Nitrogen Cycle

Term: Legume

www.sciencedaily.com/releases/2009/04/090414110818.htm

• Go to sciencedaily.com & search for soybeans & aphids

• Read the Article from April 20th

• Are rhizobia specific to legume species• Do naturally occurring rhizobia or

commercially inoculated rhizobia provide better resistance?

• Does the source of the rhizobia influence the effectiveness of nitrogen fixation?

Legumes

• Plants that have a symbiotic relationship with bacteria that grow in nodules on their roots.

• Soybeans – widely rotated with corn crops to revitalize soil

• Vetch & Clovers – Cover crops – green manure

• Clovers & Alfalfa - grown to improve hay crops & pasture.

Other Legumes

• Red bud trees• Locust Trees• Beans• Peas• Most plants that form Pods

Rhizobia• Rhizobia are specific to legume

species• The naturally occurring bacteria

were more effective at creating resistance against the aphid than the imported bacteria.

• Older generations of rhizobia are more effective than the new ones

• The rhizobia compete for space to grow on the roots

• Naturalized rhizobia are as effective in nitrogen fixation as the commercial ones.

Nitrogen & Fertilizer• Industrialized countries began producing

chemical fertilizers after WWI– Haber Process

• Manufactured nitrogen begins with the production of ammonia (NH3).

• Ammonia is a gas that is produced when atmospheric N2 is combined with hydrogen from fossil fuels - also called anhydrous ammonia

• Stored under pressure• When fuel prices ↑, fertilizer prices ↑

Ammonia Fertilizers

NH3

Nitric Acid

Sulfuric Acid

Phosphoric Acid

Carbon Dioxide

Anhydrous Ammonia

Ammonium Nitrate

Ammonium Sulfate

Ammonium Phosphate

Urea

Nitrogen Fixation

• Conversion of Inert N2 into bio-usable forms

• atmospheric fixation by lightning • biological fixation by certain microbes

– alone– symbiotic relationship with some plants and

animals • industrial fixation – Haber Process

Nitrification

• Nitrification – whatever form of ammonia is applied to the soil, it must be converted to be used by plants– NH3 + Nitrosomonas → NO2

– NO2 + Nitrobacter → NO3

• NO3 Very soluble form– Only held loosely by soil colloids (clay & humus)– ~ 5% of NO3 dissolves in water and is lost by leaching

Denitrification

• In saturated soils• Anaerobic Conditions (No oxygen)• Bacteria convert NO3 to elemental

Nitrogen (N2) which is a gas and returns to atmosphere

• 30 -40% of applied Nitrogen if area has been flooded for 3-5 days

Volatilization

• Occurs when urea is converted to ammonium carbonate

• Ammonium carbonate breaks down in warm moist soils to ammonia gas and carbon dioxide

• Ammonia gas evaporates into the atmosphere.

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.

Carbon – Nitrogen Ratio• The rate of decomposition of decaying plants

depends on the C:N ratio• C:N ratios of 25:1 or less decompose quickly

and make Nitrogen available to plants (mineralization)

• C:N ratios of 25:1 or more cannot decompose with out drawing nitrogen out of the soil (immobilization)

• Humus has a C:N ratio of 12:1 and has completely decomposed

• Implications for mulch and composting

The Nitrogen Cycle

• N2 - Nitrogen Gas 78% of air - Inert

• NH4 – Ammonium ion – Made by bacteria

• NH3 – Ammonia - Made by Haber-Bosch process. Industrial N fixation

• NO2 – Nitrite ion – Very toxic to plants – Bacteria convert it to Nitrate

• NO3 – Nitrate ion – Form used by plants Very soluble – quickly leaches out of soil

• Video