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3. NITROGEN CYCLE. SOIL 5813 Soil-Plant Nutrient Cycling and Environmental Quality Department of Plant and Soil Sciences Oklahoma State University Stillwater, OK 74078 email: [email protected] Tel: (405) 744-6414. GLOBAL WARMING. ATMOSPHERE. 3H 2 + N 2 2NH 3. - PowerPoint PPT Presentation
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3. NITROGEN CYCLE
SOIL 5813Soil-Plant Nutrient Cycling and
Environmental QualityDepartment of Plant and Soil Sciences
Oklahoma State UniversityStillwater, OK 74078
email: [email protected] Tel: (405) 744-6414
SOIL 5813Soil-Plant Nutrient Cycling and
Environmental QualityDepartment of Plant and Soil Sciences
Oklahoma State UniversityStillwater, OK 74078
email: [email protected] Tel: (405) 744-6414
ORGANICMATTER
MESQUITERHIZOBIUMALFALFASOYBEAN
BLUE-GREEN ALGAEAZOTOBACTERCLOSTRIDIUM
PLANT AND ANIMAL RESIDUES
R-NH2 + ENERGY + CO2
R-NH2 + H2O
R-OH + ENERGY + 2NH3
MATERIALS WITH NCONTENT < 1.5% (WHEAT STRAW)
MATERIALS WITH NCONTENT > 1.5%(COW MANURE)
MICROBIAL
DECOMPOSITION
HETEROTROPHICAMINIZATION
BACTERIA (pH>6.0)FUNGI (pH<6.0)
AMMONIFICATION
GLOBAL WARMING
pH>7.0
2NH4+ + 2OH-
FIXED ONEXCHANGE SITES
+O2
Nitr
osom
onas
2NO2- + H2O + 4H+
IMMOBILIZATION
NH3 AMMONIA -3NH4
+ AMMONIUM -3N2 DIATOMIC N 0N2O NITROUS OXIDE 1NO NITRIC OXIDE 2NO2
- NITRITE 3NO3
- NITRATE 5
OXIDATION STATES
ATMOSPHERE
N2ONON2
N2O2-
NH3
SYMBIOTIC NON-SYMBIOTIC
+ O2Nitrobacter
FERTILIZATION
LIGHTNING,RAINFALL
N2 FIXATION
DENITRIFICATION
PLANTLOSS
AMINOACIDS
NO3-
POOL
LEACHING
AMMONIAVOLATILIZATION
NITRIFICATION
NH2OH
Pseudomonas, Bacillus,Thiobacillus Denitrificans,and T. thioparus MINERALIZATION
+ NITRIFICATION
IMMOBILIZATION
NO2-
MICROBIAL/PLANT SINK
TEMP 50°F
pH 7.0
LEACHING LEACHING
DENITRIFICATIONLEACHING
LEACHINGVOLATILIZATIONNITRIFICATION ADDITIONS
LOSSES
OXIDATION REACTIONS
REDUCTION REACTIONS
HABER BOSCH
3H2 + N2 2NH3
(1200°C, 500 atm)
Joanne LaRuffaWade ThomasonShannon TaylorHeather Lees
Department of Plant and Soil SciencesOklahoma State University
INDUSTRIALFIXATION
Nitrogen cycle not well understood
Temperature and pH included
reduction/oxidation
tillage (zero vs. conventional)
C:N ratios (high, low lignin)
Fertilizer source and a number of other variables.
Mechanistic models would ultimately lead to many 'if-then' statements/decisions that could be used within a management strategy.
Denitrification Volatilization
Leaching Leaching
>50°F
<50°F
7.0soil pH
Assuming that we could speed up the nitrogen cycle what would you change?
1. Aerated environment (need for O2)
2. Supply of ammonium
3. Moisture
4. Temperature (30-35C or 86-95F) <10C or 50F
5. Soil pH
6. Addition of low C:N ratio materials (low lignin)
Is oxygen required for nitrification?
Does nitrification proceed during the growing cycle? (low C:N ratio)
Plants remove O2 to incorporate N into amino forms
aminoaminoaminoaminoacidsacidsacidsacidsNHNHNHNH 3333
nitrite reductasenitrite reductasenitrite reductasenitrite reductasenitrate reductasenitrate reductasenitrate reductasenitrate reductase
NONO 22NONO 33
N recommendations
1. Yield goal (2lb N/bu)
a. Applies fertilization risk on the farmer
b. Removes our inability to predict 'environment' (rainfall)
2. Soil test
a. For every 1 ppm NO3, N recommendation reduced by 2lbN/ac
3. Potential yield
Nitrite accumulation?
