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Notes on the terrestrial N cycle:
Oxidation states of N in the environment:
N is found in a wide range of oxidation states. NH3 is highly reduced (-3), while
NO3- is highly oxidized (+5). Intermediate compounds including N2O (nitrous oxide),
NO (nitric oxide), N2O5, and NO2- (nitrite) exist. The pathways of nitrification and
denitrification pass through some of these intermediate oxidation states as N is
progressively reduced or oxidized.
Nitrogen is essential for all forms of life, because of it’s incorporation into nucleic acids
and amino acids which form the genetic coding molecules RNA and DNA, and proteins.
The atmosphere contains 79% N2 by volume, but this nitrogen is of little use to most
organisms. The di-nitrogen triple bond is very strong, and for the most part N2 acts as an
inert gas. It does effectively absorb short wavelength UV radiation in the upper
atmosphere. A small number of bacteria are know as “nitrogen fixers” and can convert
N2 to biologically useful forms. These bacteria may live symbiotically with other
organisms. A commonly cited example is the legume family (beans), which have
symbiotic N fixer colonies in their rhizomes, and typically have high protein levels in the
fruiting bodies (thus soybeans are a good dietary substitute for meat).
Sources of “fixed” (biologically available) nitrogen:
Nitrogen fixation (conversion of N2 to NH2- amine groups) by specialized bacteria
is the main “new” source of fixed nitrogen in the terrestrial environment. Nitrate (and in
some cases ammonium) in precipitation also contribute, especially in areas affected by air
pollution. Small amounts of N are fixed in abiotic process, including lightning
discharges and reaction of N2 on hot lava surfaces. Given that N fixation is an
energetically demanding process, and that only a certain class of bacteria is capable of it,
efficient recycling of fixed is fundamental to the operation of any ecosystem.
Nitrogen recycling
“Mineralization” during decomposition
Ammonia (NH3) or ammonium can be produced by bacteria during decomposition of
N-containing organic matter (“mineralization”). Ammonium may have several fates;
1) NH4+ may be incorporated into microbial biomass (“immobilization”)
2) NH4+ may be taken up by plant roots
3) NH4+ may be incorporated into or adsorbed onto clay minerals.
4) NH4+ may be converted to NO3- during chemoautotrophic carbon fixation (i.e.
CO2 is reduced to form biomass (CH2O), and NH4+ is oxidized to form
nitrate). This process is known as “nitrification”, and is carried out by
bacteria such as Nitrosomonas and Nitrobacter. It is believed that much of the
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nitrate found in a soil-water system is produced by these type of bacteria
during decomposition. These “nitrifying” bacteria are critically important to
the N-cycle.
5) NH4+ may react to form NH3 and be lost as the gas phase (“volatilization”).
NH4+ + OH-  NH3(gas)↑ + H2O. This process is obviously favored at high
pH. It is also favored at high T, and fires cause significant N losses.
The net NH4+ and NO3- produced (total minus that taken up – immobilized- by
bacteria) is sometimes referred to as “net mineralization”, and is the quantity of
fixed N released to the environment during decomposition.
Nitrate sinks and denitrification:
Nitrate is readily taken up by most plants, but some can be leached out of the soil system
in ground and stream waters. The other major sink for nitrate is “denitrification”, during
which bacteria use NO3- as an oxidant to break down organic matter and release energy:
5CH2O(org) + 4H+ + 4NO3-  2N2(gas)↑ + 5CO2 + 7H2O
The process of denitrification “completes the loop” for the N-cycle, returning fixed N to
the atmospheric N2 reservoir. Denitrification is a form of anaerobic respiration, i.e.
respiration with an oxidant other than oxygen.
During the denitrification process, the intermediate compound N2O is produced.
Nitrous oxide is volatile, and so N can be lost during denitrification as nitrous oxide
before the process is complete, i.e. reaches N2. A common means of measuring
denitrificaiton rates is to measure N2O production by gas chromatography, as this process
is the main source of natural N2O.
We can summarize the major inputs and outputs of fixed N to an ecosystem as follows;
Inputs: nitrate and ammonium in precipitation, nitrogen fixation
Outputs: leaching of nitrate, ammonia volatilization, denitrification
As with the other nutrients we have considered, fixed N is extensively recycled within the
plant-water-microbial-soil system, undergoing a variety of transformations between the
most reduced forms (NH4+, NH3), to intermediate forms (N2O, NO) produced during
“nitrification”, to oxidized forms (NO3-, HNO3). Some N2O(gas) can be lost to the
atmosphere. While this nitrous oxide is not usually a major loss compared to the
production of N2 via denitrification, N2O is an important greenhouse gas even at low
concentrations.
Minerals may play an important role in the N-cycle, but how important is still not
clear. Clay and mica minerals can incorporate NH4+ in place of OH- or Cl-, and
weathering of those clays and micas can be a significant source of ammonium. NH4+
can also be adsorbed onto clays just like other cations. Other minerals act as anion
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exchangers (analogous to the cation adsorption we measured in the lab), and take up
NO3- in exchange sites.
Figure 1) A simplified N cycle, modified from Fenchel et al., 1998. The arrows and
process names are color coded. Ammonification and nitrification are processes of
“mineralization”. Uptake of ammonia and reductive uptake of nitrate by bacteria are
known as “immobilization”, as N is incorporated into bacterial biomass. Denitrification
takes place in low O2 environments, where NO3- is used as the electron acceptor
(oxidant) for the respiration of organic matter. N fixation is the process of “fixing” N2
from the atmosphere.
Pollutant sources of N
Internal combustion engines produce quantities of fixed N in the form of various oxides
of nitrogen, usually written as NOx. The high temperatures and high pressures within the
combustion chamber promote the oxidative reaction of N2 with O2, a process that is
vanishingly slow under normal conditions. The NOx consists of NO, N2O, N2O5, etcetera,
each of which can be oxidized to NO3- in the atmosphere (usually by reaction with OH
radical). Concentrations of nitrate in rainfall in industrialized areas are far above natural
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levels, and a great deal of NO3- is added each year to ecosystems around the world in
precipitation. Efforts to reduce NOx production from vehicles included lowering the
compression ratio in the cylinder (1970’s), and more recently adding catalytic converters
to the exhaust system.
Agriculture is another important source of fixed N. Annual global nitrate fertilizer
production now exceeds all natural sources of N-fixation, and application of this fertilizer
is a major source of N in ecosystems near agricultural fields and in streams. Livestock
release significant quantities of N in urea, which can be converted to NH4+ or NO3-, and
finds its way into streams and the atmosphere. It is likely that much of the high ammonia
levels in the Aurora Farm precipitation data come form local livestock sources.
Rates of global nitrogen cycling
A schematic diagram of the global N cycle (from Schlesinger, 1997) is below. Note that
in both the terrestrial and marine environments, the largest fluxes are internal recycling.
Also not that the presumed rate of biological N fixation in the terrestrial environment is
much higher that that in the oceans. Recent work suggests that the N fixation rate in the
oceans is higher than presented here. Finally, note that “Human activities” are estimated
to fix approximately 100 Tg of N/yr. Again, more recent work (e.g. Vitousek et al.,
Science, 1997) indicates that human activities now exceed biological fixation. The
impacts of high rates of anthropogenic N fixation are manifold, and are still being
explored. They include fertilization, acidification, species shifts, and human population
growth, just to name a few of the major ones.
Figure from Schlesinger, 1997, Biogeochemistry, Academic Press
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