3.NITROGEN

• The gas, nitrogen occupies 78 percent of our atmosphere.

• It was not known until "mephitic air" was discovered by Daniel Rutherford in Scotland in 1772.

• The inability of nitrogen to support combustion was the cause of its discovery.

• Nitrogen is the first fertilizer element of the macronutrients usually applied in commercial fertilizers.

• Nitrogen is very important nutrient for plants and it seems to have the quickest and most pronounced effect.

NITROGEN IN SOILSSOURCESThe earth's atmosphere • Consists of 78 percent nitrogen

• It is the ultimate source of nitrogen.

• The nitrogen found in soil minerals is negligible.

• Nitrogen may be added to or lost from soil by a number of processes.

• In the soil, nitrogen can undergo a number of transformations.

2) Rainfall

• It adds about 10 pounds of nitrogen to the soil per acre per year.

• The nitrogen oxides and ammonium that are washed to earth are formed during electrical storms, by internal combustion engines and through oxidation by sunlight.

3) Crop residues • Decompose in the soil to form soil organic matter.

• This organic matter contains about 5 percent nitrogen.

4) Legumes • Fix atmospheric nitrogen through their symbiotic

association with Rhizobium bacteria.

• Perennial legumes, such as alfalfa, can fix several hundred pounds of nitrogen per acre per year.

5) Manure

• It contains an appreciable amount of nitrogen.

• Most of this nitrogen is in organic forms: protein and related compounds.

• Cattle manure contains about 10 to 40 pounds of nitrogen per ton.

• About half of this nitrogen is converted to available forms to plants during the first growing season.

• Lesser amounts are converted during succeeding seasons.

• Each ton of applied manure is equal to about 5 to 20 pounds of commercial fertilizer nitrogen.

6) Commercial fertilizer • Nitrogen comes in three basic forms: gas, liquid and

dry. • All forms are equally effective when properly applied.

• Once applied, fertilizer nitrogen is subject to the same transformations as other sources of nitrogen.

• There is no difference between the ammonium (NH4+) or

nitrate (NO3-) that enters the plant from commercial

fertilizer and that produced from natural products such as manure, crop residues or organic fertilizers.

GAINS OF NITROGEN IN SOILS• Soils gain nitrogen by nitrogen fixation process.

Nitrogen fixation refers to the conversion of elemental nitrogen, N2, to its compound forms.

• The fixation processes by which the soils gain atmospheric nitrogen are classified as shown below:

• Symbiotic fixation of atmospheric nitrogen.

• Non-symbiotic fixation of atmospheric nitrogen.

• Industrial fixation of atmospheric nitrogen.

• Fixation of atmospheric nitrogen by atmospheric electrical discharge and addition of ammonia, oxides of nitrogen and organic nitrogen compounds

SYMBIOTIC FIXATION OF ATMOSPHERIC NITROGEN• It is the process by which elemental nitrogen, N2, is

converted to its organic compounds by the association (or symbiosis) between two kinds of plants.

• The enzyme nitrogenase activates the mechanism of biological nitrogen fixation (BNF) by catalyzing the reduction of dinitrogen gas into ammonia under anaerobic conditions.

• Then host systems combine ammonia with organic acids, and form amino acids and proteins.

N2 + 8H+ + 6e- Nitrogenase 2 NH3 + H2(Fe, Mo)

• The legumes and rhizobia, the two dissimilar organisms, live together in intimate association (i.e Symbiosis).

• Members of the genus Rhizobium bacteria infect the roots of appropriate legumes to develop nodules.

• Effective nodules cluster on primary roots and have pink to red centers.

• The red colour of the nodule is attributed to the occurrence of leghemoglobin.

• The quantity of N fixed by properly nodulated legume averages about 75% of the total N used for the plant growth.

• The amount of BNF varies with Rhizobium strain, host plant, and environment.

• From the legumes, the rhizobia obtain their requirements for growth such as carbohydrates, mineral nutrients and other growth factors.

• The rhizobia take free nitrogen (i.e. elemental nitrogen, N2) from the soil air and synthesize complex compounds of nitrogen (i.e glutamic acid).

• Thus, nitrogen is fixed. The legumes absorb this nitrogen.

• The amount of nitrogen fixed depends on several factors as follows;

• Soil properties : aeration, moisture, drainage, pH • Content of nutrients: calcium, Ca, boron, B,

molybdenum, Mo, cobalt, Co, sulphur, S, magnesium, Mg.

• Leguminous species: alfalfa, soybean, cowpea, bean, pea etc. Thus, for a given legume the amount of fixed nitrogen varies from land (i.e., soil) to land and for a given land.

Legumes Nitrogen fixed(kg ha -1 year -1)

Range Average

Alfalfa 50 – 450 194

Soybean 58 – 160 100

Cowpea 58 – 116 90

Pea 30 – 140 72

The amount of fixed nitrogen varies with leguminous species as shown below:

• Here nitrogen fixing capacity of the legumes is graded as

• Alfalfa > soybean > cowpea > pea

• Nitrogen fixation is high when the soil contains minimum inorganic nitrogen.

• The application of large amount of nitrogen

fertilizers reduces the activity of Rhizobia and hence, reduces the extent of nitrogen fixation

• However, a very small amount of nitrogen fertilizer may be applied so that the legume seedlings get required (never excess) nitrogen until the Rhizobia establish on the root nodules.

• Nitrogen fixation by bacteria in symbiosis with legume trees

• Some leguminous trees also cause nitrogen fixation. The trees develop nodules.

• The bacteria in these nodules fix free nitrogen by the

symbiosis process. Some of these tree species are;- Mimosa, Acacia, Black locust

Like legumes some non-legumes develop root nodules. The bacteria existing in nodules fix free nitrogen by symbiosis. The families of these non-legumes capable of causing nitrogen fixation are;

-Betulaceac, Casurinaceae, Coriariaceae, Elaeagnaceae, Myricaceae, Rhamnaceae

Nitrogen fixation by bacteria in symbiosis with non-legumes

• However, these are not cultivated as crop plants.

