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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 N N 2 2 O NO N2 FIXATION

SYMBIOTIC MESQUITE RHIZOBIUM ALFALFA SOYBEAN NON-SYMBIOTIC BLUE-GREEN ALGAE AZOTOBACTER CLOSTRIDIUM

INDUSTRIAL FIXATION

3H HABER BOSCH (1200 °C, 500 atm) 2 + N 2 2NH 3

LIGHTNING, RAINFALL FERTILIZATION PLANT LOSS AMINO ACIDS

NH 3 NH 2 OH

ORGANIC MATTER PLANT AND ANIMAL RESIDUES

MATERIALS WITH N CONTENT > 1.5% (COW MANURE) MATERIALS WITH N CONTENT < 1.5% (WHEAT STRAW)

AMMONIA VOLATILIZATION

AMINIZATION HETEROTROPHIC R-NH 2 + ENERGY + CO 2 BACTERIA (pH>6.0) FUNGI (pH<6.0) R-NH 2 + H 2 O AMMONIFICATION pH>7.0

IMMOBILIZATION R-OH + ENERGY + 2NH 3 Pseudomonas, Bacillus, Thiobacillus Denitrificans, and T. thioparus N 2 O 2 NO 2 MINERALIZATION + NITRIFICATION

MICROBIAL/PLANT SINK

FIXED ON EXCHANGE SITES 2NH 4 + + 2OH +O 2 OXIDATION STATES NH 3 NH 4 + AMMONIA AMMONIUM N 2 DIATOMIC N N 2 O NITROUS OXIDE NO NITRIC OXIDE NO 2 NO 3 NITRITE NITRATE -3 -3 0 1 2 3 5 DENITRIFICATION LEACHING LEACHING LEACHING VOLATILIZATION NITRIFICATION TEMP 50 °F LEACHING

NO 3 POOL LEACHING

pH 7.0

NITRIFICATION 2NO 2 + H 2 O + 4H + Nitrobacter + O 2

Joanne LaRuffa Wade Thomason Shannon Taylor Heather Lees Department of Plant and Soil Sciences Oklahoma State University ADDITIONS LOSSES

OXIDATION REACTIONS REDUCTION REACTIONS

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.

>50 °F Denitrification Volatilization <50 °F Leaching Leaching

7.0

soil pH

Assuming that we could speed up the nitrogen cycle what would you change?

1. Aerated environment (need for O 2 ) 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 NO 3 nitrate reductase NO 2 nitrite reductase NH 3 amino acids

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 NO 3 , N recommendation reduced by 2lbN/ac 3. Potential yield Nitrite accumulation?

1. high pH 2. high NH 4 levels (NH 4 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.

Soil-Plant Inorganic N Buffering

4000 Point where increasing applied N no longer increases grain yield 500 400 3000 2000 1000 0 0 40 Range (buffer) where increasing applied N does not increase grain yield, but also where no increase in soil profile inorganic N is observed 80 120 160 300 200 applied N increases soil 100 accumulation 0 240 Annual Nitrogen Fertilizer Rate, kg/ha

NH 4 , NO 3 Fertilizer Organic Matter Pool Inorganic Nitrogen

Udic Argiustoll, 0-240 cm, #502 Udic Argiustoll, 0-300 cm, #505

30 0 60 90 120 150 180 210 240 270 300 NO 3 -N, kg ha -1 100 200 300 400 N Rate kg ha -1 0 22 45 67 90 112 30 0 60 90 120 150 180 210 240 270 300 NO 3 -N, kg ha -1 100 200 300 400 N Rate kg ha 0 34 67 134 269 -1

If the N rate required to detect soil profile NO 3 accumulation always exceeded that required for maximum yields, what biological mechanisms are present that cause leaching?

excess N

applied to be lost via other pathways prior to

Nitrogen Buffering Mechanisms 1. Increased Applied N results in increased plant N loss (NH 3 )

Table 3. Forage, grain and straw N uptake and estimated plant N loss, experiments 222, 1996 1997, and 502, 1997 Location