1. high pH
2. high NH4 levels (NH4 inhibits nitrobacter)
Inorganic Nitrogen Buffering
Ability of the soil plant system to control the amount of inorganic N accumulation in the rooting profile when N fertilization rates exceed that required for maximum yield.
So
il P
rofi
le In
org
an
ic N
A
cc
um
ula
tio
n, k
g/h
a
Gra
in y
ield
, kg
/ha
Annual Nitrogen Fertilizer Rate, kg/ha
0 40 80 120 160 200 240
Point where increasing applied N no longerincreases grain yield
500
400
300
200
100
0
Point where increasing applied N increases soilprofile inorganic N accumulation
Range (buffer) where increasingapplied N does not increasegrain yield, but also where noincrease in soil profile inorganic N is observed
Soil-Plant Inorganic N Buffering
4000
3000
2000
1000
0
FertilizerFertilizer
NHNH44, NO, NO33NHNH44, NO, NO33
Organic Matter PoolOrganic Matter PoolOrganic Matter PoolOrganic Matter Pool
InorganicInorganicNitrogenNitrogenInorganicInorganicNitrogenNitrogen
00
2222
4545
6767
9090
112112
00 100100 200200 300300 4004003030
6060
9090
120120
150150
180180
210210
240240
270270
300300
NONO33---N, kg ha-N, kg ha-1-1
Udic Argiustoll, 0-240 cm, #502
N Rate kg haN Rate kg ha-1-1
00
3434
6767
134134
269269
N Rate kg haN Rate kg ha-1-1 N Rate kg haN Rate kg ha-1-1
00 100100 200200 300300 4004003030
6060
9090
120120
150150
180180
210210
240240
270270
300300
NONO33---N, kg ha-N, kg ha-1-1
Udic Argiustoll, 0-300 cm, #505
De
pth
, cm
De
pth
, cm
De
pth
, cm
De
pth
, cm
If the N rate required to detect soil profile NO3 accumulation always exceeded that required for maximum yields, what biological mechanisms are present
that cause excess N applied to be lost via other pathways prior to leaching?
Nitrogen Buffering Mechanisms
1. Increased Applied N results in increased plant N loss (NH3)
Table 3. Forage, grain and straw N uptake and estimated plant N loss, experiments 222, 1996-1997, and 502, 1997
Location Fertilizer Applied Total N Uptake 1996 1997 N P K Forage Grain Straw Loss/
Gain Forage Grain Straw Loss/
Gain ----------kg ha-1 yr-1--------- -------------------------------------------kg N ha-1---------------------------------------- 222 0 29 38 29.40 23.47 12.74 -6.81 18.76 22.54 8.04 -11.82 45 29 38 38.59 32.10 18.54 -12.05 42.81 23.13 21.43 -1.75 90 29 38 70.72 40.63 27.50 2.59 96.62 31.01 55.02 -6.32 135 29 38 102.49 48.41 39.41 14.67 143.61 51.69 71.93 5.1 SED 8.20 4.40 2.79 19.91 2.90 11.91 N rate linear *** ** *** ** *** ** N rate quadratic ns ns ns ns ** ns 502 0 20 56 29.46 32.83 11.08 -14.45 23 20 56 56.21 50.01 26.68 -20.48 45 20 56 127.96 57.05 47.54 23.37 67 20 56 132.12 63.56 40.15 28.41 90 20 56 182.29 90.54 63.05 28.70 112 20 56 191.84 105.39 44.90 41.55 SED 24.79 14.65 9.55 N rate linear *** *** *** N rate quadratic ns ns * Loss/gain determined by subtracting forage N uptake at flowering from total N in the grain and straw at maturity. *, **, *** significant at the 0.05, 0.01, and 0.001 probability levels, respectively. SED = standard error of the difference between two equally replicated treatment means.
Lees, H.L., W.R. Raun and G.V. Johnson. 2000. Increased plant N loss with increasing nitrogen applied in winter wheat observed with 15N. J. Plant Nutr. 23:219-230.
photosynthesis carbohydrates
respiration
carbon skeletons
aminoacidsNH3
reducing power
nitritereductase
nitratereductase
ferredoxinsiroheme
NO 2NO 3
NADH or NADPH
Bidwell (1979), Plant Physiology, 2nd Ed.Metabolism associated with nitrate reduction
Francis, D.D., J.S. Schepers, and M.F. Vigil. 1993. Post-anthesis nitrogen loss from corn. Agron. J. 85:659-663.
