Roadside Revegetation

IMPLEMENTATION GUIDES
Inset 10-10 | How does biological nitrogen fixation work?
The symbiotic partnership between plants and their nitrogen-fixing microsymbionts
works this way: The bacteria live in nodules on roots of the plant. Each nodule
contains millions of the bacteria that accumulate atmospheric nitrogen and share
this nitrogen with the plant. In exchange, the plant provides energy in the form of
carbohydrates to the bacteria.
The bacteria must come in contact with the root systems early in the plant’s life, ideally
within the first 2 to 6 weeks of growth. For nursery-grown materials, nitrogen-fixing
bacteria must be introduced in the nursery. For direct field sowing of seed, inoculants
should be applied as the seeds are being sown.
When the “nitrogen-fixing” plant sheds its leaves, dies, or dies back, the nitrogen
stored in the plant’s tissues is cycled to other plants and through the ecosystem.
This process, part of the nitrogen cycle, is the major source of nitrogen fertility in
most natural ecosystems.
Other Beneficial Microorganisms
In nature, communities of bacteria, fungi, algae, protozoa, and other microorganisms in the soil
make nutrients available to plants, create channels for water and air, maintain soil structure,
and cycle nutrients and organic matter. A healthy population of soil microorganisms can also
maintain ecological balance, preventing the onset of major problems from viruses or other
pathogens that reside in the soil. The practice of protecting and re-establishing beneficial
microorganisms is a key one for revegetation. As a science, however, the use of beneficial
microorganisms is in its infancy. Although thousands of species of microorganisms have
been recognized and named, the number of unknown species is estimated to be in the
millions. Almost every time microbiologists examine a soil sample, they discover a previously
unknown species (Margulis and others, 1997). Revegetation specialists should keep an eye
on developments in this field and see how their plants can benefit from new insights into the
roles of microorganisms. Conserving and utilizing healthy topsoil will also help to sustain the
natural populations of beneficial microorganisms.
10.1.8 TOPOGRAPHIC ENHANCEMENTS
Introduction
Topographic enhancements are alterations to the roadside landscape designed to improve
the growing environment for plants. Topographic enhancements should be considered when
site resources such as topsoil, organic matter, and water are limited (see Section 5.11, Inventory
Site Resources). It is often better to concentrate limited resources in key areas where resources
can be most effective, rather than spread them across the larger project area and dilute them
to the point of having little benefit to reestablishing native vegetation.
Topographic enhancement integrates three components into the roadside design – soil
improvement, site stability, and water harvesting (Figure 10-43). Soil improvement can occur
when limited topsoil and organic matter are strategically used to create growing areas with
optimum rooting depth (see Section 5.3, Available Water Storage and Accessibility, and Section
5.5, Nutrient Cycling). Stable landforms are created that reduce surface erosion and increase slope
stability (see Section 5.6, Surface Stability, and Section 5.7, Slope Stability). Water harvesting
can result when local topography is modified to capture runoff water and concentrate it in
areas where it can be used by plants (see Section 5.2, Water Input) (For background on water
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