Soil Fertility Unit 14 Nitrogen

What are the two major nitrogen reservoirs?

The atmosphere contains more than 98%

How much nitrogen is in soil compared with the atmosphere?

Soil contains only about 1/10,000th of the N in the atmosphere.

What are the major ways nitrogen enters soil?

Plant/animal residues, atmospheric inputs, biological fixation, and industrial N inputs.

What happens when nitrogen enters the soil from residues?

Organic N in residues enters the soil and can later be mineralized.

What is nitrogen fixation?

Conversion of atmospheric N₂ into reactive nitrogen forms.

What are the four major sources of N fixation?

Legume symbioses, free-living microbes, lightning, and industrial Haber-Bosch fixation.

What is mineralization?

Soil organisms convert organic N into NH₄⁺(ammonium).

What is nitrification?

Bacteria convert much of the NH₄⁺ (ammonium) into NO₃⁻(nitrate).

What forms of nitrogen can plants take up?

NH₄⁺ (ammonium) and NO₃⁻ (nitrate)

What is immobilization?

NH₄⁺ and NO₃⁻ are converted back into organic N.

What is nitrate leaching?

NO₃⁻ moves below the root zone with downward-moving water.

What is NH₄⁺ fixation?

2:1 clay minerals can fix NH₄⁺.

What is volatilization?

NH₄⁺ can be converted to gaseous NH₃ and escape to the atmosphere.

What is denitrification?

Bacteria convert NO₃⁻ into N₂, N₂O, or NO gases that escape to the atmosphere.

What is the basic nitrogen cycle sequence?

Organic N → NH₄⁺ → NO₃⁻ → plant uptake.

Which process converts organic N into NH₄⁺?

Mineralization.

Which process converts NH₄⁺ into NO₃⁻?

Nitrification.

Which process converts inorganic N back into organic N?

Immobilization.

Which process removes NO₃⁻ through downward water movement?

Leaching.

Which process can remove N as NH₃ gas?

Volatilization.

Which process returns N to the atmosphere as gases?

Denitrification.

What happens to organic N after fish meal is added to soil?

It can be mineralized to NH₄⁺, nitrified to NO₃⁻, and taken up by plants.

What is atmospheric N deposition?

Nitrogen from the atmosphere that enters ecosystems through deposition.

How does lightning contribute nitrogen to soil?

Lightning splits N₂; nitrogen reacts with O₂ to form nitrogen oxides that can become nitrate.

How does lightning-produced nitrate enter soil?

Nitrate dissolves in rain and enters soil during rainfall.

What are sources of atmospheric N deposition besides lightning?

Fossil-fuel combustion, biomass burning, fertilizer NH₃, poultry litter, and manure.

How can fertilizer contribute to atmospheric N deposition?

NH₃ can volatilize from fertilizer and later be deposited elsewhere.

How can poultry farms contribute nitrogen to nearby fields?

NH₃ from poultry litter/manure can enter the atmosphere and be deposited downwind.

Why might a corn field downwind of a poultry farm have high yields with little fertilizer?

Atmospheric N deposition from the poultry operation can supply additional N.

Why did corn yields decline after the poultry operation closed?

The additional atmospheric N input from the poultry operation was lost.

What is biological nitrogen fixation by legumes?

Rhizobia living in legume root nodules convert atmospheric N₂ into usable N.

What bacteria are involved in legume nitrogen fixation?

Rhizobium bacteria.

Where does biological N fixation occur in legumes?

Inside nodules on legume roots.

What does the legume provide to rhizobia?

Carbon/photosynthate.

What does the legume receive from rhizobia?

Fixed nitrogen.

What is the approximate range of N fixation by legumes?

About 40–300 lb N/acre/year.

How does increasing soil N affect biological N fixation?

Increasing soil N decreases N fixation.

Why does excess NO₃⁻ reduce biological N fixation?

NO₃⁻ reduction competes with N fixation for plant photosynthate.

Do all rhizobia work with every legume?

No. Different rhizobia species are associated with different crops.

Why should seeds receive the correct rhizobium inoculum?

The correct rhizobium is needed for effective nodulation and N fixation.

What does a red nodule center indicate?

An active, working N-fixing nodule containing leghemoglobin.

What does a green or white nodule center indicate?

An inactive N-fixing nodule.

What is Azolla?

A small water fern associated with blue-green algae that can fix atmospheric N.

How much N can Azolla add to rice?

Up to about 50 lb N/acre.

What is Haber-Bosch fixation?

An industrial process used to produce synthetic reactive N fertilizer.

What is a nonsymbiotic nitrogen fixer?

A free-living soil microorganism that fixes atmospheric N₂.

How does soil acidity affect biological N fixation?

Acidity restricts rhizobial survival and growth and affects nodulation.

What problems can occur in acidic soils?

Al, Mn, and H toxicity can occur, while Ca and P availability can be low.

How can liming affect biological N fixation?

Liming can improve acidic soil conditions for rhizobia and nodulation.

Which micronutrient is most likely to limit N fixation in acid soils?

Molybdenum (Mo). It is required by the microbes, so a lack of it will limit N fixation.

What other micronutrients are required for N fixation?

Co, B, Fe, and Cu.

How does photosynthesis affect N fixation?

High photosynthate production is associated with high N fixation.

What conditions reduce N fixation by reducing photosynthate?

Low light, moisture stress, and low temperature.

What legume management factors can reduce N fixation?

Water/nutrient stress, weeds, insects, excessive cutting, premature harvest, and delayed harvest.

What is the relationship between N inputs and N losses?

Inputs add N to the soil system; losses remove N from the soil system.

What are the major nitrogen loss pathways?

Leaching, volatilization, and denitrification.

What are the major nitrogen storage/retention pathways?

Organic N, plant uptake, and NH₄⁺ fixation by 2:1 clay minerals.

What should you remember about fixed NH₄⁺?

Some fixed NH₄⁺ can be released slowly over time.

What is the most important overall nitrogen-cycle pathway to memorize?

N enters → organic N mineralizes to NH₄⁺ → nitrification produces NO₃⁻ → plants take up N, while N can also be immobilized, fixed, leached, volatilized, or denitrified.