• They are often found in soils having low nitrogen.

• The species (strains) of the genus Rhizobium that fix free (i.e. elemental) nitrogen are meliloti, trifolii, japanicum, lupine, phaseoli, leguminosarum. They (N fixers) are commonly referred to as nodule organism or legume organism or legume bacteria or nodule bacteria.

• NON-SYMBIOTIC FIXATION OF ATMOSPHERIC NITROGEN

• Mechanism of fixation

• Certain organisms live independently in soils (e.g. Azotobacter), in water (e.g. blue – green algae) and on leaves (e.g. Beijerenckia).

• They are not associated with higher plants. • The free-living organisms absorb elemental nitrogen,

N2, from atmosphere and synthesize organic nitrogenous compounds.

• This phenomenon is referred to as non symbiotic (or asymbiotic or free) fixation of nitrogen.

• Non-symbiotic nitrogen fixing organisms

• These organisms are aerobic, anaerobic, heterotrophic, autotropic, photosynthetic.

• They are bacteria, algae and fungi. • The genera

1.Bacteria:• Azotobacter, Aerobacter , Beijerinckia,

chlostridium, Chromatium. Chlorobium, Rgodospirillum, Rhodomicrobium

2. Blue – green algae:

• Nostoc, Calothrix

• The Anabaena blue green algae (Cyanobacteria) inhabit cavities in the leaves of the floating water fern Azolla and fix quantities of N comparable to those of the better Rhizobium-legume complex.

• It could fix about 30-105 kg N/ season taking care of 75% N requirement of rice.

3. Fungi:

• Pullularia

• The amount of nitrogen gained by soils through non-symbiotic nitrogen fixation varies widely from few kilograms to about 50 kilogram per acre per year.

• It is to be noted that the nitrogen fixing organisms absorb inorganic nitrogen (NH4+ and NO3-) present in soil.

• Until the inorganic nitrogen in soil is deficient the organisms do not absorb atmospheric nitrogen.

• If the soil contains more organic matter, the organisms are supplied with more energy (needed for their life processes) and consequently, nitrogen fixation by them increases.

• INDUSTRIAL FIXATION OF ATMOSPHERIC NITROGEN

• Nitrogen, industrially prepared from atmosphere, reacts with hydrogen to yield ammonia. From ammonia various nitrogen fertilizers are produced.

• Thus, elemental nitrogen in atmosphere is industrially (or non-biologically) fixed in fertilizer forms.

• If these fertilizers are added, soils gain nitrogen.

• FIXATION OF ATMOSPHERIC NITROGEN BY ATMOSPHERIC ELECTRICAL DISCHARGE:

• Soils gain small amount of nitrogen from atmosphere by ways as follows:

1. During atmospheric electrical discharge (lightening) the elemental nitrogen, N2, present in atmosphere is converted to nitrogen dioxide, NO2.

N2 + 02 → 2 NO2NO + O2 → 2 NO2

• With rain water nitrogen dioxide, NO2 forms nitric acid, HNO3 (i.e., nitrate).

4 NO2 + 2 H2O + O2→4 HNO3

• This nitric acid, HNO3, (i.e., nitrate) is brought down by rainfall.

2. The ammonia, NH3, emanating from industrial sites or volatilized from soils is brought down by rainfall.

3.The ammonia, NH3, may be directly absorbed by soils (not by rainfall) in locations where the concentration of atmospheric ammonia is above normal.

4. The nitrogen dioxide, NO2, emanating from industrial sites is likely to be added to soils by rain water

• The amount of nitrogen gained by soils varies with seasons and climatic zones.

• For example, the gain of nitrogen is higher in tropical zone than in temperate zone.

• Again the gain is higher in temperate zone than in semiarid zone.

• However, the amount of nitrogen gained is very small. It

varies from 1 to 60 kg per hectare per year.

• FORMS OF NITROGEN IN SOILS

• Nitrogen is in organic and inorganic forms in soils.

• ORGANIC FORMS

• The organic forms arise from plant and animal residues. • More than 90 percent of soil N is associated with soil

organic matter.

• They occur as protein, free amino acid, amino sugar and other complexes.

• 20 – 40% of total soil nitrogen occurs as protein.

• INORGANIC FORMS• The inorganic forms can be classed as ionic forms and

gaseous forms.

• The ionic forms include ammonium ions, NH4+, nitrate

ions, NO3, and nitrite ions, NO2.

• 2-5% of total soil nitrogen occurs as these ionic forms. • They arise from the mineralization of organic forms of

nitrogen.

• Also, the fertilizers contribute these ionic forms.

• Plants may use either ammonium (NH+4 or nitrate (NO-3), which behaves quite differently in soils.

• Positively charged NH4+ is attracted to negatively charged sites on soil particles, as are other cations.

• It is available to plants, but it will not leach.

• Negatively charged NO3- remains in the soil solution and moves with soil water.

• Consequently NO3- may leach out of the root zone when rainfall is excessive, or it may accumulate at the soil surface when conditions are dry.

• The gaseous forms include elemental nitrogen,(N2); nitrous oxide, (N2O); nitric oxide, (NO); nitrogen dioxide, (NO2) and ammonia (NH3).

• These gaseous forms arise from chemical reaction, de-nitrification and volatilization.

FORMS OF NITROGEN ABSORBED BY PLANTS

• Nitrate generally occurs in more concentration than NH4+, and freely moves into roots by mass flow and diffusion.

• Ideally, NH4+ is the preferred source of N since energy will be saved for the synthesis of protein, instead of NO3-.

• Most plants absorb both NH4+ and NO3-. Rice prefers N as NH4+. • Certain rice varieties are sensitive to high NO3- concentration.

Solanaceous plants prefer both at the same time.