Nitrogen Buffering Mechanisms
1. Increased Applied N results in increased plant N loss (NH3)
2. Higher rates of applied N - increased volatilization losses
Nitrogen Buffering Mechanisms
1. Increased Applied N results in increased plant N loss (NH3)
2. Higher rates of applied N - increased volatilization losses
3. Higher rates of applied N - increased denitrification
Burford and Bremner (1975) found that denitrification losses increased under anaerobic conditions with increasing organic C in surface soils (0-15 cm) (wide range in pH & texture).
Denitrifying bacteria responsible for reduction of nitrate to gaseous forms of nitrogen are facultative anaerobes that have the ability to use both oxygen and nitrate (or nitrite) as hydrogen acceptors. If an oxidizable substrate is present, they can grow under anaerobic conditions in the presence of nitrate or under aerobic conditions in the presence of any suitable source of nitrogen
Burford and Bremner, 1975
Aulakh, Rennie and Paul, 1984
Nitrogen Buffering Mechanisms
1. Increased Applied N results in increased plant N loss (NH3)
2. Higher rates of applied N - increased volatilization losses
3. Higher rates of applied N - increased denitrification
4. Higher rates of applied N - increased organic C, - increased organic N
0.040.040.040.04
0.050.050.050.05
0.060.060.060.06
0.070.070.070.07
0.080.080.080.08
0.090.090.090.09
0.10.10.10.1
0000 40404040 80808080 120120120120 160160160160 2002002002000.40.40.40.4
0.50.50.50.5
0.60.60.60.6
0.70.70.70.7
0.80.80.80.8
0.90.90.90.9
TSNTSNTSNTSN
OCOCOCOC
#406#406
To
tal
So
il N
, %
To
tal
So
il N
, %
To
tal
So
il N
, %
To
tal
So
il N
, %
Org
anic
Car
bo
n,
%O
rgan
ic C
arb
on
, %
Org
anic
Car
bo
n,
%O
rgan
ic C
arb
on
, %
N Rate, kg/haN Rate, kg/haN Rate, kg/haN Rate, kg/ha
SED TSN = 0.002SED TSN = 0.002SED TSN = 0.002SED TSN = 0.002
SED OC = 0.03SED OC = 0.03SED OC = 0.03SED OC = 0.03
Raun, W.R., G.V. Johnson, S.B. Phillips and R.L. Westerman. 1998. Effect of long-term nitrogen fertilization on soil organic C and total N in continuous wheat under conventional tillage in Oklahoma. Soil & Tillage Res. 47:323-330.
Nitrogen Buffering Mechanisms
1. Increased Applied N results in increased plant N loss (NH3)
2. Higher rates of applied N - increased volatilization losses
3. Higher rates of applied N - increased denitrification
4. Higher rates of applied N - increased organic C, - increased organic N
5. Increased applied N - increased grain protein
Gra
in N
up
take
, kg
/ha
Annual Nitrogen Fertilizer Rate, kg/ha
0 40 80 120 160 200 240
80
60
40
20
0
Point where increasing applied N no longerincreases grain yield
Increased grain N uptake (protein) at N rates in excess of that requiredfor maximum yield
Continued increase ingrain N uptake, beyond thepoint where increasingapplied N increases soilprofile inorganic Naccumulation
0 20 40 60 80 100 120 14020
30
40
50
60
70
80
# 222# 222
N rate, kg/ha
Gra
in N
Up
take
, kg
/ha
Y = 29.7 + 0.28x - 0.00055x2
r2=0.90
9.4 =19%
Nitrogen Buffering Mechanisms
