Chapter 10
Nitrogen nutrition of plant
and Nitrogen fertilizers
I Plant nitrogen
1.1 Uptake of N
1. N is required by plants in large amounts.
Most plant material contains 2-4% N and
40% C.
2. N is usually the limiting nutrient in
unfertilized systems.
3. Most N is taken up from the soil in the
form of NH4+ or NO3-. A small amount of
NH3 can be absorbed through the leaves.
N2 can be used by legume plant via
biological nitrogen fixation.
I Plant nitrogen

Most plants grow best with a combination
of NO3- and NH4+.

Some plants are specialists: e.g. use NH4+
& amino acids (e.g. mature forests, arctic
tundra北极冻塬, rice)

Some take up more NH4+ than NO3- when
both supplied together in equal amounts,
but NO3- is prime form of N in soil solution

Over-supply of NH4+ can be toxic to plants
I Plant nitrogen
Uptake of NH4+ may be brought about by the
facilitated diffusion (by the electropotential
difference and cation selective channels).
With NH4+ nutrition recycling of H+ back into
the cytosol is restricted and the H+/pumped out of
the cell and remain mainly outside and hence the
pH is depressed.
Mechanism of uptake NO3 The uptake of NO3- is mainly a H+/ NO3cotransport with the pumped out of the
cell by the PM proton pump being
recycled back into the cytosol.
 Hence the nitrate uptake is associated
with an pH increase in the outer
medium.
1.2 Uptake of N and Rhizosphere pH changes
 NH4+ assimilation: pH  (acid) - often whole root surface
 NO3- assimilation: pH often  (alkaline) - sometimes in
patches; may be also be pH  patches elsewhere on same
root
Maize (in soil & agar + indicator)
NO3-
Marschner &
Römheld (1983)
NH4+ low NO3-
NH4+ assimilation in roots (and pH regulation)
protein etc
sucrose
shoot
xylem
phloem
‘Excess’
cation uptake
root
amino-N
Raven & Smith
(1976); modified
light
CO2
protein etc
NH4+
H+
pH 
NH4+ assimilation in roots (and pH regulation)
protein etc
sucrose
shoot
root
xylem
light
CO2
phloem
‘pH nonperturbing’
amino acids
& amides
amino-N
protein etc
NH4+
H+ pH 
Ammonium
cytoplasm
outside
-ve PD
 NH4+
H+ 
co H+ )
transport  NH4+)
 NH4+ [& K+]
High Affinity Transport
Systems; Km=2040mmol/m3
H+
pH 
As well as net H+ efflux,
some H+ ‘re-cycles’ by cotransport with NH4+
Low Affinity TS
channel  NH4+ [& K+]?
NH3? (Diffusion at high concentrations?)
Energetics, kinetics, regulation, genes : Forde & Clarkson (1999); Glass et al. (2002)
pH regulation during NO3- assimilation in roots
protein etc
sucrose
shoot
xylem
light
CO2
phloem
‘Excess’
anion uptake
root
amino-N
protein etc
NO32H+ pH 
pH regulation during NO3- assimilation in shoots: 1
vacuole
C+ RCOO-
RCOOH
suc.
light
H+
NO3-
shoot
xylem
CO2
amino-N etc
phloem
root
Ignores root NO3reduction & protein
synthesis
NO3-
NO3C+
pH 
pH regulation during NO3- assimilation in shoots: 2
NO3K+ is ‘re-cycled’
to roots with
RCOO- which is
decarboxylated
K+
amino-N etc
suc.
H+ +
RCOO-
xylem
light
CO2
K+
phloem
root
Ignores root NO3reduction & protein
synthesis
K+
[RH]
RCOO-
NO3-
CO2
H+
NO3-
pH 
pH regulation during NO3- assimilation in shoots: 2
NO3K+
amino-N etc
suc.
H+ +
RCOO-
xylem
synthesis
CO2
K+
phloem
‘Excess’
anion uptake
root
Ignores root NO3reduction & protein
light
K+
[RH]
RCOO-
NO3-
CO2
H+
NO3-
pH 
Nitrate
cytoplasm
outside
-ve PD
H+ 
 2H+ )
cotransport  NO3 )
High Affinity TS
高亲合力的运输系统
or
H+ ‘re-cycles’ by cotransport
pH 
cotransport
 2H+ )
 NO3-)
Low Affinity TS
低亲合力的运输系统
NO3-  channel
Energetics, kinetics, regulation, genes : Forde & Clarkson (1999); Glass et al. (2002)
Plasma membrane transporters: summary
Nitrate
cytoplasm
outside
-ve PD
H+ 
 2H+ )
cotransport  NO3 )
Balance sheet outside:
+1H+ -[2H+ + 1NO3-]
= -1[H+ + NO3-]
or: +1OH- -1NO3-
or
( = ‘excess anion influx’)
cotransport
 2H+ )
 NO3-)
pH 
Ignores C+ uptake: as
when NO3- is reduced in
roots
Plasma membrane transporters: summary
Nitrate
cytoplasm
outside
-ve PD
2H+ 
 2H+ )
cotransport  NO3 )
cotransport
 2H+ )
 NO3-)
 C+
or
pH
unchanged
With C+ (K+ etc) uptake: the
balance sheet outside is:
-1[C+ + NO3-]
