Environmental Engineering Reference
In-Depth Information
and developed the ability to take in and use nitrogen in both
the oxidized form as nitrate, NO 3 , and the reduced form as
ammonium, NH 4 + . Ammonium is preferred by plants as it is
already in the reduced state, but nitrate has higher water
solubility and is sorbed onto soils less.
There are four main processes by which atmospheric N 2
gas can become available to plants for protein synthesis
(Fig. 11.5 ):(1) N 2 can become 'fixed' by lightning during
electrochemical reactions, to produce nitrate ions, NO 3 ; (2)
N 2 can be artificially fixed during the man-made version of
this natural reaction, called the Haber-Bosch process; (3) N 2
can be fixed into ammonia, NH 3 , in the root zone, and (4)
NH 3 can be converted into NO 3 in the root zone through
nitrification. Some plants do not need to directly rely on such
sources of nitrogen, however, such as the various pitcher
plants ( Nepenthes spp. ) and Venus fly traps that use modified
leaves to entrap nitrogen-rich insects as their nitrogen
source. These plants must rely on external nitrogen sources
because the soil that they live in is nitrogen poor.
Above ground, fixation of molecular N 2 takes the energy
of the lightning to split N 2 toN+N+O 2 to yield nitrate
ions. This nitrate can be taken up foliarly by plants during
precipitation or by the roots after precipitation. The Haber-
Bosch process, named after Fritz Haber and Carl Bosch,
revolutionized the agricultural industry in 1909 by taking
nitrogen in air in the presence of H 2 from coal to make
ammonia, NH 3 . This process was used to replace the previ-
ous source of nitrogen from guano, or bird droppings, which
was rapidly mined away by the late 1800s. That the process
is important is indicated by the fact that one-half of the
nitrogen atoms in human proteins have been produced by
an ammonia factory (The Economist 2005). This process
does essentially what nitrogen-fixing organisms do but
requires high temperatures (400 C) and pressure (200 atm),
whereas the biological process uses nitrogenase and hydrog-
enase enzymes to perform the same reaction at the lower
temperatures and pressures of the soil zone. Finally, internal
combustion engines with high pressures and temperatures
can oxidize N 2 in fuel to nitrogen oxides (NO x ), although
this is reduced in catalytic converters prior to exhaust.
Below ground, elemental gaseous N 2 must be fixed, or
reduced, much in the same way CO 2 must be reduced for use
by plants. The fixation of nitrogen is the rate limiting step of
the flow or cycling of nitrogen as is evident by the large
Fig. 11.5 A representation of the
flow of nitrogen. Plants are
surrounded by nitrogen in the
atmosphere, but unlike their
ability to take up carbon dioxide,
they cannot directly take up
nitrogen. This need is met by
bacteria living in the roots. The
energy needed by bacteria to
perform nitrogen uptake is
provided by plants, however, in
the form of carbon compounds
released in the root zone . Shown
are water (H 2 O); oxygen (O 2 );
nitrogen (N 2 ); nitrate (NO 3 ),
ammonia (NH 3 ) and; ammonium
(NH 4 ).
Search WWH ::




Custom Search