The Big List of Nitrogen Fixing Plants including Australian Natives

Companion planting is the practice of planting specific plants close to others to provide them with some benefit, such as the control of pests, increased health and vigour, resistance to disease, or higher yields. Plants can also be used to add fertility to the soil.

Nitrogen fixing plants have root nodules which house the symbiotic Rhizobium nitrogen-fixing bacteria, which can take nitrogen from the air and convert it to a form usable by plants as fertiliser, helping the plants grow in poor soils.

Ecologically, this allows the plants to grow in environments with poor soils that will not support other plants, enrich the soil, and create microclimates that will support less resilient plants.

The Nitrogen Fixing Process in Plants

Nitrogen is an important plant macronutrient that is required for leafy green vegetative growth. The nutrient nitrogen is used to synthesise (produce) proteins, nucleic acids and other nitrogen-containing compounds.

Even though nitrogen (N₂) is the most abundant naturally occurring gas in earth’s atmosphere, which makes up around 78% of the air, it can’t be used in this form by most living organisms until it has been fixed, that is reduced (combined with hydrogen), to form ammonia (NH₃).

The reduction of nitrogen gas to ammonia is energy intensive and is facilitated by a specialized energy molecule known as adenosine triphosphate or ATP. It requires 16 molecules of ATP and specific enzymes (nitrogenase) produced by bacteria to break the nitrogen bonds between the nitrogen atoms to combine them with hydrogen.

Nitrogenase is a complex enzyme consisting of two key components, dinitrogenase reductase (Fe protein) and dinitrogenase (MoFe protein), which work together to reduce atmospheric nitrogen and convert it into ammonia.

The chemical reaction can be written as:

N₂ + 3H₂ —energy, nitrogenase–> 2NH₃

This nitrogen fixing process in the root nodules helps the plant to grow, and when the plants drop their leaves, or when the roots die back (when the foliage is pruned), usable nitrogen is returned to the soil.

Nitrogen fixing annual plants can also be grown as green manures, then cut down as soon as they begin to flower so they release all their nitrogen into the soil. If they are not cut down, the nitrogen goes into their beans, peas, etc, contributing to their high protein content.

Which Plants Can Fix Nitrogen?

The most common nitrogen fixing plants are the plants and trees from the family Fabaceae (bean and pea), which includes legumes such as alfalfa, beans, clovers, lupines, peanuts, peas, and leguminous trees, such as tagasaste (tree lucerne) and acacias.

Plants from the Fabaceae (legume, bean & pea) family host symbiotic nitrogen fixing Rhizobium bacteria in their root nodules.

Note, there are some members of the Fabaceae family which do no fix nitrogen, such as carob and honey locust trees.

Conversely, some non-leguminous species have the ability to fix nitrogen, such as alder, bayberry, casuarina, elaeagnus as well as the tiny floating aquatic fern azolla (fairy moss).

The aquatic fern Azolla forms a symbiotic relationship with the cyanobacterium Anabaena azollae which fixes atmospheric nitrogen.

Interestingly, as an aside, Azolla has been used as a companion plant and to control mosquito larvae in rice fields and has a long history of being used as a as a rice biofertilizer. It is also used in bioremediation to remove chromium, nickel, copper, zinc, and lead from effluent. Quite an amazing fast-multiplying little plant.

How Nitrogen Stored in Plants is Released Back into the Soil

In perennial legumes around 75­-80% of the plant’s nitrogen content estimated to be stored in the top growth, the rest in the roots.

Fixed nitrogen is released by decomposition of decaying roots, root nodules, crowns, stems and leaves.

We can ‘chop & drop’ plants into soil when they start flowering, before nitrogen is used to form pods, or cut and incorporate (dig) into the soil after harvest. Perennials can be pruned and prunings buried.

With ‘chop & drop’ technique, cut plants at soil level, leaving roots to break down in the soil, then lay cut plant on soil surface (chopped/mulched or whole), and cover with mulch or bury.

For more information on this topic, see article – What is Chop and Drop Gardening (Sheet Composting)?

Nitrogen Fixing Legumes for Various Climates

Edibles legume crops such as peas (garden, snow and sugarsnap), alfalfa (lucerne), broad beans (fava beans), common beans (bush beans, climbing beans), mung beans, soybeans, perennial beans (7-year beans, scarlet runners, climbing lima beans), peanuts, chickpeas, lentils.

Nitrogen fixing legume crops such as alfalfa/lucerne (Medicago sativa), birdsfoot trefoil (Lotus corniculatus), clover (Trifolium species, such as crimson, red, white, rose, subterranean), cowpea (Vigna unguiculata), fenugreek (Trigonella foenum-graecum), gorse (Ulex europaeus), hairy vetch (aka woolly pod vetch, Vicia villosa), lablab (Lablab purpureus), lespedezas (aka Japanese clovers), liquorice (Glycyrrhiza glabra), lupin (Lupinus species), medics, perennial peanut (Arachis glabrata), pigeon pea (Cajanus cajan), pinto peanut (Arachis pintoi), scotch broom (Cytisus scoparius), wynn’s cassia (Cassia rotundifolia).

Australian Native Nitrogen Fixing Legumes

The Fabaceae (legume) family can be divided into three subfamilies:

• Sub-family Faboideae – typical pea-flowered plants
• Sub-family Mimosoideae – Acacias & related plants
• Sub-family Caesalpinioideae – Senna, Cassia & related plants

Australian Native Pea-Flowered Legumes

The sub-family Faboideae have characteristics pea-like flowers, comprised of 5 irregularly shaped petals, with the largest lobed petal (the standard or banner) uppermost, two identical petals on either side (the wings), and two centre petals fused at the upper edges forming a boat-shaped structure (the keel), which encloses 10 pollen-producing stamens.

