What factors influence levels of nitrogen fixation
in legumes?
The two biggest factors influencing nitrogen
fixation in legumes are the cultivar
and species of legume host,
and the strain of bacteria
used as inoculant. Part
of the research in the Rhizobium Research Laboratory is on
identifying cultivars of bean and soybean that are better able to
fix nitrogen, and to better understand the basis for such differences.
- Nitrogen fixation is also an energy requiring process, so that
anything which influences plant photosynthesis
will also affect nitrogen fixation. Such influences can include
shading from other plants, enhanced levels of carbon dioxide,
or even adequate supplies of phosphate fertilizer, since phosphate
fertilizer is used to make adenosine triphosphate (ATP), an energy-rich
substance.
- Plants that are fixing nitrogen in symbiosis
will also differ from N-fertilized plants in terms of their nutrient
requirements. Thus in addition to phosphorus, iron is needed for
the hemoglobin associated
with nitrogen fixation and as a component of the enzymes that
are responsible for nitrogen fixation. Molybdenum is also a component
of one of these enzymes.
- The presence of high N levels in soil will affect the number
of nodules produced,
but will also affect rates of nitrogen fixation in already-existing
nodules. Why spend energy in developing additional combined N,
when you already have sufficient?
- Environmental stresses, particularly temperature, pH, and water
stress, can also limit nitrogen fixation.
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