222

SED  N ----------kg ha -1 yr -1 --------- 0 45 90 135 Fertilizer Applied P 29 29 29 29 K 38 38 38 38 Forage -------------------------------------------kg N ha -1 ---------------------------------------- 29.40 38.59 70.72 102.49 8.20 Grain 23.47 32.10 40.63 48.41 4.40 1996 Straw 12.74 18.54 27.50 39.41 2.79 Total N Uptake Loss/ Gain  -6.81 -12.05 2.59 14.67 Forage 18.76 42.81 96.62 143.61 19.91 N rate linear N rate quadratic

502

0 23 45 67 20 20 20 20 56 56 56 56 *** ns ** ns *** ns ** ns 29.46 56.21 127.96 132.12 SED  N rate linear 90 112 20 20 56 56  N rate quadratic 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. 182.29 191.84 24.79 *** Grain 22.54 23.13 31.01 51.69 2.90 *** ** 32.83 50.01 57.05 63.56 90.54 105.39 14.65 *** ns 1997 Straw 8.04 21.43 55.02 71.93 11.91 ** ns 11.08 26.68 47.54 40.15 63.05 44.90 9.55 *** * Loss/ Gain  -11.82 -1.75 -6.32 5.1 -14.45 -20.48 23.37 28.41 28.70 41.55 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.

Bidwell (1979), Plant Physiology, 2nd Ed.

Metabolism associated with nitrate reduction photosynthesis carbohydrates respiration reducing power NADH or NADPH NO 3 nitrate reductase NO 2 nitrite reductase

NH

3 carbon skeletons amino acids ferredoxin siroheme

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 (NH 3 ) 2. Higher rates of applied N - increased volatilization losses

Nitrogen Buffering Mechanisms 1. Increased Applied N results in increased plant N loss (NH 3 ) 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

#406

0.1

0.9

0.09

0.8

0.08

0.7

0.07

0.06

0.05

0.04 0 0.6

TSN OC 40 SED TSN = 0.002

SED OC = 0.03

80 120 N Rate, kg/ha 160 0.5

0.4

200

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 (NH 3 ) 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

Increased grain N uptake (protein) at N rates in excess of that required for maximum yield Point where increasing applied N no longer increases grain yield 80 60 40 20 0 0 40 80 120 160 Continued increase in grain N uptake, beyond the point where increasing applied N increases soil profile inorganic N accumulation 200 Annual Nitrogen Fertilizer Rate, kg/ha 240

# 222

80 70 60 50 40 30 20 0 9.4 =19% Y = 29.7 + 0.28x - 0.00055x2

r2=0.90

20 40 60 80 N rate, kg/ha 100 120 140

Nitrogen Buffering Mechanisms 1. Increased Applied N results in increased plant N loss (NH 3 ) 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

N Buffering Mechanisms

4 15-40 kg N/ha/yr

NO N 2 O N 2

Denitrification 3 7-80 kg N/ha/yr

NH 3 , N 2

Fertilizer N Applied Microbial Pool 1 NH 4 +OH NH 3 + H 2 O Urea 0-50 kg N/ha/yr

NH 3

Volatilization 2 NH 4 fixation (physical) 10-50 kg N/ha/yr Organic Immobilization

NH 4 NO 3 NO 2 1 Mills et al., 1974 Matocha, 1976 DuPlessis and Kroontje, 1964 Terman, 1979 Sharpe et al., 1988

Olson and Swallow, 1984 Sharpe et al., 1988 Timmons and Cruse, 1990

NO 3

5 Leaching 3

Francis et al., 1993 Hooker et al., 1980 O’Deen, 1986, 1989

0-20 kg N/ha/yr

Daigger et al., 1976 Parton et al., 1988

Chaney, 1989 Sommerfeldt and Smith, 1973 Macdonald et al., 1989 Kladivko, 1991 Aulackh et al., 1984 Colbourn et al., 1984 Bakken et al., 1987 Prade and Trolldenier, 1990

NITROGEN Cycle Links

Industrial view of the Nitrogen Cycle Nutrient Overload: Unbalancing the Global Nitrogen Cycle Carbon Cycle

Urea

Urea is the most important solid fertilizer in the world today.

In the early 1960's, ammonium sulfate was the primary N product in world trade (Bock and Kissel, 1988).

The majority of all urea production in the U.S. takes place in Louisiana, Alaska and Oklahoma.

Since 1968, direct application of anhydrous ammonia has ranged from 37 to 40% of total N use (Bock and Kissel, 1988) Urea: high analysis, safety, economy of production, transport and distribution make it a leader in world N trade.

