The nitrogen (N) is present in atmosphere. Therefore, plants are exposed to the impact of the nitrogen. However, the nitrogen does not impact the plants directly, because it is highly stable as it is triple bonded (NN). At the same time, the nitrogen is an important component of many structural, genetic and metabolic compounds in plant cells.
The nitrogen contained in the soil may have three forms, including organic nitrogen compounds, ammonium (NH4+) ions, and nitrate (NO3-) ions. A very small amount of organic nitrogen is contained in soluble organic compounds, including urea, that may be slightly available to plants. The larger part of plant-available nitrogen is contained in the inorganic NH4+ and NO3-forms. Ammonium ions bind to the soil’s negatively-charged cation exchange complex (CEC) and behave much like other cations in the soil. Nitrate ions do not bind to the soil solids because they carry negative charges, but exist dissolved in the soil water, or precipitated as soluble salts under dry conditions.
Atmospheric nitrogen is the major source of nitrogen in soils. N2 existing in atmosphere is converted into useful form, which is used by the plant. To complete this process effectively, some bacteria are needed. These bacteria should be able to use the enzyme nitrogenase to break the bonds among the molecular nitrogen and combine it with hydrogen.
Nitrogenase can function only on the condition of the absence of oxygen in the atmosphere. Rhizobium exists in oxygen-free zones in nodules on the roots of legumes and some other woody plants.
Legume -water relations
Specialists recommend legume cultivation using legume-water relations carefully. Normally specialists recommend that plants should be cultivated in a split plot design with 3 replicate and two water treatments (weekly irrigation, and under stress with same amount per 15 days interval) (Keshavarzi., 2010).. In such a case, the water entrance is measured by Partial flow. The measurements should be done on the regular basis once a week. After that plants should be gathered and weighed separately for each vegetative parts. An Area Meter (Vista) should be used to measure root surface and leaf area. In addition it is necessary to measure the total shoot and root fresh and dry weight, leaf area index and soil coverage. (Keshavarzi., 2010).
The legume-temperature relationships should be measured when they are five weeks past-inoculation. Specialists recommend using the heat stress to the roots for 2, 4 or 6 h (2 h daily) at 20°C, 30°C and 40°C in a water bath. The pots should be covered by a plastic lid with two holes, one for the plant shoot and the other for injecting and withdrawing gas samples. The pots should be sealed with electrician water-proofing compound (Centaure MFG) and transferred to the water bath for the heat stress treatment. At the beginning of the experiment, the heat stress should be measured every 30 min after transfer to the water bath. The researcher should measure the temperatures inside the pot with a thermocouple probe inserted into the vermiculite. After heat treatment, the pots should be removed from the water bath and nitrogen fixation should be measured. The acetylene reduction method should be used to determine the nitrogen fixation rate (Keerio, 2001). Nitrogen fixation should be measured every 10, 20 and 30 min after injecting the acetylene. The experiment should involve three replicates for each treatment. The rooting system will be removed from the pot and washed carefully after each nitrogen fixation. On the ground of the results obtained, the changes in plants should be revealed along with the impact of nitrogen fixation on the plants.
In such a way, legume have a wide application in farming. However, scientific studies on the effective use of legume and the application of nitrogen and phosphorus are very important for the effective farming. In this regard, the development of modern science and agriculture has already reached a tremendous progress but still further experiments and studies may help to reveal new ways to the effective use of legumes, nitrogen and phosphorus.
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