The Nitrogen Cycle
(with ocean emphasis dissolved…)
Updated Schedule
This week:
• Wed: 11/10 Oxygen Cycle- take 2 (Jered)
• Fri 11/ 12: Discussion on N-Cycle (Priya)
Next week:
• Monday: Nov-15, P-Cycle (Wendy)
• Wed: Sulfur- (Priya)
• Fri: paper on S or P? ( Jared and Don?)
N Cycle Readings/ References
Hand outs:
Background:
•Galloway, (2007) The Global Nitrogen Cycle, Chapter 8.11, vol. 8
(Biogeochemical cycles) Treatise on Geochemistry (eds. H.D. Holland and K.K.
Turekian).
•For Friday:
•Falkowski, P (2010) Review: Evolution and future of the Earth’s N Cycle,
Science 330, p 192.
•Gruber, N. (2008) An Earth System Perspective of the Global N Cycle,
Nature. 451, p293.
Additional on Website:
•Gruber (2008), The Global Ocean N Cycle: Overview and Challenges (book
chapter)
The Nitrogen Cycle
“N is so fundamental…
Without N there would be no peptide
bonds, to alpha helixes, no beta
sheets, no protein folding.
No us.”
Nitrogen: why we care
 Biochemically Critical to all life (cell structure and function)
o N in enzymes for photosynthesis/respiration..everything
*Limiting (macro) nutrient
* N generally thought to be main limiting nutrient on biological time scales ‐ (on geological time scales, likely is P) *Massive human changes to N cycle (to increase primary production on land.. But lots of unintended consequences)
Nitrogen: controls the short term C cycle?
 IF N is generally the limiting nutrient.. That means N availability in fact regulates (controls) the supply side of the C cycle.. Average Biomass Stoichiometry: tells how much N is needed
REDFIELD RATIO for marine organic matter
C:N:P of 106:16:1
TERRESTRIAL SYSTEMS C:N (and C:P generally) larger
Recall: “ transfers of carbon between Atm, Oceans, and Life”
N (etc)
N
Main Processes:
•Most Biological: Primary Production & Remineralization
•But these differ in ocean vs. land
Amazon Basin
Terrestrial vs. Ocean C/N ratios
Fresh
Leaves
Amazon
Soils
Amazon DOM
Mainstem
Rio
Negro DOM
50 – 60 !
10
0
Ocean)
Amazon
coarse POM
Amazon
Fine POM
Bacteria
Plankton
Ocean Seds
20
Ocean
DOM
30
40
Marine Humics
Ocean POM
C/N Continuum: Amazon River vs Ocean
The Central Reactions of N Cycle: “And now for something..completely different..” More reduced form
Ab
(and d iotic progre
egrad
ation f ssion
ate in
atm..)
More
Oxidized
form
For N Cycle: More oxidized forms
NOx
Kine
tic B
ar
rier
NO
(and n Abiotic pro
o degr
g
adatio ression
n fate
in atm
..)
More
reduced
Form (N2)
The Central Issue of the N Cycle
Triple bond => extremely stable
(226 kCal/mol activation )
 N2 = Highly abundant (~78% Atm), but also almost totally unreactive
 Leads to concept of “fixed” or “reactive” N (Nr) = any form of N which triple bond has been broken‐ can now be used by biology or undergo other chemistry
Over Geologic time, most N has ended up in atmosphere‐ largely inert
Over Geologic time, most N has outgassed‐ and ended up in atmosphere
Most reactive N is as nitrate in subsurface ocean
Unreactive vs. Reactive (fixed) N in active surface pools
 Reduced
 NH3, NH4+
 Oxidized
 NOx, HNO3, N2O, NO3‐, NO2
 Simple‐Organic Forms
 Urea
 1‐amines
Two Natural Pathways for N fixation
“The Adam and Eve of Nr..”
