environment. Nitrates in the soil are
taken up by plant roots and may
then pass along food chains into ani-
mals. Decomposing bacteria convert
nitrogen-containing compounds (es-
pecially ammonia) in plant and ani-
mal wastes and dead remains back
into nitrates, which are released into
the soil and can again be taken up by
plants (see nitrification). Though ni-
trogen is essential to all forms of life,
the huge amount present in the at-
mosphere is not directly available to
most organisms (compare carbon
cycle). It can, however, be assimi-
lated by some specialized bacteria
(see nitrogen fixation) and is thus
made available to other organisms
indirectly. LightningÛashes also
make some nitrogen available to
plants by causing the combination of
atmospheric nitrogen and oxygen to
form oxides of nitrogen, which enter
the soil and form nitrates. Some ni-
trogen is returned from the soil to
the atmosphere by denitrifying bac-
teria (see denitrification).
nitrogen dioxide See dinitrogen
tetroxide.
nitrogen ÜxationA chemical
process in which atmospheric nitro-
gen is assimilated into organic com-
pounds in living organisms and
hence into the nitrogen cycle. The
ability toÜx nitrogen is limited to
certain bacteria (e.g. Azotobacter, Ana-
baena). Rhizobiumbacteria are able to
Üx nitrogen in association with cells
in the roots of leguminous plants,
such as peas and beans, in which
they form characteristic root nod-
ules; cultivation of legumes is there-
fore one way of increasing soil
nitrogen. Various chemical processes
are used toÜx atmospheric nitrogen
in the manufacture of fertilizers.
These include the *Birkeland–Eyde
process, the cyanamide process (see
calcium dicarbide), and the *Haber
process.nitrogen monoxide (nitric oxide)
A colourless gas, NO; m.p. –163.6°C;
b.p. –151.8°C. It is soluble in water,
ethanol, and ether. In the liquid state
nitrogen monoxide is blue in colour
(r.d. 1.26). It is formed in many reac-
tions involving the reduction of ni-
tric acid, but more convenient
reactions for the preparation of rea-
sonably pure NO are reactions of
sodium nitrite, sulphuric acid, and ei-
ther sodium iodide or iron(II) sul-
phate. Nitrogen monoxide reacts
readily with oxygen to give nitrogen
dioxide and with the halogens to give
the nitrosyl halides XNO (X = F,Cl,Br).
It is oxidized to nitric acid by strong
oxidizing agents and reduced to dini-
trogen oxide by reducing agents. The
molecule has one unpaired electron,
which accounts for its paramagnet-
ism and for the blue colour in the liq-
uid state. This electron is relatively
easily removed to give the nitrosyl
ionNO+, which is the ion present in
such compounds as NOClO 4 , NOBF 4 ,
NOFeCl 4 , (NO) 2 PtCl 6 and a ligand in
complexes, such as Co(CO) 3 NO.
In mammals and other vertebrates,
nitrogen monoxide is now known to
play several important roles. For ex-
ample, it acts as a gaseous mediator
in producing such responses as dila-
tion of blood vessels, relaxation of
smooth muscle, and inhibition of
platelet aggregation. In certain cells
of the immune system it is converted
to the peroxynitrite ion (–O–O–N=O),
which has activity against tumour
cells and pathogens.nitrogen mustards A group of ni-
trogen compounds similar to *sul-
phur mustard. They were used as
chemotherapy agents in cancer treat-
ment. Like sulphur mustard, they are
powerful blistering agents. Large
quantities were made during World375 nitrogen mustards
n