• Plants absorb nitrogen in such forms as follows:

• Nitrate ions, NO3

• They predominate in warm and well – aerated soils. Hence, plants grown in well-aerated soils absorb nitrate ions, NO3.

• Ammonium Ions NH4

• Urea, CO (NH2)2

• Urea is absorbed through leaf epidermis, when urea solution is spread over the foliage. Usually, it is not absorbed by roots.

• Nitrite ions, NO2

• Plants absorb nitrite ions, NO2. They are toxic.

• Amino acid and nucleic acid

• They are absorbed by higher plants. However, they accumulate in soils in small quantities.

• Most of the nitrogen is absorbed by plants as ammonium ions, NH4

+ and nitrate ions, NO3-. Some plants such as

tobacco, tomatoes etc., prefer nitrate ions, NO3- for absorption.

• Some plants such as potato, rice, corn, pineapple, sugar beets etc., prefer equally ammonium ions, NH4

+ , and nitrate ions, NO3

-.

• Thus, the preferences for ammonium, NH4+ , or nitrate,

NO3-, ions differ with different plant species.

• However, plat growth is often improved when plants are nourished with both ammonium, NH4

+ , and nitrate,NO3- ,

ions rather than with sole ammonium ions, NH4+ or with

sole nitrate ions, NO3-.

NITROGEN CONTENT OF SOILS• On world –wide basis the nitrogen content is 0.02 – 0.4%

in surface soils.

• In peat soils, nitrogen content is above 2.5%. In Indian soils, nitrogen content is 0.03 – 0.06% in surface soils.

In soils profile: • The surface layer of soil profile contains highest amount

of nitrogen and with increasing depth of profile, the nitrogen content decrease.

THE NITROGEN CYCLE• The nitrogen cycle represents one of the most important

nutrient cycles found in terrestrial ecosystems (Figure -1). Nitrogen is used by living organisms to produce a number of complex organic molecules like amino acids, proteins, and nucleic acids. The store of nitrogen found in the atmosphere, where it exists as a gas (mainly N2), plays an important role for life. This store is about one million times larger than the total nitrogen contained in living organisms. Other major stores of nitrogen include organic matter in soil and the oceans. Despite its abundance in the atmosphere, nitrogen is often the most limiting nutrient for plant growth.

• This problem occurs because most plants can only take up nitrogen in two solid forms: ammonium ion (NH4+) and the ion nitrate (NO3- ).

• Most plants obtain the nitrogen they need as inorganic nitrate from the soil solution.

• Ammonium is used less by plants for uptake because in large concentrations it is extremely toxic.

• Animals receive the required nitrogen they need for metabolism, growth, and reproduction by the consumption of living or dead organic matter containing molecules composed partially of nitrogen.

• In most ecosystems nitrogen is primarily stored in living and dead organic matter.

• This organic nitrogen is converted into inorganic forms when it re-enters the biogeochemical cycle via decomposition.

• Decomposers, found in the upper soil layer, chemically modify the nitrogen found in organic matter from ammonia (NH3) to ammonium salts (NH4+).

• This process is known as mineralization and it is carried out by a variety of bacteria, actinomycetes, and fungi.

• Nitrogen in the form of ammonium can be absorbed onto the surfaces of clay particles in the soil. The ion of ammonium has a positive molecular charge is normally held by soil colloids. This process is sometimes called micelle fixation (see Figure 9s-1). Ammonium is released from the colloids by way of cation exchange. When released, most of the ammonium is often chemically altered by a specific type of autotrophic bacteria (bacteria that belong to the genus Nitrosomonas) into nitrite (NO2- ). Further modification by another type of bacteria (belonging to the genus Nitrobacter) converts the nitrite to nitrate (NO3- ).

• Both of these processes involve chemical oxidation and are known as nitrification.

• However, nitrate is very soluble and it is easily lost from the soil system by leaching.

• Some of this leached nitrate flows through the hydrologic system until it reaches the oceans where it can be returned to the atmosphere by de-nitrification. De-nitrification is also common in anaerobic soils and is carried out by heterotrophic bacteria.

• The process of de-nitrification involves the reduction of nitrate into nitrogen (N2) or nitrous oxide (N2O) gas.

• Both of these gases then diffuse into the atmosphere.

• This process is important to the bacteria because it supplies them with oxygen for respiration.

• Almost all of the nitrogen found in any terrestrial ecosystem originally came from the atmosphere.

• Small proportions enter the soil in rainfall or through the effects of lightning.

• The majority, however, is biochemically fixed within the soil by

specialized micro-organisms like bacteria, actinomycetes, and cyanobacteria.

• Members of the bean family (legumes) and some other kinds of plants form mutualistic symbiotic relationships with nitrogen fixing bacterial.

• In exchange for some nitrogen, the bacteria receive from the plants carbohydrates and special structures (nodules) in roots where they can exist in a moist environment.

• Scientists estimate that biological fixation globally adds approximately 140 million metric tons of nitogen to ecosystems every year.

• The activities of humans have severely altered the nitrogen cycle. Some of the major processes involved in this alteration include:

• The application of nitrogen fertilizers to crops has caused increased rates of denitrification and leaching of nitrate into groundwater.

• The additional nitrogen entering the groundwater system eventually flows into streams, rivers, lakes, and estuaries.

• In these systems, the added nitrogen can lead to eutrophication.

• Increased deposition of nitrogen from atmospheric sources because of fossil fuel combustion and forest burning.

• Both of these processes release a variety of solid forms of nitrogen through combustion.

• Livestock ranching. Livestock release a large amounts of ammonia into the environment from their wastes.

• This nitrogen enters the soil system and then the hydrologic system through leaching, groundwater flow, and runoff.

• Sewage waste and septic tank leaching.

• Nitrogen exists in a number of chemical forms and undergoes chemical and biological reactions.

• The following transformation processes are numbered to coincide with Figure 1.

NITROGEN TRANSFORMATIONS

N-TRANSFORMATIONS

1.Organic nitrogen to ammonium nitrogen (mineralization).