1. Increased Applied N results in increased plant N loss (NH3)
2. Higher rates of applied N - increased volatilization losses
3. Higher rates of applied N - increased denitrification
4. Higher rates of applied N - increased organic C, - increased organic N
5. Increased applied N - increased grain protein
6. Increased applied N - increased forage N
7. Increased applied N - increased straw N
VolatilizationVolatilizationVolatilizationVolatilization
DenitrificationDenitrificationDenitrificationDenitrification
LeachingLeaching
NHNH33, N, N22NHNH33, N, N22
NO3NO3
Microbial PoolMicrobial PoolMicrobial PoolMicrobial Pool
NHNH44NHNH44
NONO33NONO33
NONO22NONO22
7-80 kg N/ha/yr7-80 kg N/ha/yr7-80 kg N/ha/yr7-80 kg N/ha/yr
NONONONONN22OONN22OO
NN22NN22
15-40 kg N/ha/yr15-40 kg N/ha/yr15-40 kg N/ha/yr15-40 kg N/ha/yrNH3NH3
0-50 kg N/ha/yr0-50 kg N/ha/yr0-50 kg N/ha/yr0-50 kg N/ha/yr
UreaUreaUreaUrea
Organic ImmobilizationOrganic ImmobilizationOrganic ImmobilizationOrganic Immobilization10-50 kg N/ha/yr10-50 kg N/ha/yr10-50 kg N/ha/yr10-50 kg N/ha/yr
0-20 kg N/ha/yr0-20 kg N/ha/yr
Fertilizer N Fertilizer N Fertilizer N Fertilizer N
AppliedAppliedAppliedApplied
11
22
33
44
55
55
22
Olson and Swallow, 1984Olson and Swallow, 1984Sharpe et al., 1988Sharpe et al., 1988Timmons and Cruse, 1990Timmons and Cruse, 1990
Olson and Swallow, 1984Olson and Swallow, 1984Sharpe et al., 1988Sharpe et al., 1988Timmons and Cruse, 1990Timmons and Cruse, 1990
11
Mills et al., 1974Mills et al., 1974Mills et al., 1974Mills et al., 1974Matocha, 1976Matocha, 1976Matocha, 1976Matocha, 1976DuPlessis and Kroontje, 1964DuPlessis and Kroontje, 1964DuPlessis and Kroontje, 1964DuPlessis and Kroontje, 1964Terman, 1979Terman, 1979Terman, 1979Terman, 1979Sharpe et al., 1988Sharpe et al., 1988Sharpe et al., 1988Sharpe et al., 1988
44
Aulackh et al., 1984Aulackh et al., 1984Colbourn et al., 1984Colbourn et al., 1984Bakken et al., 1987Bakken et al., 1987Prade and Trolldenier, 1990Prade and Trolldenier, 1990
Aulackh et al., 1984Aulackh et al., 1984Colbourn et al., 1984Colbourn et al., 1984Bakken et al., 1987Bakken et al., 1987Prade and Trolldenier, 1990Prade and Trolldenier, 1990
33
Francis et al., 1993Francis et al., 1993Hooker et al., 1980Hooker et al., 1980O’Deen, 1986, 1989O’Deen, 1986, 1989Daigger et al., 1976Daigger et al., 1976Parton et al., 1988Parton et al., 1988
Francis et al., 1993Francis et al., 1993Hooker et al., 1980Hooker et al., 1980O’Deen, 1986, 1989O’Deen, 1986, 1989Daigger et al., 1976Daigger et al., 1976Parton et al., 1988Parton et al., 1988
Chaney, 1989Chaney, 1989Sommerfeldt and Smith, 1973Sommerfeldt and Smith, 1973Macdonald et al., 1989Macdonald et al., 1989Kladivko, 1991Kladivko, 1991
Chaney, 1989Chaney, 1989Sommerfeldt and Smith, 1973Sommerfeldt and Smith, 1973Macdonald et al., 1989Macdonald et al., 1989Kladivko, 1991Kladivko, 1991
NHNH44+OH+OH-- NH NH33 + H + H22OONHNH44+OH+OH-- NH NH33 + H + H22OO
N Buffering MechanismsN Buffering Mechanisms
NHNH44 fixation (physical) fixation (physical)NHNH44 fixation (physical) fixation (physical)