The flowers form legume pods which contain hard seeds varying in number and shape depending on species.

In Australia there are about 1,100 species in the sub-family Faboideae that inhabit every state and location, from coastal to alpine areas, and tropics to arid climates.

Sub-family Faboideae – pea-flowered plants, which includes Aotus, Bossiaea, Castanospermum, Chorizema, Crotalaria, Daviesia, Dillwynia, Gastrolobium, Glycine, Gompholobium, Goodia, Hardenbergia, Hovea, Indigofera, Jacksonia, Kennedia, Oxylobium, Phyllota, Platylobium, Podolobium, Pultenaea, Swainsona, Templetonia

Sub-family Mimosoideae – Acacias & related plants, which includes the Wattle species, ranging in size from small shrubs to large trees

Sub-family Caesalpinioideae – Senna, Cassia & related plants, ranging in size from small shrubs to large trees

Non-Leguminous Nitrogen Fixing Plants

Some nitrogen fixing plants are not from the Fabaceae (legume) family.

• Lacy phacelia (Phacelia tanacetifolia) from the Boraginaceae (borage and forget-me-not family), is an excellent bee-forage.
• Elaeagnus (silverberries), Hippophae (sea-buckthorns) and Shepherdia (buffaloberries) from the Elaeagnaceae (oleaster) family. The ebbing’s silverberry (Elaeagnus x ebbingei) is a popular fruiting permaculture hedging plant that can grow anywhere, sun or shade.
• Ceanothus, Colletia, Discaria, Kentrothamnus, Retanilla, Talguenea, Trevoa from the Rhamnaceae (buckthorn) family.
• Cercocarpus (mountain mahoganies), Chamaebatia (mountain miseries), Dryas, Purshia/Cowania (bitterbrushes/cliffroses) from the Rosaceae (rose) family.
• Myrica (babyberries) from the Myricaceae family, includes edible Chinese bayberry (Myrica rubra).
• Allocasuarina and Casuarina (she-oaks), Australian trees from the Casuarinaceae family.
• Azolla (fairy moss) a genus of seven species of tiny floating aquatic ferns from the Salviniaceae family.
• Alder trees from the genus Alnus in the Betulaceae (birch) family have a symbiotic relationship with the actinomycete Frankia alni, a filamentous, nitrogen-fixing microorganism, which resides in root nodules of the tree.

Diversity of Symbiotic Nitrogen Fixation

Several types of bacteria are known to have become nitrogen-fixing symbionts (symbiotic organisms that live in association with other organisms), and these include:

• Rhizobia – a group of soil bacteria that form a symbiotic association with legumes (plants from the family Fabaceae). Thay are usually rod-shaped, flagellated (have a whip-like appendages called flagella which provides a means of propulsion) and motile (capable of motion), but change to a different shape when inside their host.
• Actinobacteria – a group of filamentous spore-forming bacteria belonging to the order Actinomycetales, which share the characteristics of both bacteria and fungi, and are widely distributed in both terrestrial and aquatic ecosystems, mainly in soil, where they play an essential role in decomposing organic matter.
• Cyanobacteria – (also known as blue-green algae) are algae-like bacteria which inhabit freshwater, coastal and marine waters, are able to photosynthesise like plants and have similar requirements for sunlight, nutrients and carbon dioxide to grow and produce oxygen.

Even though the rhizobia bacteria are associated with legumes (plants from the Fabaceae family), plants from the genus Parasponia, which belong to the Cannabaceae (hemp) family are the only non-legume host plant known to be nodulated by rhizobia.

Actinobacteria colonize the roots of several plant species of the order Fagales, which includes the Betulaceae (birch) family, Casuarinaceae (she-oak) family and Myricaceae (bayberry) family, in a similar way to the rhizobia.

Cyanobacteria can associate in symbioses with a wide range of plants, which include species from various groups such as the liverworts, hornworts, mosses, ferns, gymnosperms (conifers, cycads and gingko), and angiosperms (flowering trees and plants). The only cyanobacteria involved in these associations belong primarily to the Nostocaceae family.

The Nostocaceae interact with various types of plants in different ways.

• The mosses associate only epiphytically (growing on the surface another plant but not feeding from it) with the cyanobacteria which still fix nitrogen for their hosts.

In other associations, the cyanobacteria are kept in specific plant structures:

• In the liverwort Blasia pusilla, the cyanobacteria colonize specific cavities on the upper side of the liverwort’s thallus (body).
• The hornwort Anthoceros punctatus forms the cavities on the underside of its thallus for the the cyanobacteria to inhabit.
• The water fern Azolla hosts the cyanobacteria in mucilage-filled leaf cavities.

• Cycads keep the cyanobacteria between cells, in mucilage-filled zones in coralloid root (a highly branched roots system that contains symbiotic nitrogen-fixing cyanobacteria).

• In plants from the genus Gunnera, of which one of the most familiar species is the Gunnera manicata (Giant Rhubarb), the Nostoc species of cyanobacteria is attracted to a mucilage-filled tract and comes to reside within specially developing cells, surrounded by the host’s plasma membrane.

References

• de Vries, S., de Vries, J. (2018). Azolla: A Model System for Symbiotic Nitrogen Fixation and Evolutionary Developmental Biology. In: Fernández, H. (eds) Current Advances in Fern Research. Springer, Cham. <https://doi.org/10.1007/978-3-319-75103-0_2>
• Society for General Microbiology, Rhizobium, Root Nodules & Nitrogen Fixation. <http://labs.bio.unc.edu/Vision/pmabs/rhizobium.activity2.pdf>