In 1978, developed countries accounted for 44% of the world N market (Bock and Kissel, 1988).

By 1987, developed countries accounted for less than 33%

Koch Industries

7.5 million metric tons of N fertilizer/year

World

Total Production N, P, and K 216 million metric tons

Share of world N consumption by product group

1970 1986 Ammonium sulfate Ammonium nitrate

Urea

Ammonium phosphates Other N products (NH 3 ) Other complex N products 8 27

1 36 16 5 15

5 29 8 30

Urea Hydrolysis

increase pH (less H + ions in soil solution)

urease enzyme required

CO(NH 2 ) 2 + H + + 2H 2 O --------> 2NH 4 + + HCO 3 pH 6.5 to 8 HCO 3 + H + bicarbonate ---> CO 2 + H 2 O (added H lost from soil solution) CO(NH 2 ) 2 + 2H + + 2H 2 O --------> 2NH 4 + + H 2 CO 3 (carbonic acid) pH <6.3

H 2 CO 3  CO 2 carbonic acid + H 2 O 2004 2 14

During hydrolysis, soil pH can increase to >7 because the reaction requires H + the soil system.

from (How many moles of H + are consumed for each mole of urea hydrolyzed?) 2 In alkaline soils less H + having low H + .

is initially needed to drive urea hydrolysis on a soil already In an alkaline soil, removing more H + (from a soil solution already low in H + ), can increase pH even higher NH 4 + + OH ---> NH 4 OH ---->NH 3 + H 2 O pH = -log[H + ] Calculate pH of 2.0x10

-3 M solution of HCl HCl is completely ionized so [H+] = 2.0 x 10 -3 M 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% NH 4 pH 7.3

8.3

9.3

10.3

11.3

Base (NH3) 1 10 50 90 99 and 50% NH 3 Acid (NH4) 99 90 50 10 1 Chemicals A and B react to form C and D A + B = C + D Equilibrium Constant (K) K = [C][D] / [A][B]

10 9 NH 3 pH 8 7 + NH 4 6 0 20 40 60 80 100

H

0



H

+ OH

Equilibrium relationship for ammoniacal N and resultant amount of NH for a dilute solution.

3 and NH 4 as affected by pH

As the pH increases from urea hydrolysis, negative charges become available for NH and Kissel) 4 + adsorption because of the release of H + (Koelliker Decrease NH 3 loss with increasing CEC (Fenn and Kissel, 1976) Assuming that pH and CEC are positively correlated, what is happening?

CEC pH

** on soils where organic matter dominates the contribution to CEC then there should be a positive relationship of pH and CEC. Relationship of pH and BI (?) none In acid soils, the exchange of NH 4 + 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, NH 4 of CaCO 3 (CO 3 = from HCO 3 exchanges for Ca, precipitation above) and one H + released which helps resist the increase in pH

However

, pH was already high,

N Rate = 112 kg/ha

9 8 7 6 10 0 2 4 6 8 10 12 14 16 18 20 8 6 SOIL MIX 3-High Buffering Capacity SOIL MIX 2-Moderate Buffering Capacity SOIL MIX 1-Low Buffering Capacity 4 2 0 0 2 4 6 8 10 12 14 16 18 20 DAYS AFTER APPLICATION

Soil surface pH and cumulative NH 3 loss as influenced by pH buffering capacity (from Ferguson et al., 1984).

Ernst and Massey (1960) found increased NH 3 liming a silt loam soil.

volatilization when 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 NH 3 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 nonexchangeable titratable acidity) of exchangeable 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 non-weathered clays is due to isomorphic independent charge) CEC’s are higher in substitution – pH

Soil with an increased H + buffering capacity will also show less NH 3 when urea is applied without incorporation.

loss 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 + Ability of a soil to supply H H + is required for urea hydrolysis is produced via nitrification (after urea is applied): acidity generated is not beneficial + is related to amount of NH 3 loss 4.

What could we apply with the urea to reduce NH 3 loss?

an acid; strong electrolyte; dissociates to produce H + ;increased H + buffering; decrease pH reduce NH 3 loss by maintaining a low pH in the vicinity of the fertilizer granule (e.g. H 3 PO 4 ) Comment: Ferguson et al. (1984).