1. Abiotic: Oxidation
+ O2
NOx
2. Biological: reduction
+ H2
NH3
2N2 + 6H2O= 4NH3 + 3O2
1) Lighting‐driven Oxidation
+ O2
NOx
•
Lightning discharges provide the 226kcal activation energy
•
Thought to be key in creating Nr in early earth
•
Still important in tropics‐ places w/ lots of convective overturn and lightning
•
But overall, on modern earth a minor source
2) N Fixing Bacteria
“Diazatrophs”= bacteria/ cyannobacteria
+ H2O
NH3
+ CO2
•
Organic matter provides the 226kcal activation energy
•
Nitrogenase Enzyme: needs Anaerobic environment‐
typically inside the cell‐ Bacteria are not restricted to anaerobic environments
•
Major source of fixed N over geologic time
•
Diazatroph abundance assumed to be major evolutionary feedback control on quantity of fixed N Un‐Natural Pathways for N fixation
Original Apparatus
1) Haber‐Bosch Process
High T
Herr Haber und Herr Bosch
+ H2
Catalyst
NH3
•
Fossil Fuels (typically) provide the energy and the H2
• Metal catalyst lowers activation energy
•
Developed in 1930s‐ took off in WWII for explosives manufacture in both Europe and US
•
Major source of fertilizer for agriculture‐ source of “green revolution” last half of 20th century. •
~ MAJOR SOURCE OF FIXED N‐ in today’s earth. In 50 yrs totally altered the Global N cycle. 2)Fossil Fuel Combustion
High T
+ 02
N0
combustion
•
NO a byproduct of fossil fuel (oil and coal) combustion •
Fossil Fuels provide the energy and to an extent the N also • If N is from the fossil fuel, it’s not technically fixation, but rather a “remobilization” – but this is minor
•
ALSO A MAJOR NEW SOURCE OF FIXED N in today’s earth. Relative anthropogenic Nr sources
C-BNF: “anthropogenic” biological N fixation from crops
Total Relative Nr sources: 1890 & 1990
N cycling reactions 1:
Atmosphere = Abiotic
•Atmosphere provides:
•A) the major global reservoir for “source”
material
•1) small abiotic source of Nr (lighting)
•2) Transit reservoir, for Nr transfers between other
compartments (like Atm does for CO2 in C-cycle)
•3) No real “sink” for Nr•Cycling Relatively Simple: no biological transformations
Cycles of oxidized and reduced species are mostly
decoupled.
Atmosphere
Oxidized N cycle
•Many species and oxidation states
•NOx = NO + NO2
•NOy = all oxidized species
Reactive oxidizedmost res. time
minutes to days
NO Oxidations pathways and subspecies are also important for
consumption of hydrocarbons, formation of particles and smog
Atmosphere
Reduced N
•“inorganic” = NH3 and NH4
•“Organic” = basically particles with OM in them..
Inorganic: NH3
emitted to atmophere
from biological
sources (strong GHH)
Can also turn into
sulfate aerosols (NH4
+ SO2)
via acid reactions
Fate: typically
deposition
N cycling reactions 2:
Terrestrial and Aquatic = Biological •Complex and microbial..
•A Central Framework of
common reactions (mostly
microbial) recycles N
•Ultimately links life (proteins)
with Fixed N pool, and back to
N2 pool in Atm.
N cycling reaction network
”most complex of the biogeochemical cycles..”
Main Processes:
1) N fixation= source
2) Biological uptake/ release: a) Assimilation, b) Amonification,
3) Microbial form changes: Nitrification and Denitrification- (bugs
get energy from doing this..)
N Uptake Rx’s 1. AMMONIA ASSIMILATION
* Uptake of NH3 or NH4‐ turn into biomass
2. ASSIMILATORY NITROGEN REDUCTION
Uptake of nitrate by organism Into biomass through nitrate reduction
3. Ammonification
*organic matter => NH3 excreted
Conversion Rx’s 1. NITRIFICATION: NH3 (ammonia) to N03 –2 (nitrate)
Two steps, each carried out by different bacteria
∙first Ammonia to NO2‐ (nitrite) – 65.7 kCal/mol
Second: N02– (Nitrate) to N03 –2 (Nitrate)
– 17.5 kCal/mol
2. DENITRIFICATION: NO3 (nitrate) back to N2 gas
∙Anaerobic process, reducing nitrate N2
Delta G: - 649 kcal/ mol
Conversions
Nitrification and de‐nitrification together represent a “bridge” from NH3 (ammonia) back to atmospheric N2
NH3 is almost always extremely low‐
because: First: it is the preferred N form for autotrophs (can use directly)
Second: nitrifying bacteria can use it..
