Introducing Nitrogen Fixing Trees: Nature’s Solution to Curing N2 Deficiency
Nitrogen deficiency is a major challenge to world agriculture. This element is one of the most important nutrients for the growth and survival of plants. Roughly 78% of earth’s atmosphere consists of this gas essential to supporting life. However, plant life is unable to derive vital nutrients from its gaseous form. Instead, plants must pull nitrogen from their soil.
The introduction of chemicals to compensate for nitrogen deficiency has created a host of environmental challenges. The enduring practice of using chemical fertilizers has resulted in extensive ecological damage. Groundwater contamination, eutrophication, acid rain and ozone depletion have been widely recognized as consequences of the extensive use of harmful chemicals in agricultural practices. Birth malformations, hypertension, respiratory ailments, cardiac disease and multiple cancers all have been associated with environmental degradation, caused by the nearly ubiquitous presence of unsafe chemical mixtures used in soil amendments and general cultivation.
Nature offers healthy alternatives to artificial treatments that contaminate the environment. Collaborating with nature as a partner is paying off beyond the balance sheet. Natural solutions are proving efficacious in resolving environmental issues, allowing communities to rely less on toxic remedies.
When it comes to nitrogen deficiency, microorganisms and root nodule trees are two essential allies.
Biological Nitrogen Fixation
Biological Nitrogen Fixation is a natural process where certain bacteria and trees with nodules in their root systems are able to convert the gas into a form that is usable for other plant life. What makes them extraordinary is their unique ability to tackle the sturdy gas molecule and unpack its compounds essential to supporting plant life. Nitrates, nitrogen dioxide and ammonia become transformed into accessible components.
Diazotrophs are one example of the type of bacteria and archaea capable of transforming the atmospheric gas into more usable forms (mainly ammonia). Others such as Azotobacter and Clostridium are non-symbiotic, and fix nitrogen without association to other plants. A different group of bacteria remedies the challenge by establishing symbiotic associations. Rhizobium, for instance, collaborate with legumes, while Frankia work with non-legumes.
Nitrogen fixing Trees
Trees with the capacity to convert the atmospheric gas into usable compounds, such as ammonia, are nitrogen fixing trees (NFTs). A limited number of plants in nature have this rare ability to use atmospheric nitrogen for their own purpose and to add it to the soil. Leguminous plants such as alfalfa and clover (perennials), and beans, peanuts, and soybeans (annuals) are superior fixers. Black Locust, Mimosa, Alder, Redbud, Autumn Olive, Kentucky Coffee Tree, Golden Chain Tree, Acacia, Mesquite and others are examples of trees that support nitrogen in soil with the help of bacteria. These NFTs pull the element out of the atmosphere and build a storehouse of the gas through their nodule root formation.
Permaculture and Nitrogen Fixing Trees
Permaculture adheres to the philosophy of working with nature, rather than against it. NFTs blend seamlessly with this core belief. NFTs provide a tool that promotes living harmoniously with our natural world. By using agricultural techniques learned from observing patterns and features that are embedded in nature, safe and efficient methods of treating nitrogen deficiency are being developed.
For example, the co-cultivation of leguminous NFTs with other plants has been found to be beneficial in Nigeria and other African countries. In developing tropical nations, leguminous trees that are nodulated – including Acacia nilotica, Dalbergia sissoo, Pongiamiaglabra and others – are planted to restore denuded land. Along with improving the fertility of marginal soil, NFTs also moderate the harsh conditions of unvegetated areas, playing a central role as the benevolent pioneer species.
Nodules: the Warehouses of Nitrogen
Bacteria reside within the nodules of NFTs, making the nodes their single most important feature. Nitrogen fixation and ammonia production are the work of these microorganisms that labor intensively inside these knobs of varying size.
Although nodules may be formed on stems or leaves, the most common are found on root systems. The morphology of nodules varies widely based upon the characteristics of the host plant. For instance, soybeans produce nodules that are spherical in shape; clover forms club-shaped nodules; and alfalfa and pigeon pea develop branched nodules.
Nodules also differ in their size and number. Most are small, usually less than 0.5 cm in diameter or length; others are as large as a baseball.
Plants and trees with more nodules tend to have smaller nodes and are less efficient in fixation. Successful nodules are larger in size and demonstrate greater efficacy in fixing higher amounts of nitrogen.
Young nodules are generally white or grey inside. They are incapable of nitrogen fixation. Mature and larger nodules are identifiable by their pink or reddish internal color due to the presence of leghemoglobin, a nitrogen and oxygen carrier. They are the most effective fixers.
Nodulation is the formation of nodules. It is significant because this part of the nitrogen fixation process differs between legumes and non-legumes, creating some important distinctions.
Nodulation in Legumes by Rhizobium
The stages of nodulation in legumes are:
1. Legumes release flavonoids whenever they experience nitrogen deficiency. This sends a signal to the rhizobia about their interest in a symbiotic association.
2. Rhizobia typically live in soil. When exposed to flavonoids, they release a nodulation factor that encourages the plant to create deformed root hairs.