2.Ammonium nitrogen to nitrate nitrogen (nitrification).

3.Nitrate or ammonium nitrogen to organic nitrogen (immobilization).

4.Nitrate nitrogen to gaseous nitrogen (de-nitrification).5.Ammonium nitrogen to ammonia gas

(ammonia volatilization).

• Organic nitrogen comprises over 95 percent of the nitrogen found in soil.

• This form of nitrogen cannot be used by plants but is gradually transformed by soil microorganisms to ammonium (NH4

+).

• Ammonium is not leached to a great extent. Since NH4

+ is a positively charged ion (cation), it is attracted to and held by the negatively charged soil clay. Ammonium is available to plants.

1.Organic nitrogen to ammonium nitrogen (mineralization).

• In warm, well-drained soil, ammonium transforms rapidly to nitrate (NO3

-).

• Nitrate is the principle form of nitrogen used by plants.

• It leaches easily, since it is a negatively charged ion (anion) and is not attracted to soil clay. The nitrate form of nitrogen is a major concern in pollution.

2. Ammonium nitrogen to nitrate nitrogen (nitrification).

• Soil microorganisms use nitrate and ammonium nitrogen when decomposing plant residues.

• These forms are temporarily "tied-up" (incorporated into microbial tissue) in this process.

• This can be a major concern if crop residues are high in carbon relative to nitrogen.

• Examples are wheat straw, corn stalks and sawdust.

• The addition of 20 to 70 pounds of nitrogen per ton of these residues is needed to prevent this transformation.

• After the residues are decomposed, the microbial population begins to die back and processes 1(mineralization) and 2 (nitrification)take place.

3. Nitrate or ammonium nitrogen to organic nitrogen (immobilization)

• When soil does not have sufficient air, microorganisms use the oxygen from NO3

- in place of that air and rapidly convert NO3

- to nitrogen oxide and nitrogen gases (N2).

• These gases escape to the atmosphere and are not available to plants.

• This transformation can occur within two or three days in poorly aerated soil and can result in large loses of nitrate-type fertilizers.

4. Nitrate nitrogen to gaseous nitrogen(de-nitrification).

• Soils that have a high pH (pH greater than 7.5) can lose large amounts of NH4

+ by conversion to NH3 gas.

• To minimize these losses, incorporate solid ammonium-type fertilizers, urea and anhydrous ammonia below the surface of a moist soil.

5. Ammonium nitrogen to ammonia gas (ammonia volatilization)

• Mineralization of nitrogen is the conversion of organic form of nitrogen to inorganic form (or mineral form) of nitrogen such as ammonium ions, NH4

+, nitrite ions, NO2, and nitrate ions, NO3.

• It takes place in three steps as follows:

1.Aminization 2. Ammonification 3. Nitrification

1.MINERALIZATION OF ORGANIC NITROGEN COMPOUNDS

1.Aminization

• In this step protein (an organic form of nitrogen ) breaks down to yield amines, amino acids, carbon dioxide, energy and other reaction products.

Protein R-NH2+CO2+Energy+Other reaction products

• This step starts after completion of aminization.

• In this step, the amines and amino acids released by aminization process are converted to ammonia as follows:

R-NH2 + H-OH NH3 + R–OH+ E

2. Ammonification

• This conversion is caused by another group of heterotrophic soil micro organisms such as bacteria, fungi and actinomycetes.

• NH3 so released is converted to ammonium ions, NH4

+. Two reactions may be suggested to demonstrate it.

Reaction 1

NH3 + H2O NH4OH

NH4OH NH4+ + OH-

• In this reaction, ammonia, NH3, reacts with water,H2O, to form ammonium hydroxide, NH4OH. Being unstable, ammonium hydroxide, NH4OH, breaks apart to yield ammonium ions, NH4

+, and hydroxyl ions. OH-.

Reaction 22NH3 + H2CO3 (NH4)2CO3 2NH4

++ CO3

NH3 + NHO3 NH4NO3 NH4+ + NO3

• In this reaction, ammonia, NH3,reacts with carbonic acid, H2CO3, or nitric acid, HNO3, formed in soils to yield ammonium carbonate, (NH4)2CO3, or ammonium nitrate, NH4NO3 which, in turn, breaks apart to yield ammonium, NH4

+, and carbonate ions, CO3-, or

ammonium, NH4+, and nitrate, NO3

-, ions.

• In both reactions the common products are ammonium ions,NH4

+.

• Hence, this step (process) is termed ammonification.

• The ammonium ions, NH4+, released in this process are

subjected to several fates.

• This step follows ammonification process.

• In this step, a part of ammonium ions, NH4+, released by

ammonification is converted to nitrate ions, NO3-, as shown below:

2NH4+ + 3O2 2NO2

- + 2H2O + 4H+ + Energy[1st Step]2NO2

- + O2 2NO3- + Energy [2nd Step]

• It is a two-step process. • In the first step, ammonium ions,NH4

+ are oxidized to yield nitrite ions, NO2

-.

• In the second step, the nitrite ions, NO2- , are oxidized to yield

nitrate ions, NO3-.

3. Nitrification (oxidation process)

• The enzymes causing nitrification process (biological oxidation) are secreted largely by obligate autotrophic bacteria (or autotrophs: They obtain energy from oxidation of inorganic salts and cabon from CO2).

• The genera of bacteria which oxidize ammonium ions, NH4

+, (ammonium oxidizers) are.

– Nitrosomonas, Nitrosococcus, Nitrosocystis, Nitrosospira, Nitrosogloea

• The genus of bacteria which oxidizes nitrite ions, NO2-, (nitrite

oxidizer) is Nitrobacter.

• Collectively, Nitrosomonas and Nitrobacter are known as Nitrobacteria.

• They are also referred to as nitrifiers or nitrifying bacteria.

• Also, numerous heterotrophic organisms (or heterotrophs) oxidize ammonium ions, NH4

+, and nitrite ions, NO2-.