Industrial view of the Nitrogen Cycle
Nutrient Overload: Unbalancing the Global Nitrogen Cycle
Carbon Cycle
NITROGEN Cycle LinksNITROGEN Cycle Links
Urea
1. Urea is the most important solid fertilizer in the world today.
2. In the early 1960's, ammonium sulfate was the primary N product in world trade (Bock and Kissel, 1988).
3. The majority of all urea production in the U.S. takes place in Louisiana, Alaska and Oklahoma.
4. Since 1968, direct application of anhydrous ammonia has ranged from 37 to 40% of total N use (Bock and Kissel, 1988)
5. Urea: high analysis, safety, economy of production, transport and distribution make it a leader in world N trade.
6. In 1978, developed countries accounted for 44% of the world N market (Bock and Kissel, 1988).
7. By 1987, developed countries accounted for less than 33%
Koch Industries7.5 million metric tons of N fertilizer/year
WorldTotal Production N, P, and K216 million metric tons
Share of world N consumption by product group
1970 1986 2004
Ammonium sulfate 8 5 2
Ammonium nitrate 27 15 14Urea 9 37 50
Ammonium phosphates 1 5Other N products (NH3) 36 29 30Other complex N products 16 8
Urea Hydrolysisincrease pH (less H+ ions in soil solution) urease enzyme required
CO(NH2)2 + H+ + 2H2O --------> 2NH4+ + HCO3
-
pH 6.5 to 8
HCO3- + H+ ---> CO2 + H2O (added H lost from soil solution)
CO(NH2)2 + 2H+ + 2H2O --------> 2NH4+ + H2CO3 (carbonic acid)
pH <6.3
H2CO3 CO2 + H2O
During hydrolysis, soil pH can increase to >7 because the reaction requires H+ from the soil system.
(How many moles of H+ are consumed for each mole of urea hydrolyzed?) 2
In alkaline soils less H+ is initially needed to drive urea hydrolysis on a soil already having low H+.
In an alkaline soil, removing more H+(from a soil solution already low in H+), can increase pH even higher
NH4+ + OH- ---> NH4OH ---->NH3 + H2O
pH = -log[H+]
Calculate pH of 2.0x10-3M solution of HCl
HCl is completely ionized so
[H+] = 2.0 x 10-3M
pH = -log(2.0x10-3)
= 3 – log 2.0
= 3 - 0.30
= 2.70
◦ pH = pKa + log [(base)/(acid)]◦ pKw = pH + pOH◦ 14.00 = pH + pOH
◦ At a pH of 9.3 (pKa 9.3) 50% NH4 and 50% NH3◦ pH Base (NH3) Acid (NH4)◦ 7.3 1 99◦ 8.3 10 90◦ 9.3 50 50◦ 10.3 90 10◦ 11.3 99 1
Chemicals A and B react to form C and D
A + B = C + D
Equilibrium Constant (K) K = [C][D] / [A][B]
6
7
8
9
10
0 20 40 60 80 100
pH
%
NH 3
4+NH
Equilibrium relationship for ammoniacal N and resultant amount of NH3 and NH4 as affected by pH
for a dilute solution.
H20 H+ + OH-
As the pH increases from urea hydrolysis, negative charges become available for NH4
+ adsorption because of the release of H+ (Koelliker and Kissel)
Decrease NH3 loss with increasing CEC (Fenn and Kissel, 1976)
Assuming that pH and CEC are positively correlated, what is happening?
Relationship of pH and BI (?) none
In acid soils, the exchange of NH4+ is for H+ on the exchange
complex (release of H here, resists change in pH, e.g. going up)
In alkaline soils with high CEC, NH4 exchanges for Ca, precipitation of CaCO3 (CO3
= from HCO3- above) and one H+ released which helps
resist the increase in pH
However, pH was already high,
pH
CEC** on soils where organic matter dominates the contribution to CEC then there should be a positive relationship of pH and CEC.
5
6
7
8
9
0 2 4 6 8 10 12 14 16 18 20
SOIL MIX 3-High Buffering Capacity
SOIL MIX 2-Moderate Buffering Capacity
SOIL MIX 1-Low Buffering Capacity
0
2
4
6
8
10
12
0 2 4 6 8 10 12 14 16 18 20
3SO
IL S
URFA
CE p
H
DAYS AFTER APPLICATION
kg N
H -N
/ha
VOLA
TILI
ZED
Soil surface pH and cumulative NH3 loss as influenced by pH
buffering capacity (from Ferguson et al., 1984).
N Rate =
112 kg/ha
Ernst and Massey (1960) found increased NH3 volatilization when liming a silt loam soil. The effective CEC would have been increased by liming but the rise in soil pH decreased the soils ability to supply H+
Rapid urea hydrolysis: greater potential for NH3 loss. Why?
Management:
•dry soil surface
•Incorporate
•localized placement- slows urea hydrolysis
H ion buffering capacity of the soil:Ferguson et al., 1984
(soils total acidity, comprised of exchangeable acidity + nonexchangeable titratable acidity)
A large component of a soils total acidity is that associated with the layer silicate sesquioxide complex (Al and Fe hydrous oxides). These sesquioxides carry a net positive charge and can hydrolyze to form H+ which resist an increase in pH upon an addition of a base.
H+ ion supply comes from:
1. OM
2. hydrolysis of water
3. Al and Fe hydrous oxides
4. high clay content (especially 2:1, reason CEC’s are higher in non-weathered clays is due to isomorphic substitution – pH independent charge)
Soil with an increased H+ buffering capacity will also show less NH3 loss when urea is applied without incorporation.