3. To enter the root cells through the deformed root hairs, rhizobia then form an “infection thread.”
4. Rhizobia penetrate the curled root hairs, multiplying within the infection threads.
5. The infection thread reaches the cortical region and divides into multiple branches.
6. Under rhizobial infection, the cortical region shows meristematic growth and young nodules are formed. Small nodules are visible to the naked eye within a week of infection.
7. Finally, mature nodules develop, packed with both advanced and young bacteroids (the misshapen form of the rhizobium bacteria).
Nodulation in Non-Legumes by Frankia
Actinorhizal plants (trees and shrubs) form a symbiotic association with Frankia, the nitrogen fixing actino bacteria, to produce root nodules. Depending upon the host plant, Frankia beneficially infects actinorhizal plants in two different ways: (1) by intracellular infection, or (2) by intercellular infection.
• An intracellular infection starts when Frankia use an unknown signal to stimulate the root hairs to curl.
• Frankia makes its way through the root hairs and infects the cells. When cell divisions occur inside of the cortex, small external protuberances called the prenodules begin to form.
• After the nodule lobe premordium forms, Frankia Hyphae proceed towards the young nodule lobe to create mature nodules.
• Intercellular infection is a nodule formation process that occurs without any root hair deformation or the development of prenodules.
How NFTs Add Nitrogen to the Soil
As nitrogen accumulates in the roots of NFTs, new nodules grow allowing for increased storage capacity. Furthermore, the NFTs collect the nutrient from the air and store it in the nitrogen-friendly warehouses, expanding the size of the nodes.
A large variety of nitrogen fixing legumes can accumulate an estimated 100-300kg of nitrogen per hectare per year. Some higher yielding legumes are capable of contributing up to 500kg N/ha/year.
The process is not without trade-offs for the NFTs. In this symbiotic interaction, the trees provide carbohydrates (succinate and malate) to the rhizobia, obtaining the ammonia needed to form amino acids. The nodules hold the majority of the nitrogen, enabling host plants to grow. However, excess gas accumulates in plant tissues ultimately causing the trees to die.
Still, this process benefits the surrounding soil and neighboring plants. The decomposed trees provide valuable nitrogen enrichment to the soil that later becomes available to future plant growth. Also, using permaculture methods, rather than harmful commercial treatments, serve to protect the integrity of overall environmental health.
Additional NFT Contributions to Soil
• NFTs are reliable food suppliers, producing a significant number of food grains and vegetables. They are also the mulch banks for home gardens.
• NFTs play a vital role as a pioneer species in harsh, arid and infertile regions. Through their ability to moderate environmental harshness and restore fertility, they create land that is cultivable and productive.
• Their constant addition of organic matter to the soil reduces the demand for fertilizers.
• With their extensive root system, they improve the soil structure and prevent erosion.
• Both growing and atrophying roots create many channels that enhance healthy soil aeration.
• NFTs provide fodders, fuel wood, wind protection, living fences and timber.
• Some NFTs offer microclimates for crops that require shade.
• NFTs can also serve as a trellis for vine crops.
How to Introduce NFTs into a System
Alley cropping, contour hedgerows, clump plantings, shelter belts and single distribution plantings are some practices commonly used to integrate NFTs into a system. The variety should be selected with extreme care to avoid unwanted invasion by NFTs.
The following prerequisites must be satisfied before and during the planting of NFTs:
• NFTs offer a wide variation of characteristics. A number of NFT species are small, while many NFTs are large with a rapid growth rate. Some produce edible shoots and pods and are well suited for home gardens. Others are very useful for providing fuel wood or poles. Accordingly, it is paramount to survey both a region’s particular environmental characteristics as well as its population’s needs before deciding which species to plant.
• Cutting the seed coat using abrasion, thermal stress, or chemicals to stimulate germination is known as scarification. Hard seed coats of many NFTs impede the germination of the seeds. To avoid this problem, scarification using hot water is widely used. Seeds are placed into water with a temperature of 70-90°C (160°F) for one to three minutes and rinsed afterwards.
• Sowing the seeds of NFTs requires inoculation. Just a small amount of vegetable oil or pure water is applied to the seeds allowing inoculums to be dusted afterwards. Note: Once seeds are inoculated, they become sensitive to high temperature and sunlight, requiring prompt sowing.
• Stump cuttings, branch cuttings or bare root seedlings can be used to grow NFTs, but must be kept moist and uninjured until planting. Stump cuttings require scarification with a sharp tool to promote rooting.
• Prepare a tiny hole in the soil that is approximately the same diameter as the plant material. Seedlings should be positioned so that their root/shoot collar is at ground level.
• Soil amendments and the application of mulch at planting facilitate the growth of healthy young plant by ensuring they receive the required amount of moisture and nutrients. These practices also protect seedlings from weed competition.
Proceed With Care
NFTs are not always the best solution. The introduction of a single non-native NFT species can bring the total collapse of an invasion-resistant ecosystem, paving the way for other invaders. Because they are durable of growing even in the most unfavorable conditions, they have the potential to become an invasive species, negatively impacting local primary crops. Additionally, an excess number of NFTs could over nitrify the soil and pollute surface and ground water. To avoid this unwanted invasion, only those species that are known for their compatibility with local conditions should be considered.
All images are from Zaytuna Farm, taken by Ingrid Pullen.