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Rhizobium Symbiosis with Woody Plants: Leguminous Nitrogen-Fixing Trees

Soil improvement through vegetation

February 26, 2009 by Douglas Barnes 1 Comment

Key points:

  • 3 subfamilies of the legume family can fix nitrogen
  • Symbiotic bacteria (Rhyzobium) convert nitrogen in the air to a form plants can use
  • Repair damaged land in tropical and arid regions with initial plantings of 90% nitrogen fixers

As mentioned in the previous article in this series, beneficial partnerships are the way of nature. In particular, some microbes (Frankia and Rhizobium) form associations with certain plants allowing them to fix atmospheric nitrogen into a form that plants can use. These symbiotic partners can help us to rehabilitate damaged landscapes, preparing the soil for a succession of more long-term plants.

Rhizobium nodule

This piece focuses on woody plants that associate with the bacteria of the genus Rhizobium. We can see from the diagram below that there are 3 subfamilies of the family Fabaceae (AKA Leguminosae). These families are Faboideae (AKA Papilionoideae), Mimosoideae, and Caesalpinoideae. Note that not all the trees in these subfamilies are nitrogen-fixers. Among the Caesalpinioideae, 23% are nitrogen fixers. For Mimosoideae, the figure is 90%, and for Faboideae, 97% are nitrogen-fixers.

Nitrogen fixers using Rhizobium

As the diagram shows, Mimosoideae contains the nitrogen-fixers Acacia, Albizia, Calliandra, Enterolobium, Leucaena, Mimosa, Paraserianthes, and Pithecellobium. Caesalpinoideae‘s nitrogen-fixers are Chamaecrista, Cordeauxia; and Faboideae has Cajanus, Dalbergia, Erythrina, Flemingia, Gliricidia, Pterocarpus, Robinia, Sesbania, and Tephrosia.

To rapidly revegetate a damaged landscape, be sure to include plenty of these species to help quickly build up the soils. In areas of very problematic soil, such as arid, tropical and subtropical regions, make 90% of your initial planting of trees nitrogen fixing, pioneer species (associating with either Frankia or Rhizobium), and 10% of species your long-term canopy overstory species. When the system reaches maturity, the proportions will be reversed with 10% nitrogen-fixing, support species and 90% canopy species. The same formula could be followed for humid temperate regions, but the soils in these area are not so fragile and can stand a lower percentage of nitrogen fixers. A 70/30 or even lower may suffice in these areas, as the seasonal cycles of death and regrowth feed these soils well.

As the diagram below shows, the nitrogen-fixing support trees can be pruned (coppiced, pollarded, shredded or sacrificed) to provide mulch, fodder, fuel or fibre. As this is done, the roots of the tree self-prune, releasing nitrogen into the soil.

Chop and drop

The highest concentrations of nitrogen are to be found in descending order in the seeds, the seed pods, the flowers, the leaves and then the woody parts of the tree. Inter-planting with fruit or nut trees naturally provides more soil nitrogen. But interplanting also makes the job of chop-and-drop mulching that much easier.

Nitrogen-fixing support trees

Filed Under: Article Tagged With: soil, trees

About Douglas Barnes

I'm a sustainable designer. I live in the countryside in Tweed, Ontario after calling different parts of Ontario and Japan home.

Comments

  1. Karan says

    September 4, 2021 at 10:26 pm

    The amount of nitrogen in the atmosphere is 78%, but despite having such a high amount, atmospheric nitrogen is unusable for plants. Plants completely lack the ability to take nitrogen in gaseous form. Plants take up nitrogen in the form of nitrates (NO3) and nitrites (NO2). The nitrogen present in the form of compounds is called fixed nitrogen.
    https://science-hub.tech/what-is-nitrogen-fixation-how-do-plants-absorb/

    Reply

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