Plasma membrane transporters: summary
 Ammonium uptake depends much on the carbohydrate
status in the roots.
 NH4+ and NO3- uptake is sensitive to pH. NH4+ is takes place
best in a neutral medium and depressed as the pH fall. The converse
is true for NO3- adsorption, a more rapid uptake occurring at low pH.
 The uptake rate of nitrate is dependent on the energy
status of plant.
 In contrast to NH4+ ,nitrate can be transported at high rate
into shoot and can stored at high concentrations in the
vacuoles.
 NH4+ uptake is promoted by NO3-; but NH4+ inhibited NO3uptake.
 N uptake rate is highest when both N forms are in
the nutrient solution.
1.3 Forms and Functions of N in the Plant
Dry plant materials contains about 0.3 to 50g N/kg.
1.
2.
3.
4.
5.
Amino acids (~5% of N in plants), proteins (~8085% of N in plants), nucleic acids (~10% of N in
plants)
Proteins/enzymes regulate biochemical reactions in
plants.
DNA and RNA are genetic matter.
N is an key component of chlorophyll.
Vitamins and secondary plant metabolism such as
alkaloids(生物碱）.
1.4 N assimilation in plant
1.4.1 Nitrate reduction
1. Before it can be assimilated, NO3- must be
reduced to NH3.
Step 1 – the reduction of NO3- to NO2-. This
requires the enzyme nitrate reductase. This
step occurs in the cytoplasm of the cell.
Step 2 – the reduction of NO2- to NH3. This
requires the enzyme nitrite reductase. This
step occurs in the chloroplast or the plastid of
the cell
Nitrate reduction
cytoplast
Step 1 – the
reduction of
NO3- to NO2-.
This requires
the enzyme
nitrate
reductase.
chloroplast
Step 2 – the
reduction of
NO2- to NH3.
This
requires the
enzyme
nitrite
reductase.
Prosthetic groups辅基 of the nitrate reductase
and sequence of reactions (Guererro et al. 1981)
The characteristics of NR and its regulation
 Nitrate induced the synthesis of mRNA coding for
NR
 The turn over of NR is rapid
 Light play an important role in nitrate assimilation
owing to lack of reducing power in night.
 The assimilation of nitrate by plants is influenced
by plant nutrition and in particular by Mo.
 The activity of the NR is rapidly modulated by
environmental conditions, such as light intensity,
CO2 concentration and oxygen supply.
Nitrate reductase activity in maize roots after an
exposure of young plants to nutrient solutions
containing NO3-, NH4+, or NH4NO3(Mengel et al 1985)
-
Treatment
NH4 +
NR activity, μmol NO2 /h×g fresh weight
NH4NO3
0.95b
NO3
-
0.10a
3.71c
Effect of time of day on the nitrate concentration of
spinach (SteingrÖver 1982)
Time of day
stems
leaf
Petiole
Mg NO3--N/kg fresh matter
8:30
13:30
372
207
228
101
830
546
17:30
189
91
504
The effects of pre-treat with Mo on the NR of
wheat leaves (Randall，1969）
Mo supply in Pre-treat of
the plant
leaves
growth
（µg/plant） （µgMo/L）
NR activity（µmolNO2-/gFW）
24h
70h
0.005
0
0.2
0.3
0.005
100
2.8
4.2
5.0
0
/
8.0
5.0
100
/
8.2
Site of nitrate reduction in plant
 Although most of plant species are able
to reduce NO3- both in the roots and the
shoots, there is differs between species.
 Most of nitrate is reduced in trees and
shrub’ roots .
 NO3- reduction must take place primarily
in green plant part in tomato.
 Oats>maize>sunflower>barley>oil radish
 Root ----------------------------------shoot
Nitrite reduction
Nitrite reduction in the chloroplast
In chloroplast, NO2- is reduced by the
nitrite reductase which is probably located
at out side of the thylakoid(类囊体）
membrane and thus may directly accept the
e- from ferredoxin（铁氧还蛋白） which in
turn receives e- from photosystem І.
 NO2- + 6Fdred + 8H+--->NH4+ + 6Fdox +2H2O
 There is thus a direct relationship between
photosynthetic activity and nitrite reduction.
Nitrite reduction in plastids(质体）
 Nitrite is also reduced in plastids of root where
ferredoxin-like e- carrier enzyme transfer e- from
NADPH to nitrite reductase.
 NO2- + 3NADPH + 5H+--->NH4+ + 3NADP+ +2H2O
 The synthesis of nitrite reductase is induced by
nitrite as well as by nitrate.
 Nitrite reductase activity is also dependent on the
supply of photosynthesis.
