ESS Nitrogen Cycle

most common form of nitrogen

N2 gas

N2 molecule is very difficult for organisms to use directly because

has a triple covalent

chemical bond that requires a significant amount of energy to break.

forms of organic and inorganic nitrogen other than N2 are referred to as

combined nitrogen

two most important forms of dissolved inorganic N in natural waters

nitrate (NO3) and ammonium (NH4).

The sum of nitrate, nitrite, and ammonium

dissolved inorganic nitrogen

Primary producers and bacteria generally use

nitrate, nitrite, or ammonium

nitrogen fixation

assimilate N2 by bacteria and cyanobacteria, some archaea, requires high activation energy

Conversion of N 2 to ammonium is accom-

plished with

he enzyme nitrogenase

nitrogenase can be…

inactivated by O2, so organisms either have to inhibit anoxic habitats or protect the enzyme from exposure

heterocysts do what

protect nitrogenase from O 2

Denitrification

conversion of inorganic combined nitrogen (nitrate) to the relatively unavail- able N2 gas.

ammonification

When ammonium is remineralized

N fixation:

• Pathway: N2 → NH3

• Energetically expensive due to triple bond, requires

15 ATP/mole to break. Aerobic oxidation of glucose

yields 26 ATP/mole, anaerobic yields less than 5. The

industrial process uses high temperature and

pressure to make NH3.

• Where do we find N fixation – whenever you have

abundant energy sources, e.g. Legumes-Rhizobia,

Cyanobacteria, Frankia-alders, rotting logs?

• Non-symbiotic fixation is rare (in soils but not soil

crusts), but there is still uncertainty about this.

N Mineralization:

Pathway: Organic N → NH4+

• Organic to inorganic (mineral, or simple)

transformation. Release of NH4+ from amino acids,

nucleotide bases.

• Should we redefine to include simple organics?

• An energy-driven process. Think like a microbe.

• Occurs under both aerobic and anaerobic conditions.

Immobilization:

• Pathway: NH4+ → Organic N

• Uptake of inorganic N to support growth.

• Again, energy driven. Microbes reluctantly need N to acquire

carbon and energy.

• Aerobic and anaerobic.

• Balance between mineralization and immobilization

controlled by the C:N ratio of the substrate:

– 25:1 is considered to be breakpoint

– Sawdust = 225:1, oat straw = 80:1, Compost = 10:1.

• Microbes and plants will produce enzymes to acquire specific

nutrients that they need.

Nitrification:

• Pathway: NH4+ → NO2- → NO3-

• Unique process carried out by strange aerobic chemoautotrophic

bacteria. They acquire energy from the oxidation of ammonia.

• Strong regulation by ammonia, and especially by the competition with

roots and heterotrophs (immobilizers). They are lousy competitors

because of slow growth rates.

• Key to losses. Without NO3-, the N cycle would be very conservative.

• Source of N2O

• The physiology literature is a pack of lies, albeit generally true:

– Anaerobic activity,. E.g. nitrifier denitrification

– PH sensitive

– Limited number of genera

– Limited substrate range (TCE, methane)

Dissimilatory processes:

• Anaerobic microbial processes that convert

nitrate into more reduced forms (ammonia or N

gases).

–Denitrification - anaerobic respiration of nitrate to

produce nitrogen gases.

–Dissimilatory nitrate reduction to ammonia

(DNRA)

–Anaerobic ammonium oxidation (Annamox)

Denitrification:

• Pathway: NO3- → NO2- → NO → N2O → N2

• Anaerobic (mostly), heterotrophs (sort of

mostly), nitrate as electron acceptor.

• Thought to be low in most terrestrial

ecosystems, but should balance fixation on a

global basis, e.g. very high rates in oceans.

• Staggeringly high rates (25 g N/m-2/y) in

wetlands with high nitrate

DNRA:

• Pathway: NO3- → NH3

• Anaerobic

• Carried out by fermenters and/or S-oxidizers.

• Dumps more electrons than denitrification, may

be favored under high C, low NO3- conditions.

• May contribute to N retention because NH3 is

more stable than NO3-

Annamox:

• Pathway: NH4+ +NO2- → N2

• Anaerobic

• Carried out poorly characterized group of

bacteria, driven by hydrazine (rocket fuel).

• Discovered in waste treatment, shown to be

important in ocean.

• May be most important in anaerobic

ecosystems with limited labile C, e.g., deep

ocean, deep lakes.

Leaching:

• NO3- is more mobile than NH4+.

• Some plants may be adapted to this mobility.

• Are ecosystems “adapted” to minimize

hydrologic loss?

• DON – may be an unregulatable loss, the

source of persistent N limitation.

Uptake/detritus dynamics by primary producers:

producers:

• Litterfall is the main pathway for terrestrial nutrient recycling.

If everything was translocated, there wouldn’t be any need

for nutrient cycling.

• Uptake of organic N is hot topic (maybe too hot).

• Terrestrial: Low site nutrient availability leads to lower

concentrations in litter, higher resorption which makes

positive feedback to low site nutrient availability.

• Aquatic: Redfield dominates.

• Root turnover is of great current interest. How fast? Is it

much more functionally important than leaf litter?

Nutrient poor site → Nutrient Poor vegetation → ____ → ____ → _____

Poor litter quality→ Low nutrient availability→ Low productivity Low loss following disturbances

Nutrient rich site → nutrient rich vegetation → ___→ ____→____, ____, ____

High Litter quality → High nutrient availability → High productivity, high loss following disturbance, sensitive to saturation

Fate of NH4+

1. Immediately taken up (immobilized) by another microbe

2. Lost downstream or out of lake’s ouGlow

3. Taken up by a plant or algae

4. VolaKlized as ammonia (NH3)—lost from ecosystem

5. Oxidized by microbes to nitrate (NO3-)

NH4+ NO3- are

the most reactive forms of N

i.e., stimulate algal growth

n aquatic ecosystems N cycles through a

variety

of pathway most of which are mediated by microbes

The presence or absence or oxygen and the presence

of or absence of organic C can

determine which

transformative pathway is favored