chapter at a glance (p. 69) to review the material from the chapter.