NO3- assimilation: summary
• Assimilation can be in roots; glutamate, aspartate,
glutamine, etc delivered to the xylem
• These have (small) negative charge, balanced by some
C+ (not shown above for simplicity)
• Some plants assimilate some (or nearly all) NO3- in
shoots. NO3- in xylem is balanced by C+
• Carboxyl/RCOO- (malate, oxalate etc) is then
accumulated in shoot vacuoles, with C+.
• Sometimes RCOO- is precipitated - e.g. in arid-zone
plants – oxalate.
•
Sometimes , RCOO- + K+ moves down phloem; RCOOis then decarboxylated, giving external pH 
More on NO3- assimilation
• NO3- assimilation in roots versus shoots is
often genetically programmed
• Woody plants usually assimilate NO3- in roots; many
herbs assimilate partly or almost entirely in shoots
• NO3- assimilation in shoots (plus ‘surplus’ in leaf
vacuoles) often increases as external concentrations
increase
• Carboxylic acids (malate, oxalate etc) can be a ‘drain’
on photosynthesis: e.g. 15% in beet and other
chenopod(黎科）
• Balance sheets of cations, organic N, and carboxyl ,
along with rhizosphere pH changes can indicate which
assimilation/pH regulation ‘strategy’ is operating.
1.4.2 Ammonia Assimilation in Plants
 There are three important enzymes in the
ammonia assimilation
 Glutamate dehydrogenase (谷氨酸脱氢酶）
 Glutamine synthetase (谷酰胺合成酶）
 Glutamate synthase (GOGAT谷氨酸合成
酶）
1.4.2 Ammonia Assimilation in Plants
Glutamate dehydrogenase
1.4.2 Ammonia Assimilation in Plants
Incorporation of ammonia into the plant cells in primarily by the
GS-GOGAT pathway.
Step 1
Glutamate + NH3+ + ATP
Glutamine + ADP
enzyme is glutamine synthetase (GS)
Step 2
Glutamine + alpha ketoglutarate + NAD(P)H
2 Glutamate + NAD(P)+
enzyme is glutamate synthase
GS-GOGAT Pathway
Specific activities of the ammonium-assimilating enzymes
of C. glutamicum wild type and its GDH mutanta
Special activity (mU/mg of protein) of :
GOGAT
GS
GDH
Strains
Wild type(ATCC 13032)
N
2.5
C
1.8
N
11
C
0.3
N
0.05
C
<0.003
GDH mutant
0.0
0.0
6.5
0.3
0.15
0.07
a:
Cells were taken from both ammonium (N)- and carbon (C)-limited continuous cultures.
1.4.2
Ammonia
Assimilation
in Plants
Ammonia
Assimilation
in Plants
The amino N in glutamate can be transferred to other oxo-acids
by the process known as transamination(转氨基作用). The
enzymes involved in this process are called amino
transferases(转氨酶).
Ammonia transamination in Plants
Asparagine
Lysine
Glutamate
天门冬氨酸
Aminotransferases Alanine
丙氨酸
Glutamate
Glycine
synthase
甘氨酸
Leucine
NH3
Threonine
苏氨酸
Methionine
甲硫氨酸
Cysteine
亮氨酸
Glutamine
Glutamine
synthetase
赖氨酸
半胱氨酸
Isoleucine
Serine
异亮氨酸
Asparagine
天门冬氨酸
Tryptophan
色氨酸
Histidine
组氨酸
Phenylalanine
苯丙氨酸
Valine
缬氨酸
Tyrosine
珞氨酸
Proline
脯氨酸
Arginine
精氨酸
These represent the 20 amino acids that make up proteins
丝氨酸
N translocation in plant
 N taken up by plant root is translocated in the
xylem to the upper plant parts.
 The forms of translocation: amino acids, nitrate,
and sometimes allantoin尿囊素 and allantoic
acid尿囊酸.
 In the phloem, amino acids are the
predominant form of N-transport.
 The transport of amino acids in the phloem can
also act as a feedback signal to control the
uptake of NO3-.
 Nitrogen translocation is an important process
in plant life.
1.5.1 Plant Deficiency Symptoms
1. The plant is stunt or poor growth rate.
2. General loss of chlorophyll and
disturbance of chloroplast development.
3. Yellowing of leaves – the oldest ones
become yellow first.
4. All parts of the plant gradually turn
yellow
1.5.1 Plant Deficiency Symptoms
5. Yield is very low
6. The length of the vegetative growth
stage may be shortened
7. Severe deficient may result in necrosis
(death of plant tissue) during the late
stages of plant growth
1.5.2 Plant response to excess N
1. Excess N stimulates vegetative growth
Vegetative growth may result in
competition between plant shoots and
storage organs for photosynthate
2. Excess N may also result in:
a. Delayed maturity
b. Lodging
c. Disease susceptibility
I I. Forms of Nitrogen in the Environment








