biogeochem: nitrogen cycle

ammonia

NH₃

ammonium

NH₄⁺

Dinitrogen gas (and characteristics)

N_{2
relatively inert and inactive, most stable form, 78% of the atmosphere (largest reservoir)}

Nitrous oxide

N₂O

Nitric oxide

NO

Nitrite

NO₂^-

Nitric acid

HNO₃

Nitrate ion

NO₃^-

describe the simple N cycle

N₂ in atmosphere is fixed by bacteria where it’s used in organic compounds with amino groups (-NH₂) in plants or animals. or decomposition of proteins into ammonia or converted into nitrate. the ammonia and nitrate go to plans. ammonia also goes through nitrification by bacteria and archaea to become nitrite which is nitrified into nitrate and nitrate goes through denitrification by bacteria into N2 in the atmosphere

what are the 5 steps of the N cycle

1. nitrogen fixation

2. nitrification

3. assimilation

4. ammonification (also called mineralization or remineralization)

5. denitrification

what is nitrification

going from ammonium to nitrate via intermediate nitrite

what is denitrification

going from nitrates to atmospheric nitrogen

what is N₂ fixation

making atmospheric nitrogen into a usable form (ammonia)

its performed by many prokaryotes.

biological N fixation is done by nitrogenase enzyme which is very oxygen sensitive.
cyanobacteria in aquatic systems and symbiotic N fixers in terrestrial

it’s very expensive and uses 16 ATP per N2 reduced

N fixing in oxic conditions

need to make little anoxic houses. multiple ways to protect nitrogenase:

moving to lower O2, making proteins to protect nitrogenase to oxygen exposure, form special cells to perform N fixation (nodules in plants, heterocytes in cyanobacteria) separation by time, fix at night when oxygen is lower due to decreased photosynthesis

types of N fixers

symbionts

• rhizobia and legumes

• actinomycetes and woody plants

associative

• azotobacter

• live in rhizosphere

free-living

• cyanobacteria

• also in soils

N assimilation and ammonification (aka mineralization)

assimilatory and dissimilatory

assimilatory:

• uptake and transformation for biomass incorporations

• biosynthesis, often energy requiring

dissimilatory

• uptake and transformation for energy generation and storage

• energy producing reaction

assimilation

formation of organic nitrogen compounds like amino acids from inorganic compounds. process where living organisms convert.

ammonium assimilation (“immobilization”

• NH4+ update from decompositions and NH4+ generation from N fixation

• preferred method for getting N for biosynthesis

• easily kind to use - looks like kind to use in biomass

NO3- Assimilation

• reducing NO3- to NH4 before incorporating

• requires energy - better to just use NH4+

• in a system with a lot of it - plants have adapted to just use it

mineralization (ammonification)

conversion of organic N (sugars, proteins) to inorganic forms

done as larger decomposition process = heterotrophs

mineralization is closely linked to decomposition

Nitrification

NH4+ to NO3-

ammonium to nitrate

two step process:

1. ammonia oxidation

2. nitrite oxidation

sets of organisms to do this are found together

who does ammonia oxidation? what are you converting?

NH₄⁺ + O₂ → NO₂^-

done by bacteria or archaea who are chemolithoautotrophs (means that this is how they make their living)

Nitrite oxidation

NO₂^- + H₂O → NO₃^-

nitrite oxidase

nitrite to nitrate

controls on nitrification

long term controls:

state factors: biota, time, parent material, climate

short-term controls:

direct controls: oxygen concentration and oxygen concentration

indirect controls:

plant NH4 uptake, litter quantity, carbon quality, root/microbial respiration, termperature, soil texture

NO₃^- reduction processes

denitrification and annamox

denitrification

converts nitrate and nitrite into nitrogen gases and happens under anaerobic conditions

• thought to be the predominant form of NO3- removal

• cycles the reactive N back to N2 and removes limiting nutrient from environment

• multi-step process where each step is conducted by a different enzyme

• contributes gaseous byproducts include N₂O which is a greenhouse gas

NO3- → NO2- → NO → N2O → N2

where there is higher oxygen levels or lower PH, there is poor activity of the nitrous oxide reductase so increases N2O production.

controls on denitrification

short term controls:

direct: nitrate concentration, labile carbon, oxygen concentration

indirect: plant NO3- uptake, litter quantity, carbon quality, room/microbial respiration, temperature and water, soil texture

long term:

state factors: biota, time, parent material, climate

Annamox (Anaerobic Oxidation of Ammonium)

Nitrite is combined with ammonium to remove combined N as N₂

combines nitrification and denitrification

some organisms can do this and they are chemolithoautotrophic

strictly anaerobic process

discovered in wastewater from a dutch yeast factory

N reservoirs

atmosphere is the largest, not very available

rocks and sediments, not available

soils and vegetation, relatively available

N fluxes

N-fixation

denitrification

runoff/leaching

internal transformations and cycling are greater than fluxes between reservoirs

what are the biological and abiotic fluxes of the N cycle

biological: fixing, denitrification, nitrification

abiotic: industrial fixation, lightning fixation, fossil fuel and biomass burning, deposition

Reactive N in the atmosphere

NO_x

• NO, NO₂

• produced by combustion, denitrification, nitrification,

• can remove zone

Nitrous oxide (N2O)

• 2nd most abundant N species

• relatively unreactive

• produced by denitrifiaction/nitrification

describe human alternation of N cycling

• doubled the amount of reactive N on earth

• three activities:

  • fertilizer productive

  • increase in N fixing crops

  • fossil fuel combustion

what is the haber-bosch process

discovered this to make bombs. makes fertilizer.

combines N with H at high temps

groundwater N contamination

toxic at high levels. possible carcinogen. causes blue baby syndrome

hyperthyroidism and diabetes