Nitrogen Dynamics of SoilsNitrogen Dynamics of Soils

IntroductionForms and Role of Nitrogen in Soils & Plants Nitrogen FixationDistribution of Nitrogen (N-Cycle) Nitrogen Transformations Fate of Nitrogen in SoilsNitrogen Fertilizer and Management

IntroductionIntroduction

Nitrogen is an essential plant nutrient It is mainly in organic forms in soil In it’s ionic form, it is very mobile in soils

and plants It is also responsible for some

environmental problems.

1. Forms of Nitrogen taken by plants

Nitrogen in the soil solution is in three forms NO3

-

NH4+

SON

NO3- and NH4

+ ions behave differently in soils and are differently preferred by plants.

NO3- exchange with HCO3

- and OH- at root surface –increase pH

NH4+ exchange with H+ at root surface –reduce pH

Component of essential plant compounds Amino acids ~ building blocks of proteins, enzymes nucleic acid ~ hereditary control Chlorophyll ~ photosynthesis

Plants respond to good available N by having deep green color of leaves Increased protein content increased plumpness of grains Increased plant productivity in general

2. Role of nitrogen in plants

Nitrogen is quite mobile (easily translocated) within plants, so available N is sent to newest foliage first

Deficiency exhibits the following:

Pale yellowish green color (chlorosis) in older leaves Have stunted growth and thin stems (low shoot:root

ratio) Plants mature more quickly than healthy plants Protein content is low and sugar content is high There is reduced productivity in general

3. Deficiency of nitrogen in plants

When too much N is applied, the following may occur:

Excessive vegetative growth (but weak plant stems) that lead to lodging with rain or wind

Delays maturity and cause plants to be prone to diseases

General decline of plant product quality Environmental aspects (build up of nitrates)

4. Oversupply of nitrogen in plants

Nitrogen FixationNitrogen Fixation

Biological Fixation Biological fixation which is by far more important

than lightening is catalyzed by enzyme nitrogenase.N2 + H+ +6e- = 2NH3 + H2

NH3 is formed which combines with organic acids to form amino acids and ultimately protein.NH3 + Organic Acids → Amino Acids → Proteins

The two types of biological fixation are:1. Symbiotic fixation2. Non symbiotic fixation

N-Fixation is the process of converting the inert dinitrogen gas of the atmosphere to nitrogen containing organic compounds that becomes available to all forms of live

N-Fixation is accomplished in nature biologically by a) certain microorganisms, and b) by lightening.

Symbiotic Fixation (Legumes & Non legumes)

Legumes and bacteria enter into symbiotic relationship. The bacteria (Rhizobium) infects the root hair and cortical cells inducing the formation of root nodules that serve as the site of nitrogen fixation. Plant provides carbohydrate

Some non legumes have also been observed to develop nodules that form site of N fixation.

Some nitrogen fixation have even been described as fixation without nodules.

Nonsymbiotic Nitrogen Fixation Certain free living organisms present in soil that

are not directly associated with higher plants are able to fix nitrogen.

Because these organisms are not directly associated with higher plants, the transformation is referred to as nonsybiotic or free living N-fixation

Some heterotrophs e.g. Azotobacter Autotrophs e.g. Photosynthetic bacteria

Distribution of NitrogenDistribution of Nitrogen(The Nitrogen Cycle)(The Nitrogen Cycle)

Nitrogen TransformationsNitrogen Transformations

Mineralization Conversion of organic forms of N into inorganic forms

(NO3- and NH4

+)

Immobilization Conversion of inorganic N forms (NO3

- and NH4+) to

organic NNitrification

Conversion of NH4+ ions into NO3

-

Denitrification Conversion of NO3

- to N gas (NO, NO2, N2O)

Volatilization Transformation of NH4

+ ions into ammonia gas

Fate of SON, NHFate of SON, NH44++ and NO and NO33

-- in in SoilsSoils

a)a) Soluble organic Nitrogen (SON) Soluble organic Nitrogen (SON) Not much information is available on this N

form Comprise of amino sugars and amino acids It is taken up directly by plant roots OR, it is leached and carried in groundwater to

streams where they cause environmental problems

b)b) NH NH44++

Like other positively charged ions NH4+ is

attracted to negatively charged soil colloids NH4

+ can be fixed by 2:1 minerals because of its unique size

NH4+ can volatilize into NH3 gas

NH4+ can also be converted into nitrates (NO3

-) directly (nitrosomonas bacteria)

OR through an intermediary step (NO2-)

(nitrobacter bacteria)

c)c) NO NO33--

Plants utilize NO3- directly

NO3- is negatively charged and so is not adsorbed

by the negative charges that dominate most soils.This makes it move down freely with drainage water causing several environmental problems (eutrophication and hypoxia).

Nitrate can also be converted to gaseous forms of nitrogen by series of reduction reactions. Denitrification

Nitrogen FertilizerNitrogen Fertilizer

Fertilizers supply nitrogen in soluble forms such as nitrate or ammonium, or as urea.

Nitrate or ammonium from fertilizer are taken up by plants and participate in the N cycle in exactly the same way as nitrate or ammonium derived from organic matter mineralization or other sources.

Fertilizer-N has much more concentration in time and space than N from other sources.

Management of Soil NitrogenManagement of Soil Nitrogen

Objectives of good N management1. Maintain adequate N supply2. Regulate the soluble forms of N to ensure enough is

readily available3. Minimizing leakage from soil-plant system

Strategies for Achieving the Objectives: Taking into account N contribution from other

sources so as not to oversupply N Improving efficiency with which fertilizer is applied Improving crop response knowledge Avoiding overly optimistic goals of meeting crop

needs that are higher than possible