Environmental Engineering Reference
In-Depth Information
magnesium, or iron, especially in the presence of oxygen,
which renders the phosphate unavailable for plant uptake.
Alkaline conditions also immobilize phosphate, whereas
acidic conditions tend to mobilize phosphate. Phosphate
also can be mobilized by microbes. Converse to the immo-
bile phosphate salt, plant fertilizers that have mobile forms
of phosphate are derived from phosphate-rich rocks, such as
apatite, which are chemically treated to a form in which only
one of the phosphate molecules is linked to a metal.
11.1.6 The Flow of Iron
As discussed in Chap. 8, terrestrial plants must harvest from
the soil through the solvent action of water the necessary
essential elements, trace elements, and micronutrients
needed for survival. This is more difficult for terrestrial
plants compared to aquatic plants that already live in a
watery solution of such elements in the dissolved form.
From a biological and biomechanical standpoint, the process
of concentrating dilute trace elements, either by aquatic or
terrestrial plants, is similar to how a filter can concentrate
material, and this process is the ultimate source of these
nutrients in all other animals in the food chain. This is one
reason why root systems are characterized by a large surface
area. This process of nutrient concentration and uptake is
facilitated when the needed element is already in solution,
such as bicarbonate ion, dissolved oxygen, or nitrate.
What happens, however, when the source of the element
is a solid phase, such as for iron (Fig. 11.7 )? Although it has
long been recognized that iron is not part of the chlorophyll
molecule itself (Willst
Fig. 11.7 A representation of the flow of iron. Shown are oxygen (O 2 );
ferric iron (Fe(III)); ferrous iron (Fe(II)), and; iron sulfide (FeS 2 ).
often contains up to 10,000 times the amount of iron found in
plants growing in the soil. On the other hand, too much iron
accumulation can be detrimental to plant survival. Strong
oxidants like peroxides generated by the TCA cycle interact
with iron and form free radicals. These radicals can damage
cellular components, including DNA. This is why plants
regulate total iron uptake.
As a result of this limited iron bioavailability, many food
crops do not provide a sufficient source of iron for humans,
and iron must be added or the food “enriched” with iron
supplements in order to meet the nutritional needs of
humans. This is especially true for foods consumed by
infants during the first years of life, and for pre-natal
nutritional needs. In developing parts of the world, reliance
on iron-deficient crops and its effect on local populations are
termed “hidden hunger.” Some vegetables contain higher
amounts of iron than cereal grains, but even so, iron from
plants is much less (2%) absorbed by the human body than
iron from animal meat (20%).
Because of the low solubility of oxidized, ferric forms
of iron and the need for iron to be in the reduced state
of dissolved Fe(II) for plant uptake, plants that can
increase the solubility of iron have a selective advantage.
Plants do this mainly in two ways, referred to as Strategy I
and Strategy II. As energy is expended by plants to facilitate
iron uptake using these strategies, it is tightly controlled.
atter and Stoll 1913), iron is required
by the enzyme ferredoxin to make chlorophyll. It also is
important to both plants and animals in electron transport
to derive energy for growth from the oxidation of organic
matter. In this case, iron is used to transport electrons
through reversible redox reactions. Iron is part of one of
the enzymes (FAD) needed during the conversion of ace-
tyl-CoA into ATP (Fig. 11.2 ), or succinate dehydrogenase
(FADH to FAD).
Iron is present naturally in most near-surface soils where
plant roots grow and is necessary for plant growth and
human health. Plant iron concentrations range from 10 to
100
M (micromolar) as total Fe (Bauer and Hell 2006; Kim
and Guerinot 2007). Iron in the soil of the root zone, how-
ever, is predominately in its oxidized, solid phase and is not
bioavailable for uptake by plant roots, especially in the
concentration range needed by plants. This situation is anal-
ogous to the comment made by the poet Samuel T. Coleridge
in that a sailor is surrounded by water, but without a drop to
drink. Plants are similarly surrounded by iron present in the
earth's crust (4.5% by weight and is the fourth most abun-
dant element) but cannot directly use it in most cases. Soil
m
Search WWH ::




Custom Search