Factors affecting nitrificationThe factors affecting nitrification are as follows;

• Aeration• Moisture• Temperature• Soil pH (Soil reaction)• Lime• Fertilizer• Carbon: Nitrogen ratio

• Aeration: Nitrification is an oxidation process. It takes place readily in well – aerated soils.

• Coarse – textured soils (e.g. sandy soils), granulated soils and ploughed soils ensure a good supply of oxygen. Hence they promote nitrification.

• Moisture: In wet soils with very high moisture content (exceeding field capacity) nitrification is retarded because high moisture occupies the larger soil pores, thereby reducing soils aeration (i.e. oxygen supply).

• In dried soils with very low moisture content (below wilting coefficient) also, nitrification is impeded because nitrifying bacteria are killed in dried soils.

• The optimum moisture content for higher plants may be considered optimum for nitrification.

• Temperature: The ranges of temperature over which nitrification takes place in different soils varies widely.

• 0 – 2oC• 5 – 35oC• 16 – 30oC• 27 – 32oC

• However, nitrification increases with increasing temperature, until the optimum temperature is reached.

• The optimum temperature varies with various soils.

Nitrification does not take place at freezing point or below.

• It ceases at 51oC or above.

• Soil pH (Soil Reaction): Nitrification does not take place in strongly acid soils, probably due to the toxic effect of aluminium ions, Al3+, on nitrifiers.

• In some other acid soils with a pH above 5, nitrification takes place only after the soils are limed.

• In strongly alkaline soils, the nitrite, NO2-, oxidizers

(Nitrobacter) becomes sensitive to alkalinity.

• Thus, nitrate ions, NO3-, formation may be retarded or

stopped.

• However, range of soil pH over which nitrification takes place is wide.

• Lime: The bases such as calcium, Ca, and magnesium, Mg, increase the activity of nitrifiers.

• Hence, they enhance nitrification.

• Thus, to increase nitrification liming is recommended.

• The acid organic soils (having pH 5.5 or below) such as forest and peat soils contain high amount of lime (as high as 4% CaO).

• So, in these soils nitrification takes place.

• Fertilizers: If large quantities of urea or ammonium sulphate or other ammonium fertilizers are added to neutral or alkaline or calcareous soils free ammonia, NH3 is formed.

• This free ammonia, NH3, is much toxic to Nitrobacter (Nitrite oxidizer). Hence, nitrate formation (Second step of nitrification.) is retarded or stopped.

• However, free ammonia, NH3, is less toxic to Nitrosomonas (ammonium oxidizer).

• Hence, nitrites, NO2-, become toxic.

Carbon: Nitrogen Ratio (C: N ratio): • The ratio of carbon content (% C) and nitrogen content

(% N) of a substance (decomposed or undecomposed organic matters, soils etc.) is termed carbon to nitrogen ratio.

• It is expressed as C: N ratio or C/N ratio. • Thus, C: N ratio = %C %N.

• If the C: N ratio is wide, the amount of carbon is much higher over the amount of nitrogen.

• Note that the C: N ratio of any organic substance is not below 1.

• This C: N ratio influences the nitrification process. Organic matters having a wide C: N ratio such as

• Rice straw (C: N ratio 89: 1 or simply 89),

• Wheat straw (C : N ratio 124 : 1 or 124),

• Maize stalks and leaves (C : N ratio 53 : 1 or 53),

• Jowar stalk and leaves (C : N ratio 93 : 1 or 93) etc.

• The organic matters contain large amount of carbon as compared to nitrogen and supplies huge amount of energy to the heterotrophic microorganisms.

• Consequently, the microorganisms multiply rapidly. For multiplication (or building up tissues), they need large amount of inorganic nitrogen (NH4

+ and NO3

-).

• They assimilate this inorganic nitrogen from the soil solution. During assimilation, the inorganic nitrogen is converted to organic nitrogen such as proteins, nucleic acids etc.

• The ammonium, NH4+ and nitrate, NO3

-, ions present in soil solution are mobile.

• Once, they are assimilated by the microorganisms they become immobile.

• Thus, this phenomenon in which ammonium, NH4+ and nitrate, NO3

-, ions are assimilated and converted to organic nitrogen is termed immobilization.

• Under this condition, very little or no nitrogen is available to plants.

• The plants may suffer from nitrogen deficiency.

• After decomposition of organic matters and tissues of microorganisms, the immobilized nitrogen in microbial tissues are mineralized to form ammonium, NH4

+, and nitrate, NO3-, ions.

• Then, they become available to plants.

• Thus, the nitrogen deficiency induced by immobilization is a temporary phenomenon.

• However, to prevent nitrogen deficiency, adequate amounts of nitrogen fertilizers are to be added at the time of incorporating organic matters in soils such that the needs of both microorganisms and growing plants are met with.

• On an average, if organic matters having a narrow C : N ratio (below 20 : 1 or 20) are added to soils, mineralization takes place.

• Usually, immobilization does not take place. If organic matters having a wide C : N ratio (above 30 : 1 or 30) are added to soil immobilization takes place during initial decomposition.

• If organic maters having neither narrow nor, wide C : N ratio (between 20 and 30) neither immobilization nor mineralization may take place.

CO2 evolution

New nitrate level4-8 Weeks

Amount

Time

NO3

level

CO2

level

0 nitrate level

20 nitrate level

60 nitrate level

40 nitrate level

80 nitrate level

C/N ratio

Net immobilization

Net mineralization

Nitrogen is lost from soils in various ways as follows: De-nitrification Decomposition Gaseous losses Volatilization Leaching Crop removal Erosion

LOSSES OF NITROGEN FROM SOILS

• The terms nitrification and de-nitrification are opposite to each other.

• Nitrification means formation of nitrate, while de-nitrification means breakdown of nitrate.

• In nitrification process, nitrate is ultimately formed.

• Thus, the process in which nitrate breaks down is termed de-nitrification. escapes to atm.