1. hydroxy Al-polymers added (carrying a net positive charge) to increase H+ buffering capacity.
2. strong acid cation exchange resins added (buffering capacity changed without affecting CEC, e.g. resin was saturated with H+).
resin: amorphous organic substances (plant secretions), soluble in organic solvents but not in water (used in plastics, inks)
Consider the following
1. H+ is required for urea hydrolysis2. Ability of a soil to supply H+ is related to amount of NH3 loss3. H+ is produced via nitrification (after urea is applied): acidity generated is not beneficial4. What could we apply with the urea to reduce NH3 loss?
an acid; strong electrolyte; dissociates to produce H+;increased H+ buffering; decrease pH
reduce NH3 loss by maintaining a low pH in the vicinity of the fertilizer granule (e.g. H3PO4)
Comment: Ferguson et al. (1984).
“When urea is applied to the soil surface, NH3 volatilization probably will not be economically serious unless the soil surface pH rises above 7.5”
UREASE inhibitors
“Agrotain” n-butyl thiophosphoric triamide
http://www.agrotain.com
Nitrosomonas inhibitors
“NSERVE” 2-CHLORO-6-(TRICHLOROMETHYL) PYRIDINE
http://jeq.scijournals.org/cgi/content/abstract/32/5/1764
Computation/commodity Production, mTWorld consumption of fertilizer-N 90,000,000
Fert-N used in cereals (60% of total applied)0.60 * 82,906,340 = 54,000,000
World Cereal Production, mT
Sorghum3%
Rye1%Oats
2%Millet1%
Barley8%
Rice28%
Corn29%
Wheat28%
NEED for INCREASED NUE
World grain N removal, 1996 %N mTWheat 2.13 12,502,267Corn 1.26 7,439,266Rice 1.23 7,007,101Barley 2.02 3,154,192Sorghum 1.92 1,356,807Millet 2.01 580,032Oats 1.93 596,012Rye 2.21 508,788Total N removed in cereals 33,144,465
N removed in cereals (from soil & rain, 50% of total) 16,572,232
NUE = ((N removed - N soil&rain)/total N applied) 33%
Savings/yr for each 1% increase in NUE 489,892 mT
Value of fertilizer savings $479/mT N $234,658,462
2005 >$400,000,000
____________________________________ World cereal grain NUE
33% Developed nation cereal NUE
42% Developing nation cereal NUE
29%
____________________________________ 1% increase in worldwide cereal NUE
= $234,658,462 fertilizer savings 20% increase in worldwide cereal NUE
1999 = $4.7 billion
2005, > 10 billion
Flowchart for NUE
http://www.nue.okstate.edu/NUE_etc.htm
Role of NH4 nutrition in Higher Yields (S.R. Olsen)
•Glutamine-major product formed in roots absorbing NH4
•NO3 has to be transported to the leaves to be reduced
•Wheat N uptake was increased 35% when supplying 25% of the N as NH4 compared to all N as NO3 (Wang and Below, 1992).
•High-yielding corn genotypes were unable to absorb NO3 during ear development, thus limiting yields otherwise increased by supplies of NH4 (Pan et al., 1984).
•Assimilation of NO3 requires the energy equivalent of 20 ATP/moleNO3, whereas NH4 assimilation requires only 5 ATP/mole NH4 (Salsac et al., 1987).
•This energy savings may lead to greater dry weight production for plants supplied solely with NH4 (Huffman, 1989).
photosynthesis carbohydrates
respiration
carbon skeletons
aminoacidsNH3
reducing power
nitritereductase
nitratereductase
ferredoxinsiroheme
NO 2NO 3
NADH or NADPH
Bidwell (1979), Plant Physiology, 2nd Ed.Metabolism associated with nitrate reduction
Discussion:Global Population and the Nitrogen Cycle
p.80 nitrous oxide
Increasing use of fertilizer N results in increased N2O. Reaction of nitrous oxide (N2O) with Oxygen contribute to the destruction of ozone.
Atmospheric lifetime of nitrous oxide is longer than a century, and every one of its molecules absorbs roughly 200 times more outgoing radiation than does a single carbon dioxide molecule.
“In just one lifetime, humanity has indeed developed a profound chemical dependence.”