“When urea is applied to the soil surface, NH 3 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

NEED for INCREASED NUE

Computation/commodity World consumption of fertilizer-N Fert-N used in cereals (60% of total applied) 0.60 * 82,906,340 = Production, mT 90,000,000 54,000,000

World Cereal Production, mT

Sorghum 3% Barley 8% Millet 1% Oats 2% Rice 28% Corn 29% Wheat 28% Rye 1%

World grain N removal, 1996 Wheat Corn Rice Barley Sorghum Millet Oats Rye Total N removed in cereals %N 2.13

1.26

1.23

2.02

1.92

2.01

1.93

2.21

N removed in cereals (from soil & rain, 50% of total) NUE = ((N removed - N soil&rain)/total N applied) Savings/yr for each 1% increase in NUE Value of fertilizer savings $479/mT N 2005 mT 12,502,267 7,439,266 7,007,101 3,154,192 1,356,807 580,032 596,012 508,788 33,144,465 16,572,232 33% 489,892 mT $234,658,462 >$400,000,000

____________________________________



World cereal grain NUE 33%

 

Developed nation cereal NUE Developing nation cereal NUE 42% 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 NH 4 nutrition in Higher Yields (S.R. Olsen)

•Glutamine-major product formed in roots absorbing NH 4 •NO 3 has to be transported to the leaves to be reduced •Wheat N uptake was increased 35% when supplying 25% of the N as NH 4 compared to all N as NO 3 (Wang and Below, 1992). •High-yielding corn genotypes were unable to absorb NO 3 during ear development, thus limiting yields otherwise increased by supplies of NH 4 (Pan et al., 1984). •Assimilation of NO 3 requires the energy equivalent of 20 ATP/moleNO 3 , whereas NH 4 assimilation requires only 5 ATP/mole NH 4 (Salsac et al., 1987). •This energy savings may lead to greater dry weight production for plants supplied solely with NH 4 (Huffman, 1989).

Bidwell (1979), Plant Physiology, 2nd Ed.

Metabolism associated with nitrate reduction photosynthesis carbohydrates respiration reducing power NADH or NADPH NO 3 nitrate reductase NO 2 nitrite reductase

NH

3 carbon skeletons amino acids ferredoxin siroheme

Discussion:

Global Population and the Nitrogen Cycle p.80 nitrous oxide Increasing use of fertilizer N results in increased N 2 O. Reaction of nitrous oxide (N 2 O) 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: Anion: Cation: accepts protons negatively charged ion 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 H 2 SO 4 (strong electrolyte) CH 3 COOH (weak electrolyte) H 2 O HA --------------> H + + A potential acidity active acidity

Nitrogen Fertilization A. ammoniacal sources of N 2.

Decomposition of organic matter OM ------> R-NH 2 + CO 2 CO 2 + H 2 O --------> H 2 CO 3 (carbonic acid) H 2 CO 3 ------> H + + HCO 3 (bicarbonate) humus contains reactive carboxylic, phenolic groups that behave as weak acids which dissociate and release H +

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(H 2 O) 6 +3 + H 2 O -----> Al(OH)(H 2 O) ++ + H 2 O + 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

Aluminosilicate clays Presence of exchangeable Al Al +3 + H 2 O -----> 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-NH

+ Energy + CO

Ammonification: R-NH

+ HOH



NH

+ R-OH + energy

+H 2 O

NH

4 +

+ OH

Nitrification: biological oxidation of ammonia to nitrate 2NH

+ + 3O

2 

2NO

2 -

+ 2H

O + 4H

2NO

2 -

+ O

2 

2NO

3 -

Ion/molecule Name Oxidation State

NH 3 NH 4 + N 2 N 2 O NO ammonia ammonium diatomic N nitrous oxide nitric oxide NO 2 NO 3 H 2 S SO 4 = nitrite nitrate hydrogen sulfide sulfate

5 electrons in the outer shell -3 -3 0 +1 +2 +3 +5 -2 +6 loses 5 electrons (+5 oxidation state NO 3 ) gains 3 electrons (-3 oxidation state NH 3 )

6 electrons in the outer shell is always being reduced (gains 2 electrons to fill the outer shell)

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 1s 1 one electron in it), as found in H 2 gas.

