07 Aug2000

Overstory #65 - Biological Nitrogen Fixation

Written by Kim M. Wilkinson and Craig R. Elevitch .

Editor's Note

Biological nitrogen fixation is an important part of many agroforestry, sustainable agriculture, and land rehabilitation practices. Although the terms "nitrogen fixing plants" and "nitrogen fixing trees (NFTs)" are widely used, the plants themselves do not have the ability to make use of the nitrogen gas in the air-- it is only through the symbiotic association with rhizobia bacteria that the process takes place. Simply planting leguminous "nitrogen fixing" plants or trees will not ensure that nitrogen will be accumulated; the process of biological nitrogen fixation depends on the presence of the correct rhizobia bacteria. This edition of The Overstory introduces how biological nitrogen fixation works, why inoculation is advantageous, and how to use rhizobia inoculants.

Biological Nitrogen Fixation

Nitrogen is commonly the most limiting element in agricultural production, and one of the most expensive to purchase as fertilizer (NifTAL 1984). There is an abundant supply of nitrogen in the air (the air is 80% nitrogen gas, amounting to about 8000 pounds of nitrogen in the air over every acre of land, or 6400 kilograms above every hectare). However, the nitrogen in the air is a stable gas, normally unavailable to plants. Many leguminous plants are able to utilize this atmospheric nitrogen through an association with rhizobia, bacteria which are hosted by the root system of certain nitrogen fixing plants.

Rhizobia are able to convert the nitrogen gas in the atmosphere into amino acids, which are the building blocks of proteins. The legume is then able to use this for its nitrogen needs. Rhizobia exchange nitrogen for carbohydrates from the plant. As the plants drop organic matter, or when the plants die, the nitrogen from their tissues is made available to other plants and organisms. This process of accumulating atmospheric nitrogen in plants and recycling it through organic matter is the major source of nitrogen in tropical ecosystems. Various agroforestry practices such as alley cropping, improved fallow, and green manure/cover cropping exploit this natural fertility process by using nitrogen fixing plants.

Examples of Nitrogen Fixing Plants

Examples of tree and shrub genera and that form this relationship include: Acacia, Leucaena, Gliricidia, Erythrina, Sesbania, Inga, Dalbergia, Cajanus, and Albizia. Ground cover or annual crops crops include: Crotalaria species, Mucuna pruriens (velvet bean), Dolichos lablab (lablab, hyacinth bean), Canavalia species (jack or sword beans), and Arachis pintoi (perennial peanut).

Why Use Rhizobia Inoculants?

There are many species of rhizobia, and each will work only with certain legumes. Likewise, each legume species will only associate with the proper rhizobia. Unless the strain of rhizobia suited to the legume species is present in the soil, no biological nitrogen fixation will take place. In some cases, one Rhizobium strain may provide some nitrogen fixation, but will be less effective than another.

Rhizobia inoculants are live bacteria cultures that are applied to seeds or young plants, infecting their root systems with the beneficial bacteria. A tremendous amount of research has been done over the past few decades to match plant and rhizobia species not only for compatibility, but also to maximize nitrogen fixation and enhance growth. Using inoculants ensures that the plants receive the best Rhizobium strain for optimum effectiveness.

Whether an uninoculated plant will spontaneously associate with rhizobia in the field depends on several factors:

If the correct rhizobia for that plant species is present in the soil;
If the rhizobia are available in sufficient quantity to infect roots;
If the rhizobia in the soil are healthy and still able to fix nitrogen (researchers have found that over a period of years, rhizobia in the soil can and do lose the ability to fix nitrogen) (Keyser 2000).

However, even if the correct rhizobia is present in the soil, its nitrogen fixing effectiveness (compared to the recommended and tested inoculant strain applied in the greenhouse prior to transplanting) will be unknown—the soil population may consist of mediocre strains. Also, if the bacteria is present in the soil, it may take several weeks, months or even years for the association to develop, and this delay will cost the farmer unnecessary time and money in early maintenance.

Using rhizobia inoculants ensures that the correct rhizobia bacteria associate with the plant, and also that the association forms early in the plant's life to accelerate its early growth and establishment.

How Is Inoculant Applied?

Inoculation (infecting the plant roots with the rhizobia) should take place as early in the plant's life as possible, when the plant will most readily form the association. Rhizobia inoculant can be applied to seeds or to young plants. Usually, Rhizobium bacteria come in a peat-based inoculant, with billions (10^9) of cells per gram. The inoculant is usually coated onto the seed immediately before plating, or dispersed in clean water and soaked into the planting medium. One hundred grams of inoculant is usually sufficient for 20,000-100,000 seeds. The cost is very small per plant inoculated--a few dollars worth of inoculant can replace a hundred or more dollars worth of nitrogen fertilizer over the life of the plant.

How Can the Effectiveness of Inoculation Be Verified?

When rhizobia are present and nitrogen is being fixed, nodules can be seen on the roots of the plants. Each one of the nodules houses millions of rhizobia bacteria. When a nodule is opened, a pink or red color inside is usually a good indicator that it is active, that biological nitrogen fixation is taking place. Thus the effectiveness of inoculation can be verified easily with the naked eye. The rhizobia will survive and multiply as the plant grows.

Conclusion

Biological nitrogen fixation in legumes depends on the presence of the correct rhizobia bacteria for the plant species. Inoculating with rhizobia ensures that nitrogen fixing plants form the necessary association to be able to fix nitrogen. Using rhizobia inoculants can be a key part of accelerating rehabilitation of degraded land and ecosystem function, enhancing survival and growth of plants, and reducing costs in establishment and maintenance. A few dollars worth of inoculant can replace hundreds of dollars worth of purchased nitrogen fertilizer over the life of the plant, and return organic matter and nitrogen to the farm ecosystem naturally.

References

Keyser, Harold. 2000. Personal communication. University of Hawaii NifTAL Project, 1000 Holomua Road, Paia, HI 96779-9744 USA.

Nitrogen Fixation for Tropical Agricultural Legumes (NifTAL) and Food and Agriculture Organization of the United Nations. 1984. Legume Inoculants and Their Use. Rome, Italy.

Acknowledgements

The authors express appreciation to Harold Keyser, Paul Singleton, Padma Somasegaran, Joe Rourke, and Bruce Martin of the NifTAL Project (Nitrogen Fixation by Tropical Agricultural Legumes) of the University of Hawaii, Paia, Hawaii, for their support and training in biological nitrogen fixation and rhizobia inoculants. NifTAL Project, 1000 Holomua Road, Paia, HI 96779-9744 USA. Thanks also to Mitiku Habte of the University of Hawaii, Department of Agronomy and Soil Science for his input on this topic.