EVSC 590.N Cycle. Ntri Denitrification

7/30/2019 EVSC 590.N Cycle. Ntri Denitrification

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Nitrogen Cycle:

Nitrification and Denitrification

n



1. Definition: The biological formation

(oxidation) of NH4+ to produce nitrite

and nitrate. 2. Significance: Beneficial or Harmful?

» a. Allow for easier uptake of N by plants.

» b. Increases loss of N to leaching.» c. Increases risks of NO3- toxicity.

Nitrification: I. Introduction



Nitrification: I. Introduction

» d. Increases soil acidity.

» e. Increases N loss due to denitrification.

» f. It is a necessary intermediate in returning N

to the atmosphere.

3. End product of fertilizer N is nitrate whether

the fertilizer is added as urea, anhydrousammonia or ammonium nitrate.

4. Potential pollutant



II. Microbiology: Nitrifying Bacteria

1. Obligate aerobes

2. Derive their carbon solely from CO2 or

carbonates. 3. Derive energy from oxidation of NH4+

or NO2- (Chemoautotrophs).

4. The following genera are known tooccur in soils:



II. Microbiology: Nitrifying Bacteria

a. Oxidize ammonium to nitrate

» i. Nitrosomonas - ellipsodal or short rods

» ii. Nitrosococcus - spherical cells

» iii. Nitrosospira - spiral-shaped cells

» iv. Nitrosobolus - lobate

b. Oxidize nitrite to nitrate (NO2-

NO3-)» Nitrobacter



III. Biochemistry of Nitrification.

NH4+ + 1/2O2 NH2OH + H+ NO2- NO3-

Nitrosomonas Nitrobacter

1. Reaction involves a hydroxylamineintermediate

NH3 +O2 + 2H+ + 2e NH2OH + H2O

2. The oxygen in the nitrate product is formed

from H2O not molecular O2. NH3 NH2 OH (HNO?) NO NO2



Nitrification

3. NO2- + H2O H2O.NO2 HO3- + 2H

2H + 1/2O2 H2O

4. Above reaction is an example of

hydration and dehydrogenation.



IV . Factors Affecting Nitrification

1. Acidity (pH):

Significant correlation between NO3-

production and pH Optimum pH between 6.5-7.5

low rates at pH less than 6.0 and negligible

below pH of 4.5.



Nitrification

2. Aeration:

» O2 is an obligate requirement, making adequate

aeration essential (Nitrosomonas and Nitrobacter are strict aerobes).

3. Moisture:

» Affects indirectly by controlling O2

availability.

Proceeds readily at -0.1 to -1MPa moisture

tension.



Nitrification

4. Temperature

» Process slow below 5oC and above 40oC.

» Optimum between 30 and 35o C. 5. Organic Matter

» Starting point.

» It's decomposition may deplete supplies of available NH4+ and O2 for the nitrifiers.



V. Fate of Nitrates in Soils

Once NO3- is formed in soils it is subject to

the following:

» 1. It may undergo denitrification by micro-organisms to gaseous oxides of nitrogen

and N2

» 2. It may be taken up by organisms and used to

synthesize amino acids (assimilatory

reduction).



Nitrification

» 3. In the absence of O2 it may be used by

microorganisms as an electron acceptor

and become reduced to NH4+ (dissimilatory

reduction).

» 4. It may be leached from the upper profile to

deeper soils layers or to ground water.



VI. Control of Nitrification

1. Use of microbial inhibitors e.g. N-serve

(Nitrapyrin).

2. Use of N-compounds that release N slowly

because of limited solubility in water or slow

rate of microbial decomposition e.g. urea

formaldehyde complexes, oxamide.

3. Use nitrogen compounds with materials thatdelay the release of N.



VII. Desirable Characteristics of

Nitrification Inhibitors

1. Specific in blocking NH4+ NO3-

2. Able to move with fertilizers through

soil 3. Non-toxic to plants

4. Non-toxic to plants plant root

microenvironment 5. Not easily bio or chemical degradable



VIII. Nitrate Pollution

1. Nitrate can be a significant environmental

pollutant.

2. This is because of its potential role in

» a. eutrophication,

» b. infant methemoglobinemia associated with

nitrate rich waters and vegetable,

» c. animal methemoglobenimia and d. formation

of nitrosoamine.




a. Eutrophication:

» This is the process whereby waters are enrichedwith nutrients.

» N03- in rivers encourage algae and rooted

plants to flourish.

b. Methemoglobinemia:

» Arises when nitrate consumed in water, foodand feed is reduced in the gastrointerstinal tractto nitrite this nitrite enters the blood stream




and reacts with hemoglobin to from met-hemoglobin.

» This leads to an impairment of O2 transport.

» It is serious for infants under three months andin ruminants.

» Standards of NO3- by WHO in water is 10ppmnitrate-N.

c. Animal may die from consuming plant specieswith high levels of NO3-.




d. Nitrosoamines:

» 1. These compounds are formed from thecondensation of a secondary amine and

nitrite.