(the first electron shell has only 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 H 2 , 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 1s 2 , 2s 2 , 2p 4 .

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 O 2 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 1s 2 , 2s 2 , 2p 3 .

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 + , N 2+ or N 3+ or electron gain to form N , N 2 , or N 3 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 N 2 .

Nitrogen Cycle:

•Increased acidity?

Ammonia Volatilization

· Urease activity (organic C) · Temperature · CEC (less when high) · H buffering capacity of the soil · Soil Water Content · Air Exchange · N Source and Rate · Application method · Crop Residues NH 4 +  NH 3 + H + If pH and temperature can be kept low, little potential exists for NH 3 volatilization. At pH 7.5, less than 7% of the ammoniacal N is actually in the form of NH 3 over the range of temperatures likely for field conditions.

10 9 NH 3 pH 8 7 + NH 4

H

0



H

+ OH

6 0 20 40 % 60 80 100

Equilibrium relationship for ammoniacal N and resultant amount of NH for a dilute solution.

3 and NH 4 as affected by pH

Chemical Equilibria

A+B  AB Kf AB = AB/A x B  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)

Assume that the absorbing complex of the soil can be represented by CaX 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.

H 2 X refers to dibasic acid (e.g., H 2 SO 4 ) (NH 4 ) 2 SO 4 -----> NH 4 + to the exchange complex, SO 4 = combines with the base on the exchange complex replaced by NH 4 + Volatilization losses of N as NH 3 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) ______________________________________________________________________

Ammonium sulfate

(NH 4 ) 2 SO 4 + CaX ----> CaSO 4 (NH 4 ) 2 X + 4O 2 nitrification

+ (NH 2HNO 3 4 ) + H 2 2 X X + 2H 2 O 2HNO 3 + CaX ----> Ca(NO 3 ) 2 + H 2 X Resultant acidity = 4H + /mole of (NH 4 ) 2 SO 4

Ammonium nitrate

2NH 4 NO 3 + CaX ----> Ca(NO 3 ) 2 (NH 4 ) 2 X + 4O 2 nitrification

+ (NH 2HNO 3 4 + H ) 2 2 X X + 2H 2 O 2HNO 3 + CaX ----> Ca(NO 3 ) 2 + H 2 X Resultant acidity = 2H+ /mole of NH 4 NO 3

Urea

CO(NH 2 ) 2 + 2H 2 O ----> (NH 4 ) 2 CO 3 (NH 4 ) 2 CO 3 + CaX ----> (NH 4 ) 2 X + CaCO 3 (NH 4 ) 2 X + 4O 2 nitrification

2HNO 3 + H 2 X +2H 2 O 2HNO 3 +CaX ----> Ca(NO 3 ) 2 + H 2 X H 2 X + CaCO 3 neutralization

CaX + H 2 O + CO 2 Resultant acidity = 2H + /mole of CO(NH 2 ) 2

Anhydrous Ammonia

2NH 2NH (NH 4 3 4 ) +2H 2 O ----> 2NH 4 OH OH + CaX ----> Ca(OH) 2 X + 4O 2 nitrification

2 + (NH 4 ) 2 X 2HNO 3 + H 2 X +2H 2 O 2HNO 3 + CaX ----> Ca(NO 3 ) 2 + H 2 X H 2 X + Ca(OH) 2 neutralization > CaX + 2H 2 O Resultant acidity = 1H+/mole of NH 3

Aqua Ammonia

2NH 4 ON + CaX ----> Ca(OH) (NH 4 ) 2 X + 4O 2 nitrification

2 + (NH 4 ) 2 X 2HNO 3 + H 2 X +2H 2 O 2HNO 3 +CaX ----> Ca(NO 3 ) 2 + H 2 X H 2 X + Ca(OH) 2 neutralization > CaX +2H 2 O Resultant acidity = 1H+/mole of NH 4 OH

Ammonium Phosphate

2NH 4 H 2 PO 4 + CaX ----> Ca(H 2 PO 4 ) 2 (NH 4 ) 2 X + 4O 2 nitrification

2HNO 3 + (NH + H 2 4 ) 2 X X +2H 2 O 2HNO 3 +CaX ----> Ca(NO 3 ) 2 + H 2 X Resultant acidity = 2H+/mole of NH 4 H 2 PO 4 ______________________________________________________________________