-2O - 2O - O - O

2 NO3- 2 NO2

- 2NO N2O N2

nitrate nitrite nitric nitrous elemental nitrogen oxide oxide or

dinitrogen

De-nitrification (reduction process)

• Here, “-O” Means removal of oxygen (i.e., reduction).

• De-nitrification occurs in soils under conditions of poor drainage and poor aeration.

• It is caused by soil microorganisms (facultative anaerobes).

• They are known as denitrifying organisms or denitrifiers.

• Under anaerobic condition of soils (e.g., submerged or waterlogged and cloded i.e., compact soils) free oxygen, O2, is excluded.

• In absence of free oxygen, O, the denitrifiers use the oxygen, O2, present in nitrates, NO3

-, nitrites, NO2- and oxides of nitrogen, for their

respiration.

• Thus the nitrates, nitrites, (including hyponitrites) and nitric oxides, NO, are reduced to nitrous oxides, N2O and ultimately to free nitrogen.

• If the soil is supplied with readily decomposable (oxidizable) organic matter, the denitrifiers oxidize carbohydrate (in organic matter) with oxygen, O2, present in nitrate, NO3.

• Thus, nitrate, NO3, is reduced nitrous oxide, N2O and free nitrogen, N2, gases.

• These reactions can occur not only in anaerobic soils but also in aerobic soil).

• Thus, it may be concluded that though denitrification occurs mostly in poor aerated soils, it can also occur in well aerated soils provided the soils are well supplied with readily decomposable (oxidizable) organic matters and nitrates, NO3.

Factors influencing denitrification:• The factors influencing denitrification are as

follows:• Aeration (oxygen supply)• Moisture (water content)• Readily decomposable organic matter• Temperature• Soil pH (soil reaction)• Nitrate

1.Aeration: (oxygen supply)

• The loss of nitrogen by de-nitrification increases with decreasing content of oxygen, O2, in soils.

• It becomes severe when oxygen, O2, is excluded in soils.

• The de-nitrification loss of nitrogen excessively increases in submerged (water logged) soils and in soils well supplied with readily decomposable organic matters (e.g., green manure).

• The clods (or small aggregates or micro aggregates) of soil may be present in many soils. Inside these clods aeration (i.e.,O2 supply) is poor.

• Thus, inside the clods de-nitrification occurs. If the soil surrounding the clods is well aerated (i.e., well supplied with O2) nitrification occurs there.

• Thus, in the same soils containing clods both nitrification and de-nitrification occur simultaneously.

• An anaerobic condition is developed in the rhizosphere as the plant roots and micro organisms respire. That consumes oxygen, O2 and simultaneously releases carbon dioxide, CO2, If the rhizosphere is supplied with large amount of nitrates, NO3, de-nitrification takes place.

• 2.Moisture: (Water content) Water logging (submergence) or saturation of soil pores with water eliminates oxygen, O2.

• Thus, an anaerobic soil condition develops that induces de-nitrification.

• 3.Readily decomposable organic matter:

As discussed earlier, the readily decomposable organic matter (oxidizable carbonaceous material) containing sugar or organic acids induces de-nitrification in soils under both anaerobic and aerobic conditions.

• 4.Temperature:

• De-nitrification proceeds very slowly at very low temperature.

• Thus, de-nitrification loss of nitrogen is very low during winter when the temperature falls near freezing.

• However, de-nitrification increases vigorously as temperature rises up to 60oC.

• De-nitrification decreases at temperature above 60oC due to detrimental effect of heat on de-nitrifiers.

• However, the optimum temperatures for de-nitrification range from 25 to 30oC.

• 5.Soil pH: (soil reaction). • De-nitrification is very slow in strongly acid soils because

de-nitrifiers can not thrive in these in these soils. • Thus, de-nitrification loss of nitrogen is very low in acid

soils with a pH much below 5.0.

• De-nitrification is very rapid in calcareous, neutral and alkaline soils.

• The optimum pH for de-nitrification loss of nitrogen is between 8.0 and 8.6.

6.Nitrate:

• As discussed earlier, de-nitrification starts from nitrate, NO3

- thus the rate and extends of de-nitrification increases if the nitrate content of the soil is high.

7.Decomposition• The reduction of nitrate, NO3

-, is termed biological denitrification while the reduction of nitrite, NO2

-, is termed non- biological denitrification.

• This process is also termed non-enzymatic decomposition or chemical decomposition or non biological decomposition or chemical denitrification or chemo-denitrification. The decomposition (nitrite reduction) is a chemical process.

• Several reactions have been proposed to demonstrate chemical decomposition of nitrite.

• REACTION 1 3HNO2 HNO3 + H2O + 2NO

or

2HNO2 NO2 + H2O + NO

• In these reactions, nitrous acid, HNO2 (i.e. nitrite) spontaneously decomposes in strongly acid soils (pH below 5) to yield nitric oxide, NO and nitrogen dioxide, NO2, because it (HNO2) is unstable in acid medium.

• In presence of oxygen, O2, NO oxidizes to nitrogen dioxide, NO2.

2NO + O2 = 2NO2

• This nitrogen dioxide formed directly from nitrite, HNO2, react with water to yield nitrous and nitric acids. 2NO2 + H2O HNO2 + HNO3

• Thus, no loss of nitrogen in the form of nitric oxide, NO, gas takes place in aerated (i.e. aerobic) soils.

• In soils where oxygen, O2, is completely eliminated (i.e. in anaerobic soils) nitrogen is lost in the form of nitric oxide, NO.

• However, the decomposition of nitrite, HNO2, occurs rapidly at pH 4 or below and slowly at pH 5 to 7.

• It does not occur at pH 8 or above.

• Humic acid, a product of organic matter decomposition, accelerates the decomposition of nitrite, HNO2, Humic acid contains phenol group.