FYI
Factors Affecting Soil Acidity
Acid: substance that tends to give up protons (H+) to some other substance
Base: accepts protonsAnion: negatively charged ionCation: positively charged ion
Base cation: ? (this has been taught in the past but is not correct)
Electrolyte: nonmetallic electric conductor in which current is carried by the movement of ions
H2SO4 (strong electrolyte)
CH3COOH (weak electrolyte)
H2O
HA --------------> H+ + A-
potential active
acidity acidity
1. Nitrogen Fertilization
A. ammoniacal sources of N
2. Decomposition of organic matter
OM ------> R-NH2 + CO2
CO2 + H2O --------> H2CO3 (carbonic acid)
H2CO3 ------> H+ + HCO3- (bicarbonate)
humus contains reactive carboxylic, phenolic groups that behave as weak acids which dissociate and release H+
3. Leaching of exchangeable bases/Removal
Ca, Mg, K and Na (out of the effective root zone)
-problem in sandy soils with low CEC
a. Replaced first by H and subsequently by Al (Al is one of the most abundant elements in soils. 7.1% by weight of earth's crust)
b. Al displaced from clay minerals, hydrolyzed to hydroxy aluminum complexes
c. Hydrolysis of monomeric forms liberate H+
d. Al(H2O)6+3 + H2O -----> Al(OH)(H2O)++ + H2O+
monomeric: a chemical compound that can undergo polymerization
polymerization: a chemical reaction in which two or more small molecules combine to form larger molecules that contain repeating structural units of the original molecules
4. Aluminosilicate clays
Presence of exchangeable Al
Al+3 + H2O -----> AlOH= + H+
5. Acid Rain
NITROGEN:
Key building block of protein molecule
Component of the protoplasm of plants animals and microorganisms
One of few soil nutrients lost by volatilization and leaching, thus requiring continued conservation and maintenance
Most frequently deficient nutrient in crop production
Nitrogen Ion/Molecule Oxidation States
Range of N oxidation states from -3 to +5.
oxidized: loses electrons, takes on a positive charge
reduced: gains electrons, takes on a negative charge
Illustrate oxidation states using common combinations of N with H and O
H can be assumed in the +1 oxidation state (H+1)
O in the -2 oxidation state (O=)
Aminization: Decomposition of proteins and the release of amines and amino acids
OM (proteins) R-NH2 + Energy + CO2
Ammonification:
R-NH2 + HOH NH3 + R-OH + energy
NH4+ + OH-
Nitrification: biological oxidation of ammonia to nitrate
2NH4+ + 3O2 2NO2- + 2H2O + 4H+
2NO2- + O2 2NO3
-
+H2O
Ion/molecule Name Oxidation State
NH3 ammonia -3
NH4+ ammonium -3
N2 diatomic N 0
N2O nitrous oxide +1
NO nitric oxide +2
NO2- nitrite +3
NO3- nitrate +5
H2S hydrogen sulfide -2
SO4= sulfate +6
N: 5 electrons in the outer shell
loses 5 electrons (+5 oxidation state NO3)
gains 3 electrons (-3 oxidation state NH3)
O: 6 electrons in the outer shell
is always being reduced (gains 2 electrons to fill the outer shell)
H: 1 electron in the outer shell
N is losing electrons to O because O is more electronegative
N gains electrons from H because H wants to give up electrons
Hydrogen:
Electron configuration in the ground state is 1s1 (the first electron shell has only one electron in it), as found in H2 gas.
s shell can hold only two electrons, atom is most stable by either gaining another electron or losing the existing one. Gaining an electron by sharing occurs in H2, where each H atom gains an electron from the other resulting in a
pair of electrons being shared. The electron configuration about the atom, where: represents a pair of electrons, and may be shown as
H:H and the bond may be shown as H-H
Hydrogen most commonly exists in ionic form and in combination with other elements where it has lost its single electron. Thus it is present as the H+ ion or brings a + charge to the molecule formed by combining with other elements.
Oxygen:
Ground state of O, having a total of eight electrons is 1s2, 2s2, 2p4.
Both s orbitals are filled, each with two electrons.
The 2p outer or valence orbital capable of holding six electrons, has only four electrons, leaving opportunity to gain two. The common gain of two electrons from some other element results in a valence of -2 for O (O=). The gain of two electrons also occurs in O2 gas, where two pairs of electrons are shared as
O::O and the double bond may be shown as O=O
Nitrogen:
Ground state of N is 1s2, 2s2, 2p3.