Conversions: aerobic vs. anaerobic requirements of enzymes constitute a fundamental control
Nitrification: Aerobic: occurs in water columns
de‐nitrification An‐
Aerobic‐ occurs in sediments, low O2 water columns
“Annamox” pathway
New Pathway, fairly recently discovered..
Debated as to importance..
ANNAMOX ANaerobic AMmonium OXidation
Coupled process, converts both NO2 and NH4 to N2
NH4+ + NO2− → N2 + 2H2O.
Since NO2 is and intermediate in both nitrification and de‐
nitrification, ANNAMOX has potential to significantly alter other pathways..
Global N Budgets and Fluxes
1. Sources: Overall Nr “creation”:
Modern world- land and sea
•Most is terrestrial
*most of that anthropogenic
* Terr Cycle totally altered
•Ocean is only about 1/3
•Ocean cycle thought to be ~
unchanged
•Note: terr. flux to ocean fairly
significant- mostly affects coasts
1. Sources: Overall Nr “creation”:
Modern world- Atmosphere
•Direct emissions large
•NOx about half, ammonia half
•Fate is deposition to ocean/ land
Atm Deposition
2. Fates of Nr: 1890
Fixation from Atm:
N2 box: 5 = Lightning, 100 = natural biological N fix, 15= agricultural bio fix
Noy box:
*emissions: 6.2 = vegitation, soil, natural burning, agricultural; 0.6= coal burning
NHx box:
*emissions: 3.1 = agricultural and bio burns; 5.6= Ruminants, 8 = oceans
Fixation from Atm:
1990
N2 box: 5 = Lightning, 89 = natural bio, 85= Haber-Bosch (fertilizer); 33= agricultural bio fix
Noy box:
*emissions: 13 (2x)= vegitation, soil, natural burning, agricultural; 21= coal burning
* Fates: 33 into veg/ soil; 13 into ocean/ waters
NHx box:
*emissions: 22 (~10x) = agricultural and bio burns; 22(4x) = animals waste, 8 = oceans
*fates: 43 agriculture, river export (20); atm deposition in ocean (14)
Consequences/ feedbacks for changes to Global N cycle
Summary of major effects of adding Nr
Positive
• Increased food production‐ feeds large portion of planet Negative: diverse negative effects
• Coastal Eutrophication
• Now Largest source of coastal pollution? • Hypoxia (dead zones) loss of biodiversity
•
Air pollution‐ production of ozone and particles
• Creates Tropsheric ozone (“bad” ozone) (?) • But: Destroys stratospheric ozone (“good” ozone) NO + 03 → NO2 + O2
Stratospheric Ozone Destruction- key step
Summary of major effects of adding Nr
Negative: diverse negative effects
• Terrestrial ecosystems
• Causes “spike” in productivity through critical threshold supported by other nutrients, etc.
• Ultimate “crash”, and loss of biodiversity
•
Greenhouse warming
• Adds GHGs‐ so contributes to warming
• Destroys stratospheric ozone (“good” ozone) – so increases warming, other ozone related problems.
The “N Cascade” Concept
Because N cycle is so complex, once N is converted from
(essentially endless) N2 pool, addition of N-r to any part of system
cascades through many compartments..
Basic idea‐
• Once created‐ “pulse” of new Nr propagates through many compartments • Causes disruption/ harm to each compartment in turn
• Ultimate negative effects‐ therefore much wider than original source or compartment would suggest
•
End? Only when Nr is ultimately de‐nitrified. (can take long time)
Example of “N Cascade”
1. Add NOx
2. Increase troposphere
ozone, particle haze,
acid rain.
3.deposition to
terrestrial- increase
soil acid, alter
terrestrial ecosystem
structure
Fossil fuel Combustion NOx production example
3.discharge to aquatic
systemseutrophicationhypoxia, alter
aquatic ecosystem
structures
Future projections for Nr End