Soils 205-90

Lecture 13- Nitrogen

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A. Soil-Plant Relations

1. N is an essential plant nutrient and is found in important biomolecules such as proteins and amino acids

2. Deficiency symptoms

(a) stunting and chlorosis (yellow color)

(b) mobile in plant - def. in older portions

(c) rapid response - growth and green color

(d) crop quality - lowered protein

 

 

3. Oversupply

(a) excessive vegetation (if not desired)

(b) maturity delay

(c) reduced crop quality

potatoes

sugar beets

(d) water quality

B. N-cycle

Major Processes:

1. Mineralization

2. Immobilization

3. Nitrification

4. Denitrification

5. N-fixation

- symbiotic

- non-symbiotic

Involve oxidation and reduction (redox) of N by microorganisms

 

C. Redox Processes

1. Oxidation = loss of e^- by the element

(a) Fe⁺² ¬¾¾® Fe⁺³ + e^-

¾¾(oxidation)¾¾®

(b) Microbial processes

= gain in O^-2

+ O₂

organic-C ¾¾¾¾® CO₂ + energy

¾¾ (oxidation of C)¾®

- obtain energy with oxidation

(c) N-cycle oxidation:

-3		+5
NH4+	¾¾®	NO3-

N lost 8 e^-

gained O^-2

2. Reduction = opposite of oxidation

(a) gain in e^-

(b) loss of O^-2

(c) requires energy

N Form	name	oxidation state
	organic-N	-3
NH4+	ammonium	-3	¯	­
N2	dinitrogen gas	0	(oxidation)	(reduction)
NO2-	nitrite	+3	¯	­
NO3-	nitrate	+5	¯	­

D. Mineralization and Immobilization

1. Mineralization

(a) carried out by numerous heterotrophic organisms

= use C for energy source

org.-C ¾® CO₂ + H₂O + energy

(b) N released as NH₄⁺

oxidation state change = none

(c) releases N for plant use

2. Immobilization

 

(a) lowers N available for plants

NH₄⁺ and NO₃^- can both be immobilized

(b) N remains in soil

(c) Immobilization process

Immobilization = soil organisms use inorganic N

3. N content or C:N ratio of residues

x low C:N or high N content = rapid mineralization

alfalfa, peas, grass

x high C:N or low N content = high immobilization

straw, bark, sawdust

x stable soil C:N @ 12-15:1

E. NH₄⁺ fixation

1. Converted to form that is not available to plants

2. Trapped in interlayer like K⁺ in illite

(a) 2:1 expanding layer lattice minerals (esp. vermiculite)

(b) only very slowly plant available

(c) most in clay subsoil

F. Ammonia Volatilization - gaseous loss of N - no reduction

 

1. NH₃ gas reacts with water

NH₃ + H₂O ¬¾® NH₄⁺ + OH^-

¾ (acid + water)®

¬(alkaline + dry)¾

2. Urea, also

CO(NH₂)₂ ¾¾¾¾¾¾® NH₃ +CO₂ + H₂O

urea ¾(soil enzymes & H₂O)¾®

3. Most volatilization when

- coarse or sandy-textured soils

- low clay and organic matter (to adsorb NH₄⁺)

- dry alkaline surface

G. Nitrification

1. NH₄⁺ ¾¾¾¾® NO₂^- ¾¾¾¾® NO₃^-

ammonium

nitrite

nitrate

- oxidation of N

2. Autotrophic bacteria

(a) obtain energy from N oxidation

(b) Nitrosomonas

NH₄⁺ ¾¾® NO₂^- + energy

(c) Nitrobacter

NO₂^- ¾¾® NO₃^- + energy

3. Rapid in well-aerated, warm, moist soils

(a) aerobic organisms

= O₂ required (oxidation)

(b) little NH₄⁺ or NO₂^- accumulate in soils

4. Acid-forming process

(a) NH₄⁺ ¾¾® NO₃^- + H⁺

(b) slowed by acid soils

5. NO₃^- is highly soluble

- easily leached into groundwater

- loss from growing plants

- degraded groundwater

- careful N and water management

 

H. Denitrification

1. Gaseous loss of N upon N reduction

 

2. NO₃^- ¾¾® NO₂^- ¾¾® NO ¾¾® N₂O ¾¾® N₂

nitric oxide------- nitrous oxide

3. Microorganisms responsible:

(a) facultative anaerobes

- prefer O₂ but will use N for a terminal e^- acceptor

(b) mostly heterotrophic

- use organic-C for energy source (reductions require energy)

4. Denitrification enhanced by:

(a) low O₂ (flooding)

(b) high O M (energy source)

(c) high NO₃^-

5. NH₄⁺ will not denitrify

NH₄⁺ ¾¾® (NO₃^- or NO₂^-) ¾¾® N₂

- Nitrification Inhibitors keep N in NH₄⁺ form

less N leaching and denitrification

þ Wetlands, O₂ content, and N form

 

 

6. Metabolic reduction is not denitrification- important input to the soil N-cycle

(no N gas formation)

organisms

NO₃^- ® NO₃^- ® NH₄⁺ ® organic-N

 

- N is reduced for use in protein formation

- immobilization, if soil microorganisms

I. Nitrogen fixation

1. N₂ ¾(organisms)¾¾® NH₃ ¾¾® amino acids and proteins

2. Symbiotic with legumes

(a) Bacteria = Rhizobium genus (species specific)

R. meliloti - alfalfa

R. trifolii - clover

R. phaseoli - beans

- bacteria require plant to function

- inoculation of seed

= coat seed with proper bacteria culture

assures bacteria is present

(b) Process:

C from plant photosynthesis î

+ Þ symbiosis

N from fixation of N₂ ì

(c) Quantity fixed

- Alfalfa and clover = large

@ 100 - 250 kg/ha/yr

mature stand, good fertility & pH

- Beans and peas

@ less fixation but high protein food

with minimum N input

= N savers

- added N fertilizer = lowered N fixation

3. Symbiotic - nodules - nonlegumes

(a) Organisms

* actinomycetes - Frankia

(b) Plants

* Alders and other trees

* forests and wetlands

- increase fertility status

- large areas

4. Symbiotic - without nodules

(a) Azolla/Anabaena complex

­ ­
­ blue-green algae (N-fixer) in leaves
­
floating fern in rice paddies

(b) Rhizosphere organisms

* use root exudates (C)

* large areas

5. Nonsymbiotic N-fixation - free-living organisms

(a) Bacteria and blue-green algae

* aerobic and anaerobic

(b) energy source

* O M
or
* photosynthesis

(c) small amounts = 5 - 50 kg/ha/yr

(d) inhibited by available soil N

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