Nitrogen

Chapter 3 in the Lecture Manual

Define Nitrogen Cycle

the association of all interactive chemical and biochemical processes related to Nitrogen availability

Define Immobilization

the temporary assimilation of plant available soil Nitrogen into the organic systems of microbes

Define Mineralization

the transformation of organic soil Nitrogen forms to plant usable Nitrogen forms by microbial oxidation

Define Nitrification

the sequence of microbial oxidation processes converting NH4+ to NO3-

Define Volatilization

the loss of NH3 (gas) after amendments containing the N nutrient are applied

Define Denitrification

the loss of available soil Nitrogen by the microbial reduction of NO3- to N2O or N2

Define Leaching

the movement of soil NO3- down through the soil profile with the movement of a wetting front

Define Biological N Fixation

the biochemical reduction of atmospheric N2 to ammonium-N within roots of a host plant by infecting species specific bacteria colonies

Nitrogen Forms in Nature

Gas: NH3 or N2

Organic: R-NH2

Ions (Plant available forms): NO3- or NH4+

Use of Nitrogen in Plants

Proteins, DNA, RNA, Chlorophyll

Most N in plants is converted to Organic amine forms (R-NH2)

What is the importance of the N cycle compared to other nutrient cycles?

N is the plant nutrient in greatest demand

Soil N is involved with the largest number of nutrient cycle processes

Soil N exists in many chemical forms simultaneously

What are the main nutrient cycle components?

Gains (inputs), Losses (outputs), and Transformations (cycling)

Examples of Gains in a nutrient cycle

Biological fixation, Fertilizer/Industrial fixation, Electrical fixation

Examples of Losses in the Nitrogen nutrient cycle

Leaching, Volatilization, Denitrification

Examples of Transformations in a nutrient cycle

Mineralization, Immobilization, Nitrification

What are the reaction products of the nitrification chemical process?

NO3- (Nitrate) and H+

High vs Low C:N and its affects on Mineralization

High C:N - anything >25:1, Lack of Nitrogen limits microbial growth, OM not easily decomposed 

Low C:N10:1 to 25:1, Nitrogen is not limiting to microbial growth, OM is easily decomposed.

Trends in Nitrogen release after adding various C:N materials.

High C:N - when incorporated, observe a mid-long term decline in available soil N (think about corn cobs)

Low C:N - when incorporated, observe a short term decline in soil N, then a release of available Nitrogen (think about alfalfa straw)

What is the reaction product of mineralization chemical processes?

NH4+

What are the mechanisms and conditions responsible for denitrification?

Mechanisms: O2 is severely restricted to microbes, anaerobic respiration mechanisms are activated in microbes, NO3-N becomes the electron acceptor instead of O2, bacterial chemically reduce the oxidized N forms present

Conditions:Soil saturation for more than 2 days, soil with poor drainage and high compaction

What are practices to reduce denitrification potential?

Minimize compaction (reduce trips across the field, drive in wheel tracks)

Improve drainage (install tiles or use cover crops)

Monitor irrigation levels

What conditions responsible for enhancement of leaching?

Excess irrigation and high rainfall

Course textured soils

Mismanagement of N fertilizer applications

What practices are used to reduce leaching potential?

Apply majority of Nitrogen in the Spring

Split application throughout the season

Add nitrification or urease inhibitors

Manage irrigation more effectively 

Benefits of the symbiotic relationships in biological N fixation.

Plant roots provide carbon and protection to bacteria colonies

Bacteria produce more NH4+ than they require

Excess Nitrogen diffuses into the plant’s xylem stream

What are some management practices to enhance N production effectiveness in biological N fixation?

Inoculate with species specific bacteria

Inoculate seeds at planting time rather than inoculating early and storing

What are the popular N fertilizers.

Urea

Urea-Ammonium Nitrate Solution (UAN) 

Ammonium phosphates (DAP and MAP)

Anhydrous Ammonia (cheapest per lb of N, highest concentration of N)

Effective management practices/strategies of N fertilizer materials

Incorporate with light tillage if possible

apply before .25” rainfall (or more) or irrigate

avoid surface applications on grass/pasture or crop residue without irrigation or rainfall

limit application on calcareous soils

What are cautions associated with anhydrous ammonia for biological systems?

NH3 is toxic to biological systems

NH3 reacts with water in mucous membranes, eyes, and lungs. Greater than 0.7% NH3 air concentration = human suffocation and death.

Free NH3 gas can injure seedlings and roots

What are the environmental risks with N fertilizer mismanagement?

Water pollution (algal blooms, dead zones, and groundwater contamination)

Air pollution (ammonia and ozone)

Climate change (greenhouse gas emissions like nitrous oxide)

Ecosystem damage (loss of biodiversity, soil acidification, and degraded habitats)

Mineralization Chemical Reaction

Nitrification Chemical Reaction

Urea Hydrolysis Reaction

Ammonia Hydrolysis Reaction