Geoscience Reference
In-Depth Information
50 years, the level of fixed nitrogen in the soil will have been raised to the point where
other plants without this ability can grow quite well. Since these other plants do not
have to support a population of bacteria in their roots, they may be able to grow better
than the nitrogen-fixing plants and so oust them. Plants with the nitrogen-fixing sym-
biosis therefore tend to be colonists or plants of other poor soils.
Both legume and non-legume symbioses have as their actual N-fixing partner a
bacterium or actinomycete, for in the whole course of evolution, some 3500 million
years, only they, with their simple cell structure, have developed the ability to fix at-
mospheric nitrogen. The whole process of nitrogen cycling in soils is described in
chapter 6
but it is worth mentioning here that several other bacteria that live freely in
soil are also N-fixers, and some of these are found in the rhizosphere around roots.
A tropical grass,
Paspalum nolatum
, often has very large populations of the N-fixing
bacterium
Azospirillum brasilense
around its roots. When this association was first
discovered, it caused great excitement, as it seemed possible that crop plants such
as wheat and barley could be bred to harbour these bacteria, so reducing their need
for nitrogen fertilizer, with enormous economic and environmental benefits. Unfor-
tunately, recent studies have shown that some initial measurements of N fixation in
the rhizosphere were over-estimates, and that there simply is not enough energy (in
the form of sugars and other compounds from plants) in the rhizosphere to sustain
agriculturally worthwhile rates of fixation. A better solution to the problem of the ex-
cessive use of nitrogen fertilizer may be to increase the use of leguminous crops.
Roots in soil exist in association with large numbers of other organisms. Some
of these associations are close and symbiotic, such as the nodule bacteria and mycor-
rhizas, while others are much looser. All of them depend on the fact that the root is
the main channel by which energy arrives in the soil; in other words, just as plants are
the primary producers that start almost all food chains above the ground, so they are,
in the form of roots and also dead leaves, below the ground.