Nitrogen Gas (N2)
Organic N
Ammonium (NH4+)
Nitrate (NO3-)
Nitrite (NO2-)
Ammonia (NH3)
Nitrous Oxide (N2O)
Nitric Oxide (NO)
I I.1 Forms of Nitrogen in the Environment
A. Nitrogen Gas (N2)
1. Makes up 78% of the earth’s atmosphere
B. Organic N (95% of total soil N)
1. Soil organic matter contains about 5% N
2. Mainly in the form of:
a.
b.
c.
d.
Amino acids, peptides(30% of organic N)
Proteins
Amino sugar
Unknown substances
I I.1 Forms of Nitrogen in the Environment
C. Ammonium N (NH4+)
1. Can be taken up by plants
2. It is a cation, therefore it can be adsorbed on the
cation exchange sites of soil and organic matter
particles.
3. Can be fixed in certain clay minerals.
4. Rapidly converted to NO3-N under most
conditions
5. Volatilizes when the pH of the soil is high
I I.1 Forms of Nitrogen in the Environment
D. Nitrate N (NO3-)
1. Can be taken up by plants
2. An anion, therefore it is not adsorbed on
the cation exchange sites
3. Very susceptible to leaching and
denitrification losses
4. Most common mineral form of N in most
soils
I I.1 Forms of Nitrogen in the Environment
E. Nitrite (NO2-) – an intermediate compound
in the transformation of NH4+ to NO3- or the
denitrification
F. Ammonia (NH3)
G. Nitrous oxide (N2O) – a greenhouse gas
that contributes to global warming
H. Nitrogen oxides (NOx) – contributes to air
pollution and acid rain
II.2 Nitrogen Transformations in the Soil
A. Fixation固氮
B. Mineralization矿化作用
C. Immobilization微生物固定 and
ammonia fixation
D. Volatilization挥发
E. Nitrification硝化作用
F. Denitrification反硝化作用
A. Nitrogen Fixation
1. The atmosphere consists of 78% N2. It
can be converted into mineral N.
a. Chemical fixation
i. In nature by lightning
ii. Industrially by the Haber-Bosch Process
(used to manufacture fertilizer）
b. Biological fixation
i.
Through a symbiotic relationship between
legumes and rhizobia (N-fixing bacteria)
ii. Non-symbiotic microorganisms
Nitrogen gains from biological N2 fixation (Hauck 1971)
Ecosystem
Arable land
Range in reported values (kg/ha·year)
Pasture (non-legume)
Pasture (grass-legume)
7-114
73-865
Forest
Paddy
58-594
13-99
Waters
70-250
7-28
B. Mineralization
1. It is decomposition of soil organic
matter by soil microbes that results
in the release of mineral N
2. Soil organic matter contains about
5% N
3. About 1-4% of the total organic N in
soil is mineralized each year
B. Mineralization