Similar to that for oxygen, except there is one less electron in the valence 2p orbital. Hence, the 2p orbital contains three electrons but, has room to accept three electrons to fill the shell. Under normal conditions, electron loss to for N+, N2+ or N3+ or electron gain to form N-, N2-, or N3- should not be expected. Instead, N will normally fill its 2p orbital by sharing electrons with other elements to which it is chemically (covalent) bound. Nitrogen can fill the 2p orbital by forming three covalent bonds with itself as in the very stable gas N2.
Nitrogen Cycle:
•Increased acidity?
Ammonia Volatilization
· Urease activity (organic C) · Air Exchange
· Temperature · N Source and Rate
· CEC (less when high) · Application method
· H buffering capacity of the soil · Crop Residues
· Soil Water Content
NH4+ NH3 + H+
If pH and temperature can be kept low, little potential exists for NH3 volatilization. At pH 7.5, less than 7% of the ammoniacal N is actually in the form of NH3 over the range of temperatures likely for field conditions.
6
7
8
9
10
0 20 40 60 80 100
pH
%
NH 3
4+NH
Equilibrium relationship for ammoniacal N and resultant amount of NH3 and NH4 as affected by pH
for a dilute solution.
H20 H+ + OH-
Chemical EquilibriaA+B AB
Kf = AB/A x B
AB A+B
Kd = A x B/AB
Kf = 1/Kd (relationship between formation and dissociation constants)
Formation constant (Log K°) relating two species is numerically equal to the pH at which the reacting species have equal activities (dilute solutions)
pKa and Log K° are sometimes synonymous
Henderson-Hasselbalch
pH = pKa + log [(base)/(acid)]
when (base) = (acid), pH = pKa
Acidification from N Fertilizers (R.L. Westerman)
1. Assume that the absorbing complex of the soil can be represented by CaX
2. Ca represents various exchangeable bases with which the insoluble anions X are combined in an exchangeable form and that X can only combine with one Ca
3. H2X refers to dibasic acid (e.g., H2SO4)
(NH4)2SO4 -----> NH4+ to the exchange complex, SO4
= combines with the base on the exchange complex replaced by NH4
+
Volatilization losses of N as NH3 preclude the development of H+ ions produced via nitrification and would theoretically reduce the total potential development of acidity.
Losses of N via denitrification leave an alkaline residue (OH-)
Reaction of N fertilizers when applied to soil (Westerman, 1985)
______________________________________________________________________1. Ammonium sulfate
a. (NH4)2SO4 + CaX ----> CaSO4 + (NH4)2Xb. (NH4)2X + 4O2 nitrification >2HNO3 + H2X + 2H2Oc. 2HNO3 + CaX ----> Ca(NO3)2 + H2X
Resultant acidity = 4H+ /mole of (NH4)2SO4
2. Ammonium nitratea. 2NH4NO3 + CaX ----> Ca(NO3)2 + (NH4)2Xb. (NH4)2X + 4O2 nitrification >2HNO3 + H2X + 2H2Oc. 2HNO3 + CaX ----> Ca(NO3)2 + H2X
Resultant acidity = 2H+ /mole of NH4NO3
3. Ureaa. CO(NH2)2 + 2H2O ----> (NH4)2CO3
b. (NH4)2CO3 + CaX ----> (NH4)2X + CaCO3
c. (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2Od. 2HNO3 +CaX ----> Ca(NO3)2 + H2Xe. H2X + CaCO3 neutralization >CaX + H2O + CO2
Resultant acidity = 2H+ /mole of CO(NH2)2
4. Anhydrous Ammoniaa. 2NH3 +2H2O ----> 2NH4OHb. 2NH4OH + CaX ----> Ca(OH)2 + (NH4)2Xc. (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2Od. 2HNO3 + CaX ----> Ca(NO3)2 + H2Xe. H2X + Ca(OH)2 neutralization > CaX + 2H2O
Resultant acidity = 1H+/mole of NH3
5. Aqua Ammoniaa. 2NH4ON + CaX ----> Ca(OH)2 + (NH4)2Xb. (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2Oc. 2HNO3 +CaX ----> Ca(NO3)2 + H2Xd. H2X + Ca(OH)2 neutralization > CaX +2H2O
Resultant acidity = 1H+/mole of NH4OH
6. Ammonium Phosphatea. 2NH4H2PO4 + CaX ----> Ca(H2PO4)2 + (NH4)2Xb. (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2Oc. 2HNO3 +CaX ----> Ca(NO3)2 + H2X
Resultant acidity = 2H+/mole of NH4H2PO4
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