REACTION 2

R – NH2 + HNO2 R-CH2COOH + H2O + N2 primary aliphatic amine hydoxyacid

or,

R-NH2 + HNO2 R-OH + H2O + N2

• In this reaction, primary aliphatic amine reacts with nitrous acid, HNO2 (i.e., nitrite) to yield hydroxy acid or alcohol, water and elemental nitrogen (or molecular nitrogen or dinitrogen).

• The elemental nitrogen escapes from soil.REACTION – 3• NH3 + HNO2 NH4 NO2

• NH4NO2 2 H2O + N2

• In these reactions, ammonia, NH3, (formed during nitrogen mineralization) reacts with nitrous acid, HNO2, (i.e., nitrite) (formed in nitrification process,) to yield ammonium nitrite, NH4NO2, which, in turn decomposes to yield water and elemental nitrogen, N2, (Molecular nitrogen or di-nitrogen).

• This nitrogen, N2, is lost from soil. This loss occurs in only dried soils.

REACTION – 4

CO(NH2) 2 + 2 HNO2 CO2 + 3 H2O + 2 N2

urea

• In this reaction, urea reacts with nitrous acid (i.e., nitrite) to yield carbon dioxide, CO2, water and elemental nitrogen, N2. The nitrogen, N2, escapes from soil.

• The chemical decomposition of nitrite, NO2, occurs in acid soils only.

• The various factors influencing the loss of nitrogen by chemical decomposition of nitrite are:

– nitrite, NO2, accumulation in soil– soil reaction i.e., soil pH,– soil aeration

Agricultural practices to reduce the loss of nitrogen from acid soils

TILLAGE:

• If the soil is well aerated by tillage, the nitrate formation is more rapid than nitrite formation.

• Thus, nitrite, NO2-, does not accumulate in large amount.

• Moreover, if nitrite, NO2

-,decomposes to yield nitric oxide, NO, the nitric oxide, NO, oxidizes to nitrogen dioxide, NO2, that reacts with soil water to yield nitrous and nitric acids, HNO2 and HNO3,.

• Thus, NO can not escape from soil.

LIMING:

• As the decomposition of nitrite, NO2-,

occurs in only acid soils, liming the acid soils to neutrality retards this process and thereby reduce the formation of elemental nitrogen, N2 and its oxides, NO and N2O.

• Thus, nitrogen loss is reduced.

Volatilization:

• In this process, free ammonia, NH3 is produced from ammonium containing fertilizers viz.

– ammonium sulphate, (NH4)2 SO4, – ammonium nitrate, NH4NO3, – ammonium chloride, NH4Cl etc,ammonium forming fertilizers viz. urea, CO(NH2) 2.

• This free ammonia, NH3, escapes from soil to atmosphere. • This process occurs in alkaline and calcareous soils.• Even in acid soils too, this process occurs if large amount of urea,

CO (NH2) 2, is added.

Volatilization from alkaline soils• Few examples to demonstrate volatilization of ammonia, NH3, are

given below:

Example 1Aqua ammonia i.e., ammonium hydroxide, NH4OH, is

unstable in alkaline medium. Hence, if added to the surface of alkaline soils, it (NH4OH) decomposes to yield ammonia, NH3, gas.

NH4OH NH3 + H2OExample 2

When added to moist alkaline soils, ammonium fertilizers dissociate to yield ammonium ions.

(NH4) 2SO4 2 NH4+ + SO4

2-

NH4NO3 NH4+ +NO3

-

NH4Cl NH4+ + Cl-

The ammonium ions, NH4+ are converted to ammonia, NH3 that

volatilizes.

NH4+ + OH- + H2O NH3 + 2 H2O

dominate in present inalkaline soils moist soil

• Example 3

• When added to the surface of alkaline soil, anhydrous ammonia, NH3, volatilizes.

• NH3 (liquid) NH3 (gas)

Volatilization from calcareous soils

• If ammonium fertilizers are added to the surface of calcareous soils (containing more than 10% free CaCO3) ammonia, NH3, volatilization occurs. The possible reactions involved in volatilization are as follows:(NH4) 2 SO4 + Ca CO3 = CaSO4 + (NH4)2CO3 (NH4) 2 CO3 + H2O = 2NH3 + CO2 + 2 H2O2 NH4 NO3 + Ca CO3 = Ca(NO3) 2 + (NH4)2CO3(NH4)2CO3+ H2O = 2 NH3 + CO2 + 2 H2O2 NH4 Cl + Ca CO3 = CaCl2 + (NH4)2CO3(NH4)2CO3 + H2O = 2 NH3 + CO2 + 2 H2O

Volatilization from urea:• Ammonia volatilization from urea occurs in acid,

actual and alkaline or calcareous soils.

Volatilization in acid soils• In moist acid soils, urea, CO (NH2)2, is

hydrolyzed by urease enzyme to ammonium, NH4+.

urease

CO (NH2) 2 + 2 H2O + H + 2 NH4 + + HCO3 –

• In this reaction, hydrogen ions, H+, that predominate in acid soils are consumed and thus, an alkaline condition (pH above 7.0) at immediate vicinity of urea droplets is induced temporarily.

• Under alkaline condition, a part of ammonium ions, NH4+, form free ammonia, NH3.

NH4+ NH3 + H+

• Hydrogen ions, H+, formed are consumed in further hydrolysis of urea and free ammonia, NH3, formed volatilizes.

Volatilization in neutral, alkaline or calcareous soils:

In moist neutral, alkaline or calcareous soils, urea, CO (NH2)2, undergoes hydrolysis. CO(NH2) 2 + 2 H2O (NH4 )2 CO3 (NH4)2 CO3 +H2O 2NH3+CO2+2H2O

CO (NH2) 2 + 2 H2O + NH4COONH2 NH4COONH2 2 NH3+ + HCO3 –

• Both the reactions yield ammonia, NH3. A part of ammonia, NH3, can volatilize. The other part dissolves in soil water to yield ammonium hydroxide, NH4OH, alkali.