Factors that affect mineralization





Moisture – fastest in a moist soil
Temperature – increases as the
temperature rises
Aeration – requires oxygen
pH
C:N ratio of organic matter
Generally the lower of C/N ratio, the
higher proportion of N mineralized.
C. Immobilization生物固持
1. Immobilization is the conversion of mineral
N to organic N by microbes
2. The microorganisms that decompose
organic matter as an energy source require
nitrogen. If the N content of the organic
matter is too low, the microorganisms take
up the inorganic N from the soil.
C. Immobilization
So incorporation of straw into soils may
therefore decrease the nitrate concentration in
soils considerably and thus also inorganic N
flows into the microbial biomass. In this way,
they are competing against the plant for the N.
The result is that the crop may be deficient in N.
Relationship between C:N ratio and immobilization
When organic matter has a low C:N
ratio (<20:1), the organic matter can
supply the microorganisms with
more N than they need. As a result,
some of the N is released into the
soil. This process is known as
mineralization.
The process of hydrolysis of amino
acids or amino sugar catalyzed by
the heterotrophic异养微生物
microorganism is called
ammonification氨化作用.
Relationship between C:N ratio and immobilization
When organic matter has a
high C:N ratio (30:1), the
organic matter cannot
supply the microorganisms
with enough N. As a result,
they take up N from the soil.
This process is known as
immobilization. It is done in
competition with the plant.
Relationship between C:N ratio and immobilization
When the C:N ratio is medium (2030:1) there is little net change in the
amount of NO3- in the soil.
Typical C:N ratio of crop residue
Material
Soil
Clover
Manure
Green rye
Corn stalks
Wheat straw
Wood materials
C:N ratio
10:1
12:1
20:1
36:1
60:1
80:1
>200:1
Ammonium fixation
NH4+ is adsorbed to negatively
charged clay minerals because of its
cationic properties. The planar bound
NH4+ can be easily exchanged.
Ammonium like K+ can be bound in
the interlayer of 2:1 clay minerals, such
as illite伊利石, vermiculites蛭石, and
smectite蒙脱石 etc. it is interlayer NH4+
or non exchangeable NH4+
Ammonium fixation
For arable soils the concentration of fixed
NH4+ may be in a range of 20-1000mg NH4+ -N
/kg soil which is equivalent to a soil depth of
30cm of about 60-3000kg N/ha.
Most of fixed NH4+ is not plant-available and
one may distinguish between native interlayer
NH4+ and recently fixed.
The native interlayer NH4+ may become
available only after weathering and
decomposition of clay minerals; The recently
fixed NH4+ is plant-available.
D. Volatilization

Nitrogen is lost from the soil in the form
of gasses, such as NH3 emission.
NH4+ + H2O + OH- High pH NH3 (gas) +
H 2O
D. Volatilization
The relationship of pH and NH3 ratio in the solution (covered)
pH
7
% as NH3
0.5
8
5.0
9
35
D. Volatilization
 Ammonia volatilization occurs when
 NH4+  NH3 + H+ (pK = 9.21 )
 protonation-deprotonation
 The loss of N due to ammonia volatilization is a
common problem when Anhydrous Ammonia液氨,
aqua ammonia氨水, NH4HCO3, urea (CO(NH2)2)
fertilizer is used.
 Ammonia volatilization is most likely to take place
when soils are moist and warm and the urea is on or
near the soil surface. Ammonia volatilization will also
take place on alkaline soils (pH greater than 8).
D. Volatilization

Additional factors that affect volatilization





Incorporation – without incorporation or top
dress, as much as 30% of the N in urea
fertilizer may be lost within one week
Wind speeds up volatilization loss
High temperatures speed up volatilization loss
High soil moisture dissolves urea fertilizer and
speeds up volatilization loss.
Low cation exchange capacity (CEC) reduces
adsorption of NH4+ and increases volatilization
losses.
Agricultural NH3 emission density of various
European countries (Isermann 1987)
Country
Netherlands
Belgium
Denmark
Demark
NH3 emission density (Kg/ha)
70
55
39
Norway
German democratic republic
36
32
Federal republic of Germany
France
30
22
Great Britain
Ireland
21
20
Italy
20
Greece
10
E. Nitrification
1. The biological oxidation of ammonia
to nitrate is known as nitrification
which has two step process
+
2NH4 + 3O2
Nitrosomonas
2NO2- + H2O + 4H +
nitrobacter
2NO2- + O2
2NO3-
2. Nitrification can occur quite
rapidly under favorable conditions.
E. Nitrification
E. Nitrification

