Nitrogen fixer

Although approximately 80 percent of the atmosphere is made of nitrogen gas (N2), plants and animals alike can starve of nitrogen if it is not converted into the ammonia (NH3) that all organisms use for amino acids, proteins, and nucleic acids.^[1] Nitrogen fixing plants are able to form a relationship with Rhizobia or Frankia bacteria by producing nodes in their root systems. With the help of bacteria they are then able to transform atmospheric N2 into usable NH3. These plants feed themselves nitrogen and give themselves a competitive edge, and, when they die, NH3 nitrogen is released enriching the soil.^[2] Nitrogen is also released in the leaf fall of nitrogen fixers and into the soil through relationships with microrhizal funghi.^[3]

Nitrogen Fixers

Nitrogen fixation is a property most commonly found in the legume family, Fabaceae. Legume taxa that fix nitrogen include clovers, soybeans, alfalfa, peanuts and roobois. Not all legumes are nitrogen fixers, the lugume genera Styphnolobium do not, for example. Examples of non-legume fixers include Parasponia, a tropical Celtidaceae and plants that form Actinorhizal funghus relationships such as alder and bayberry. These plants belong to 25 genera[9] distributed among 8 plant families. The ability to fix nitrogen is far from universally present in these families. For instance, of 122 genus in the Rosaceae, only 4 genera are capable of fixing nitrogen. All these families belong to the orders Cucurbitales, Fagales, and Rosales, which together with the Fabales form a clade of eurosids. In this clade, Fabales were the first lineage to branch off; thus, the ability to fix nitrogen may be plesiomorphic and subsequently lost in most descendants of the original nitrogen-fixing plant; however, it may be that the basic genetic and physiological requirements were present in an incipient state in the last common ancestors of all these plants, but only evolved to full function in some of them.^[4]

Uses of Nitrogen Fixers

Organic Gardening

Organic garderners, following on from traditional practices, grow nitrogen fixing plants like clover or alfalfa as a green ley crop in a rotation system to restore fertility. Mineral rich fixers like comfrey are also grown and cut down for use as a nitrogen rich mulch material.^[5]

Agro Forestry

Nitrogen fixing trees may be integrated into a system in many different ways including clump plantings, alley cropping, contour hedgerows, shelter belts, or single distribution plantings. In addition to providing nitrogen to crops around they can serve many functions: microclimate for shade-loving crops like coffee or citrus (cut back seasonally to encourage fruiting); trellis for vine crops like vanilla, pepper, and yam; mulch banks for home gardens; and living fence and fodder sources around animal fields.^[6]

A-Z of plants which function as a Nitrogen fixer