• NH3 + H2O NH4OH

• Ammonium hydroxide, NH4OH, increases soil pH (i.e., soil alkalinity). Moreover, ammonium hydroxide, NH4OH, dissociates to yield ammonium, NH4+, and hydroxyl, OH-, ions.

NH4OH NH4+ + OH –

• The ammonium ions, NH4+, are absorbed by plants, immobilized, fixed or nitrified and thus, they do no accumulate in soils.

• If not leached, the hydroxyl ions, OH-, accumulate and increase soil pH (i.e. soil alkalinity).

• Under alkaline condition, a part of ammonium ions, NH4-, are converted to free ammonia, NH3.

NH4+ NH3 + H+• The hydrogen ions, H+, formed are neutralized by

calcium carbonate, CaCO3, in calcareous soils or soil alkalinity.

• The ammonia, NH3, formed in this reaction can volatilize.

Factors influencing volatilization• Factors influencing ammonia volatilization are as

follows:• Cation Exchange Capacity (CEC)• Soil pH (soil reaction)• Calcium carbonate• Calcium and magnesium• Temperature• Dryness (moisture content)• Fertilizers• Fertilizer application methods

Cation exchange capacity (CEC): • A high CEC of a soil means high magnitude of

negative charge on the surface of soil particles. The free ammonia, NH3, released by hydrolysis of urea, CO (NH2)2, reacts with soil water or other hydrogen (or proton) donors to yield ammonium ions, NH4+.

NH3 +H2O NH4OH NH4+ + OH-

• These ammonium ions, NH4+, and that dissociated from ammonium fertilizers e.g. (NH4)2SO4 2NH4+ + SO42-;

NH4Cl NH4++Cl,

NH4NO3 NH4+ + NO3-) are positively charged.

• So, they are attracted to the high negative charges) e.g soils high in clay and / or organic matter retain more ammonium, NH4+. Consequently, the concentration of free ammonia, NH3, decreases. Thus, ammonia volatilization decreases. The volatilization is high from soils having low C.E.C. e.g., soils low in clay and organic matter.

Soil pH (soil reaction):• The magnitude of ammonia volatilization

increases with increasing pH.

Calcium carbonate: • The magnitude of loss of nitrogen by ammonia

volatilization increase with increasing calcium carbonate, CaCO3, content of soils.

Calcium and magnesium: • The calcium and magnesium are alkali earth

metals (or bases). • So, their high content in soil results in high soil

pH. • So consequently, the degree of ammonia

volatilization increases.

Temperature: High temperature enhances ammonia volatilization.

Dryness (moisture content): • Free ammonia, NH3, reacts with soil moisture (water) to

yield ammonium ions, NH4+,

NH3 + H2O NH4OH NH4+ + OH-

• This reaction decreases the concentration of free ammonia, NH3 and hence, retards ammonia volatilization.

• Thus ammonia volatilization decreases as the soil moisture content increases. Soil dryness accelerates volatilization. On the other hand, ammonia volatilization from urea is retarded by soil dryness as dryness inhibits urea hydrolysis

Fertilizers: • Ammonia volatilization takes place from (i) ammonium

forming fertilizers (e.g. urea) and (ii) ammonium containing fertilizers [e.g. (NH4)2SO4, NH4NO3, NH4Cl etc].

• The ammonium containing fertilizers are more physiologically acidic than ammonium forming fertilizers and hence, they (ammonium containing fertilizers) do not raise soil pH.

• On the other hand, urea, on hydrolysis, liberates free ammonia, NH3, and thus, raises soil pH in the immediate vicinity of urea droplets.

• Thus, ammonia volatilization from urea is greater than that from ammonium containing fertilizers.

• The rise of soil pH caused by urea hydrolysis is so high (though temporary) that ammonia, NH3, volatilizes from acid soils too.

Fertilizer application methods: • Rate of fertilizer and placement of fertilizer influence

ammonia volatilization. Rate of fertilizer application:• The magnitude of volatilization loss of ammonia, NH3,

increases as the rate of application of ammonium containing fertilizers [e.g., (NH4)SO4, NH4NO3, NH4Cl] and ammonium forming fertilizers [e.g. CO(NH2)2] increases.

Placement of fertilizer:• If an ammonium containing or ammonium forming

fertilizer is incorporated with several inches of the surface soil, the volatilization loss of ammonia, NH3, reduces because ammonia, NH3, if volatilized, is reabsorbed by soil at another site before it escapes from soil.

Leaching• Removal of inorganic (or mineral) nitrogen

from surface soil by percolation of water is called leaching; Nitrate, NO3-, ammonium, NH4+, and nitrite NO2-, forms are removed by leaching. It is noteworthy that maximum leaching occurs in the form of nitrate, NO3- (more than 99% of total leached nitrogen).

• Minimum leaching occurs in the forms of ammonium NH4+, (less than 1% of total leached nitrogen) and nitrite, NO2- (traces).

Crop removal:• The ammonium, NH4+ and NO3- forms of

nitrogen present in the soil are absorbed by the growing crops. The removal of nitrogen from soil by this process increases if

• The crop is high-yielding• The crop is capable of absorbing high amount of

nitrogen• The number of harvest per year increases.

• Erosion:

• Nitrogen is lost by erosion of surface layer of soil

.

AVAILABILITY OF N IN SOILS

The nitrogen nutrition of crops is dependent more on the nitrogen –supplying power of soil for a period than on the concentration of NH4+ and NO3- at a given time. In general, the annual release of available N by the soil organic matter lies in the range of 2.4 per cent

• Recently Bajaj and Ramamoorthy (1969) introduced the concept of nitrate potential for the prediction of the availability of nitrogen under field conditions. This is the potential of nitric acid in the saturation extract of the soil, as the formation of nitric acid in the nitrification process is the key step I the mineralization and nitrification of the soil nitrogen before it becomes available to the plant. Mathematically,

• Nitrate potential=pNO3+pH• The nitrate potential is a better index of

nitrogen availability in predicting responses to the application of N fertilizers.