Conditions that affect nitrification




NH4+ supply
Microbial population
pH – most rapid at neutral and
relatively acid pH conditions
Moisture – most rapid when the soil is
moist
Rate of nitrification of NH3 in relation of soil pH. Total N
added was 20 mg in form of ammonium sulphate (Munk 1958)
Incubation duration
pH 4.4
pH 6.0
days
mg nitrate N produced
mg nitrate N produced
14
1.74
8.0
21
2.30
12.0
35
4.72
21.4
E. Nitrification
 Temperature – nitrification generally
increases as soil temperature increases,
and attains optimum at 26℃; Nitrification
rates are generally quite low when the soil
temperature is <10℃.
 Many farmers in the northern U.S. take
advantage of this by applying their fertilizer
late in the fall when the soil temperature is
cool. It saves them time during the busy
spring planting season.
G. Denitrification 反硝化作用
1. Denitrification is defined as the microbial reduction of NO3(and/or nitrite, NO2- ) to nitrogen-containing gases such as N2O
and dinitrogen (N2) in anaerobic conditions.
4H++2e
NO34H++4e-
2NO22H2O
-
2H++2e-
2H2O
2NO
N2O
H2O
N2
2H++2e- H2O
This process occurs when O2 levels in the soil are very
low. A recent study in Europe indicated that as much as
80% of the N fertilizer applied to the soil may be lost due to
denitrification.
G. Denitrification 反硝化作用
2. Conditions that affect denitrification
a. Anaerobic denitrification occurs when O2
concentrations are low
b. Organic matter as an energy source by the
organisms that carry out denitrification.
c. Denitrification rates are high when temperatures
are warm(20℃-30℃)
d. Denitrification is favored by a low pH
G. Denitrification 反硝化作用

General comments about denitrification


Denitrification losses can be as high as 1030% of the N applied as fertilizer.
Denitrification can occur very rapidly once
soil is saturated with water.
Denitrification in the paddy field soil
H. Leaching
Leaching – the process
by which nutrients are
carried downward as
water percolates through
the soil.
.
H. Leaching

Leaching is affected by the following factors:




Type of clay – K+ is held very tightly by 2:1 clays
Soil texture –Leaching potential decreases in the order:
sand > silt loam, loam > clay
Climate – leaching losses are greatest in areas of high
rainfall.
Quantity of nutrients in soluble form. If a nutrient is in a
soluble form, it means it can easily be dissolved in
water.
H. Leaching
 Leaching losses generally decline in the order:

NO3- > Na+ > Mg2+ > Ca2+ > K+ >> P
 Nitrate (NO3-) is primarily the form of nitrogen that is
leached. NO3- is very mobile and is easily moved by
water.
 Leaching losses of N are important for at least two
reasons.
 First, the loss of NO3- is an economic loss for
farmers. He/she must add additional fertilizer to make
up for the leached N. More importantly, the leached
NO3- may enter into the groundwater where it
becomes a health threat to human beings.
Rates of leaching of plant nutrients from soils
of different texture (VÖmel 1965/66)
Soil
Clay
content
N
K
Na
Kg/ha/year
Ca
Mg
Sand
Sandy loam
<3 g/kg
16 g/kg
15-52
0-27
7-17
0-14
9-52
1-69
110-300
0-242
17-34
0-37
Loam
Clay
280 g/kg
39 g/kg
9-44
5-44
3-8
3-8
11-45
3-8
21-179
72-341
9-61
10-54
Leaching rates of plant nutrients from a clay loam
soil (18% clay) under fallow and cropped treatments
(Coppenet 1969)
Fallow
Cropped
kg nutrient/ha/year
N
P
142
0.3
62
0.3
K
Ca
46
31.0
24
23.0
230
Mg
24
18
Nitrogen leaching rates in relation in soil
cover (Low and Armitage 1970)
Period
1952-1953
1953-1954
White clover
Grass
Kg N /ha/year
Fallow
27
26
1.8
1.3
114
113
3.9
2.0
105
41
1954-1955
60*
1956
131**
* White clover dying out; ** White clover removed
L. Soil erosion
Soil erosion refers to the process
of removing the topsoil by wind
or water. As the soil erodes,
nutrients are also carried away
also. Soil erosion is the major
cause for the loss of P from the
soil, but other nutrients are lost as
well. The loess plateau is the
major place of soil erosion in
China.
L. Soil erosion
 One study from the Ivory Coast (象牙海
岸) reported that under extensive
cropping with fairly poor ground cover,
nutrient loss from erosion amounted to:




98 kg/ha/yr of nitrogen,
57 kg/ha/yr of calcium,
39 kg/ha/yr of magnesium, and
29 kg/ha/yr of phosphorus and potassium.
L. Soil erosion
 The amount of nutrient loss due to erosion is
affected by:
 Rainfall – erosion increases as rainfall increases
 Rainfall intensity –As rainfall intensity goes up,
erosion losses increase.
 Slope of the land – erosion losses increase as the
land becomes more steep.
 Vegetative cover – vegetation, residue, or mulch
covers can reduce the energy of the raindrops and
result in a decrease in erosion losses.
N cycling
Summary of N inputs to and outputs
from the soil
Inputs
 Biological N fixation
 Atmospheric
deposition (NO3/NO2-, NH3 etc)
 Fertilizer
 The residues of
plant and animal
manures
Outputs





