Nitrogen Dynamics
Nitrogen Dynamics
Nitrogen Dynamics
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<strong>Nitrogen</strong> Cycling<br />
<br />
Complicated… N can form compounds that are gaseous, or volatile, also can<br />
form water-soluble compounds<br />
<br />
N 2<br />
: dinitrogen, , molecular nitrogen (gas)<br />
• 78% of atmosphere<br />
• Colorless, chemically unreactive<br />
• Zero valent (intermediate oxidation state)<br />
<br />
HNO 3<br />
/ NO -<br />
3<br />
/ NO -<br />
2<br />
: Nitric acid / nitrate / nitrite (dissolved)<br />
• Strong acid<br />
• Nitrate salts highly soluble (e.g. KNO 3 , NaNO 3 )<br />
• Used in fertilizers, explosives<br />
• Oxidized forms of N, found in oxic solutions<br />
<br />
NO : nitric oxide (gas)<br />
• Colorless, produced by high temperature combustion<br />
<br />
NO 2<br />
: nitrogen dioxide (gas)<br />
• Brown/yellow gas, irritating/toxic
<strong>Nitrogen</strong> Cycling<br />
<br />
Complicated… N can form compounds that are gaseous, or volatile, also can<br />
form water-soluble compounds<br />
<br />
N 2<br />
O : nitrous oxide (gas)<br />
• Colorless gas, anesthetic<br />
<br />
NH 3<br />
/ NH +<br />
4<br />
: Ammonia / ammonium (gas or dissolved / dissolved)<br />
• Colorless gas, irritant / toxic<br />
• Water soluble, forms ammonium… used in fertilizers<br />
• Reduced form of N, found in anaerobic solutions<br />
<br />
R-NH 2<br />
: Amines (organics)<br />
• Low molecular weight compounds are water-soluble<br />
• Important functional group in biochemistry<br />
<br />
O-C-N : Amides (organics)<br />
• Found in urea, proteins
<strong>Nitrogen</strong> as a Nutrient<br />
<br />
N is needed by all life forms<br />
• Amino acid formation<br />
• Protein synthesis<br />
• Nucleic acid formation<br />
<br />
Natural availability of fixed N is low in most ecosystems<br />
– Fixed N acts as a limiting nutrient (biological(<br />
productivity would<br />
be higher if more fixed N were available)<br />
– Not true in all systems, but often in coastal marine environments
<strong>Nitrogen</strong> Fixation<br />
<br />
Transformation of N 2<br />
to form NH 3<br />
, NH 4+<br />
, organic N<br />
• Triple bond in N 2 requires a great deal of energy to break<br />
<br />
Fixation requires heat or enzymatic catalysis<br />
• Heat<br />
– Lightning, Forest Fires<br />
• High temperatures break N 2 bonds and combine N with O<br />
• ~10<br />
Teragrams N/yr (1 Tg = 10 12<br />
g)<br />
12 g)<br />
• Biotic<br />
– <strong>Nitrogen</strong>-fixing bacteria, cyanobacteria<br />
• Use nitrogenase enzyme to cleave N 2 bond, combine with H 2 to<br />
form ammonia (NH 3 )<br />
• <strong>Nitrogen</strong>ase enzyme contains two metalloproteins: : Mo/Fe and Fe<br />
• ~130 Tg/yr (non-anthropogenic, non-agricultural)<br />
• Industrial<br />
– Fertilizer manufacture, agricultural legume cultivation, biomass burning,<br />
land draining and clearing
Anthropogenic N Fixation<br />
<br />
<br />
<br />
<br />
<strong>Nitrogen</strong> fertilizer 83 Tg/yr<br />
– Haber process<br />
Legumes 40 Tg/yr<br />
– Agricultural production<br />
Fossil Fuels 20 Tg/yr<br />
Land use changes<br />
– Biomass burning 40 Tg/yr<br />
– Wetland draining 10 Tg/yr<br />
– Land clearing 20 Tg/yr<br />