R’ R

NH + NO2 N-NO + OH-

R R’




2. They are carcinogenic, mutagenic and

teratogenic i.e. they can induce cancer,

mutation and cause birth defects or sometimes death in fetuses.

3. Reaction can be carried out by microbial

enzymes or non-enzymatically byorganic materials present in soils.



Denitrification:

Definition: Denitrification is the microbial reduction of NO3-

and NO2- and then to gaseous N2O and N2. This process is different from dissimilatory nitrate

reduction and also from assimilatory nitratereduction.

Assimilatory Nitrate Reduction: NO3- to NH4+ in the course of biosynthesis of

Amino Acids and proteins.



I. Introduction:

Dissimilatory Nitrate Reduction:

» Certain microbial species use NO3- as

electron acceptor in absence of O2 to produce NH4+, but not N2, as an end

product.

» Pathway doesn’t lead to nitrogen lossfrom the system .



+5 +4H +3 +6H -4

2HNO3 2HNO2 ? [2NH2OH] 2NH4+-H2O -2H2O

d. Extensive microbes can reduce NO3- to NO2- in the presence of organic matter and absence of O2.

This process is called nitrate respiration. NO3- NO2-

Nitrate respiration



II. Microbiology

Although the capacity for denitrification is

found in many bacteria active in proteolysis,

ammonification and other transformation,the capacity for true denitrification is

limited to certain bacteria.

The active species are largely limited to thegenera Pseudomonas, Bacillus and

Paracoccus.



II. Microbiology

Fungi and actinomycetes have not beenimplicated in N2 production.

The denitrifying bacteria are aerobic, butnitrate is used as the electron source inabsence of O2.

Thus the active species grow aerobically

without nitrate and anaerobically in its presence.



III. Biochemistry of Denitrification

Reaction

+5 +3 +2 +1

0

2NO3- ® 2NO2- ® 2NO ® N2O ® N2

nitrate nitrite nitric oxide nitrous oxide

reductase reductase reductase reductase

¬ Photoautotrophs « Chemoautotrophs ® Chemoautotrophs®




a. Under field conditions, not all the intermediategaseous products are converted to N2 by each of thespecific reductase enzymes involved.

b. Nitrate Reductase

1. Nitrate Reductase contains Mo, Fe, and labile sulfidegroups

2. Both Mo and Fe are necessary for enzyme activity.

3. Reaction:

2NO3- + 10H N2 + 4H2 +2OH-




c. Nitrite reductase (NO2-)

1. 2NO2- + 6H ® N2 + 2H2 + OH-

2. Soluble and found in soluble fraction of cytoplasm.

d. Nitric Oxide (NO) reductase :

1. Membrane Bound,

E-FeII + NO2- ® E-FeII ® E-FeII.NO ® E-FeII.(N2O3) ® E-FeII + N2O (E = enzyme, FeII=Iron Group)



IV. Factors Affecting Denitrification

1. Soil Nitrate Content.

» At low NO3- concentration the kinetics of

reduction appear to be first order.» Nitrate concentration has been observed to

influence the N2:N2O ratio in the gaseous

product of denitrification.

» At high NO3- levels N2 is the predominant, and

at low levels it is often N2O




2. Carbon Availability.

» Most denitrification is accomplished byheterotrophs and is therefore, the process is

strongly dependant on the availability of carbon.

» Available carbonaceous substrates supplyelectrons. Their decomposition also produces

CO2 and reduces the O2 levels, thus increasingthe demand for NO3- as an electron acceptor during microbial growth.




» The increased amounts of easily decomposablecarbon provided by root exudates and rootexfoliates in the rhizosphere suggests that thisregion may be more conducive todenitrification than non-rhizosphere soil.

3. Soil Water Content.

» The influence of water is linked to its role ingoverning O2 diffusion to sites of microbial

activity. Increases in soil water content to levelsthat interfere with air diffusion progressivelyincrease denitrification potential.




» Oxygen is repressive to all the nitrogen oxide

reductases, but slightly less repressive to NO3-

reductase than to the others.

» Generally, in the field, and providing there are

no abnormal high O2 consumption rate,

denitrification is lacking or insignificant at

moisture levels less than 60% of moistureholding capacity, which is approximately

equivalent to -0.01MPa moisture stress.




3. Soil pH.

» Most denitrifying bacteria grow best near

neutrality (pH 6-8).» Denitrification becomes slow but may still

remain significant below pH 5.

» It is negligible or absent below pH 4

» The degree of soil acidity also influences the

N2O:N2 ratio in the evolved gases.




4. Temperature.

» Minimum temperature for denitrification is

about 5oC, maximum temperature, about75oC.

» Optimum temperature is around 25oC and

above.

EVSC 590.N Cycle. Ntri Denitrification

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