Crop removal
Leaching
Erosion
Denitrification
Volatilization
.
III. Nitrogen
Fertilizer
Production of N fertilizers
A.The Haber-Bosch process produces
NH3 from atmospheric N2 and H2. The
reaction requires high pressure, high
temperature, and a catalyst.
CH4 + 2H20
3H2 + N2
CO2 + 2H2
2NH3
NH3·nH2O
Production of N fertilizers
B. The production of N fertilizer consumes
a lot of energy and fertilizer prices are
often affected by the price of petroleum
(oil).
Characteristics of important N fertilizers
Anhydrous Ammonia (NH3, liquid)
a.
b.
82-0-0
Reaction in the soil
NH3 + H+
NH4+
NH3 + H2O
NH4+ + OHNH4+ can be absorbed by coulombic attraction or
electrostatic force.
iv. The injection of ammonia into the soil results in a
zone with a very high concentration of NH3 and NH4+.
The zone is characterized by a high soil pH and high
osmotic potential. Microorganisms in this area may
be killed. Plants may also be damaged or even killed
if the injection zone is too close to plant roots.
i.
ii.
iii.
Anhydrous Ammonia (NH3, liquid)

Advantages of anhydrous ammonia



Most economical source of N – for this reason
it is the common fertilizer in the U.S.
Convenient – it can be applied at the same
time the farmer does tillage
Can be applied in the fall, pre-planting, or as a
side-dress fertilizer
Anhydrous Ammonia (NH3, liquid)
d.
Disadvantages of anhydrous ammonia
i. Must be incorporated into the soil
ii. Strong affinity for water – can burn skin, eyes,
and lungs therefore it requires special handling
iii. Can be used to make illegal drugs
e.
Management considerations
i. Cannot be placed very close to seeds or
growing plants
ii. If the soil is too wet or too dry, the equipment
may not work properly and NH3 may escape
through cracks（裂隙） in the soil.
Farmer applying anhydrous ammonia
Farmer applying anhydrous ammonia
Aqua ammonia solution

Water is added to anhydrous ammonia to form
aqua ammonia. It is a liquid under low pressure
21% N
Advantages of aqua ammonia





It does not need to be placed as deeply as anhydrous
ammonia.
It does not need high-pressure application equipment.
Disadvantages of aqua ammonia

It is volatile and must be incorporated into the soil to
prevent the loss of free ammonia to the atmosphere. It
is possible to lose all of the free ammonia if it is not
incorporated.
The nitrogen solution used
in America
Percentage by Weight
CaCO3
Equiv.*
(lb per
Material
N
P205
K20
Ca
Mg
S
100 lb)
32% UAN (35% urea + 45% A.N.)
32
0
0
0
0
0
-55
30% UAN (33% urea + 42% A.N.)
30
0
0
0
0
0
-52
28% UAN (30% urea + 40% A.N.)
28
0
0
0
0
0
-49
21% AN
(60% A.N. + 40% water)
21
0
0
0
0
0
-37
19% AN
(54% A.N. + 46% water)
19
0
0
0
0
0
-33
Nitrogene solution
Urea
a. CO(NH2)2, white crystalline solid
b. 46-0-0
c. Reaction in the soil:
urease
CO(NH2)2 + H2O
2NH3 +CO2
This chemical reaction (urea hydrolysis)
takes place after the urea is dissolved in
water and is complete within about 48 hours
under field conditions in warm conditions.
It can be absorbed to the soil particles by the
molecule force attraction of hydrogen bond.
Urea

Advantages of urea





Can be applied to soil as a solid
Solution to certain crops as a foliar spray.
Urea usage involves little or no fire or
explosion hazard.
Urea's high analysis, 46% N, helps reduce
handling, storage and transportation costs
over other dry N forms.
The production of urea releases few
pollutants to the environment.
Urea

Disadvantages of urea



The formation of ammonia as urea
decomposes can reduce seed germination.
As a result urea should not be placed in direct
contact with the seed.
If not incorporated into the soil urea may
volatilize and be lost to the atmosphere as
NH3 gas in alkaline soil.
Urea

Management considerations

Urea must be incorporated into the soil. Two
things can happen to the NH3 formed from the
hydrolysis of urea: (1) it may be converted to
NH4+ and held on the cation exchange sites in
the soil; (2) the NH3 may escape into the air
(volatilization).
 Urea can damage seeds and seedlings. The
fertilizer should not be put into direct contact
with seeds and seedlings.
Urease inhibitors can be used to control the
hydrolysis of urea in some cases
Urea ammonia nitrate solution (UAN)

A mixture of urea and ammonium nitrate in water

28-32% N

Advantages



They are suitable for side dressing of corn and early spring
topdressing of grasses and small grains.
May be easier to handle compared to solid fertilizers.
Disadvantages of UAN solutions

UAN solutions may volatilize depending on factors such
as temperature and pH.