<br />
<br />
Total Anthropogenic 213 Tg/yr<br />
Total Natural 140 Tg/yr<br />
<br />
Human industrial development has more than doubled the amount<br />
of fixed N moving through the biosphere, globally…
Aspects of the N Cycle<br />
<br />
Biotic transformations<br />
• NH 3 assimilation (NH 3 => organic-N)<br />
– Dissolved NH 3(aq<br />
aq)<br />
is taken up by bacteria, plants, converted to<br />
organic N (amino acids)<br />
– Normally accomplished during photosynthesis in green plants<br />
• Ammonification (organic-N => NH 3 )<br />
– Breakdown of dead organic matter by heterotrophic bacteria<br />
– Releases NH<br />
NH 3(aq<br />
aq)
Aspects of the N Cycle<br />
<br />
Biotic transformations<br />
• Nitrification (NH 3 => NO<br />
- 2 => NO 3- )<br />
– Bacterial oxidation of NH 3(aq<br />
aq)<br />
to nitrate (NO(<br />
3-<br />
)<br />
– Done aerobically<br />
– Nitrosomonas (convert ammonium to nitrite NO 2-<br />
)<br />
– Nitrobacter (convert nitrite NO -<br />
2<br />
to nitrate NO 3-<br />
)<br />
• Assimilatory Nitrate Reduction (NO<br />
- 3 => organic-N, NH 3 or<br />
NH 4+ )<br />
– Nitrate reduced to organic N compounds, ammonia or<br />
ammonium<br />
– Accomplished by green plants, some bacteria, algae
Aspects of the N Cycle<br />
<br />
Biotic transformations<br />
• Dissimilatory Nitrate Reduction (Denitrification(<br />
Denitrification)<br />
(NO 3-<br />
, NO -<br />
2<br />
=> N 2<br />
, N 2<br />
O)<br />
– Nitrate and nitrite converted to N 2<br />
, N 2<br />
O gases<br />
– Performed by anaerobic bacteria, usually coupled to organic<br />
matter oxidation<br />
– May also be coupled to sulfur oxidation (Thiobacillus(<br />
denitrificans) ) or Fe oxidation (Gallionella(<br />
ferruginea)<br />
Gallionella twisted stalks<br />
Image from:<br />
http://seismo.berkeley.edu/~jill/tennyson/tenn2.html
Pre-Industrial N Cycle (from Chameides and Perdue, 1997)<br />
Biotic fixation<br />
(Terrestrial)<br />
100 Tg/yr<br />
Terrestrial<br />
Biosphere N<br />
29,000 Tg<br />
Assimilation<br />
500 Tg/yr<br />
River runoff<br />
4,000 Tg/yr<br />
Denitification<br />
(Terrestrial)<br />
84 Tg/yr<br />
Terrestrial<br />
Soil N<br />
4,000 Tg<br />
Photochemical oxidation<br />
8.5 Tg/yr<br />
Atmosphere<br />
N O(g)<br />
2<br />
1,300 Tg<br />
Ammonification<br />
595 Tg/yr<br />
Denitrification +<br />
Nifrification<br />
(Terrestrial)<br />
6 Tg/yr<br />
Weathering<br />
10 Tg/yr<br />
Nitrification<br />
(Terrestrial)<br />
5 Tg/yr<br />
Atmosphere<br />
N (g)<br />
2<br />
3,800,000,000 Tg<br />
Photochemical oxidation<br />
0.5 Tg/yr<br />
Denitrification +<br />
Nifrification (Oceanic)<br />
3 Tg/yr<br />
Deposition<br />
5 Tg/yr<br />
River runoff<br />
15 Tg/yr<br />
Deposition<br />
3.5 Tg/yr<br />
Sedimentary<br />
(Fossilized)<br />
Organic N<br />
2,000,000,000 Tg<br />
Abiotic fixation (3 Tg/yr)<br />
Atmosphere<br />
NO (g)<br />
x<br />
0.1 Tg<br />
Ammonification<br />
700 Tg/yr<br />
Sedimentation<br />
10 Tg/yr<br />
Ocean Biosphere N<br />
530,000 Tg<br />
Assimilation<br />
650 Tg/yr<br />
Ocean Inorganic N<br />
640,000 Tg<br />
Denitrification<br />
(Oceanic)<br />
60.5 Tg/yr<br />
Biotic<br />
fixation<br />
(oceanic)<br />
50 Tg/yr
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