Deposition in low temperature –salt out盐析作用
Ammonium sulfate
a. (NH4)2SO4, solid
b. 21-0-0-24S
c. Reactions in the soil
i.
Soil
Acid soil
H
+ (NH4)2SO4
H
Soil
NH4
NH4
+ H2SO4
Exchange acids
microorganisms
(NH4)2SO4 +4O2
2HNO3 + H2SO4 + 2H20
Biological acids
H2SO4
H2S + 2O2
a. Reactions in the soil
i.
Alkaline soil
Soil-Ca + (NH4)2SO4
Soil
NH4
+ CaSO4
NH4
(NH4)2SO4 +4O2
2HNO3 + H2SO4 + 2H20
Physiological acid fertilizer
 Advantages of ammonium sulfate





It contains both N and S
It is suitable to pop-up
It is non-volatile, therefore it does not need to
be incorporated
It is stable and has low hygroscopicity
(doesn’t absorb water easily), therefore it is
relatively easy to handle.
Good for soils that have a high pH or for
crops that prefer acid conditions
 Disadvantages of ammonium sulfate
 It is more acidifying than any other source of N
 It is not good for soils that have a low pH
 It would destroy the soil characteristics if it is
applied in large amount.
 It has a relatively low N concentration, therefore it
is fairly expensive to transport
Ammonium bicarbonate
NH4HCO3, solid, ammonium bicarbonate is a very
common fertilizer in China
a.
b.
17-0-0. It is easy to make
Reaction in the soil
NH4HCO3
d.
NH3 + CO2 + H20
Uses of ammonium bicarbonate
Ammonium bicarbonate can be used as either a basal
or topdress fertilizer.
e. Disadvantages of ammonium bicarbonate
i. Ammonium bicarbonate is unstable under warm and
humid conditions. It is easy to decomposition.
ii. If not incorporated into the soil ，it may volatilize and
be lost to the atmosphere as NH3 gas.

Management considerations

Many factors affect the loss of N from
ammonium bicarbonate, such as temperature,
humidity, stored volume, sealing conditions,
and application measures. Temperature is
one of the most important factors since
volatilization rapidly increase as temperature
increases.

Ammonium bicarbonate should be covered
by soil immediately after application in the
field, especially under conditions of high
temperature.
Ammonium Chloride
NH4Cl, solid
a. 26-0-0-66Cl
b. Reaction in the soil – similar to (NH4)2SO4 , it
makes the soil acidic.
i. Acid soil
Soil H + 2NH4Cl
ii. Alkaline soil
2NH4Cl + CaCO3
Soil H + 2NH4Cl
NH4
Soil
NH4
+ 2HCl
(NH3)2CO3 + CaCl2
Soil
NH4
NH4
+ CaCl2

Advantages of ammonium chloride




Higher N concentration than (NH4)2SO4
Rice grows better with compared to (NH4)2SO4 for rice.
This is because the SO42-is converted to hydrogen
sulfide (H2S) in paddy soils. The (H2S) is toxic
Excellent source of both N and Cl- for coconut, oil
palm, and kiwifruit, which are Cl responsive crops.
Disadvantages of ammonium chloride




Ammonium chloride is as acid forming as (NH4)2SO4
per unit of N.
Not good for an acidic soil.
It has a low N analysis in comparison to urea or
NH4NO3.
High Cl- content limits its use to crops that are Cl
tolerant. It is not suitable for chlorophobic species(忌氯
作物) potatoes, yam, sugar cane, watermelon, grapes,
tobacco, or citrus crops.
Ammonia nitrate
 NH4NO3
 34-0-0
 White crystal, it is easy to be soluble. Has
higher hygroscopicity
 Higher capability of burning and explosion.
 NO3- is easy to leaching and NH3 is to
volatilization.
 It is suitable to topdress, and basal fertilizer in
arid and semi-arid area.
 It is most suitable to the tobacco and tomato
etc.
The efficiency of fertilizer usage
 Agronomy efficiency=(grain yieldF-grain
yieldC)/fertilizer N applied
 Apparent nitrogen recovery=(N uptakeFN uptakeC)/fertilizer N applied·100%
 Physiological efficiency= (grain yieldFgrain yieldC)/(N uptakeF-N uptakeC)
Management of fertilizers







Soil conditions
Crops species
The amount of available N in soil
Rate of N application
Other plant nutrients availability
Nitrification inhibitors
Improve N application methods, such as deep
incorporation of NH4+ and urea, or granular.