The Effect of Nitrate on Nodulated Soybeans

Soybean (Glycine max) is an exotic leguminous species in Eastern Asia. These annual plant may grow not higher than 20 cm (7.8 inches) and can even stiffly erect up to 2 meters (6.5 feet) in height. Brown or gray pubescence is present in the pods, stems and even in the leaves. Leaves are trifoliate in character that varies in number of leaflets. With 6-15 cm long and 2-7 cm broad of leaflets sometimes 5 leaflets in one stem. The flower found axile in the stem and seeds matured before it falls. Fruits are the hairy pod that grows in cluster attached into the stem. Soybeans contain high amount of complete proteins and fats, thus, remain important source of proteins to people in the Far East and Southeast Asia, and to the rest of the world. A complete protein source contains essential amount of amino acids which the human body needs.  Leguminous species contains Rhizobium bacteria like soybeans in their roots that convert gaseous nitrogen to nitrogenous compounds. Since nitrogen is abundant to the atmosphere only specialized species (leguminous plant) can break nitrogen. And this is the reasons why leguminous species becomes unique and different to there organisms. In addition, they are symbiotic in relationship with plants and other beneficial organisms in nature.

Bacterium (Bradyrhizobium japonicum) is a gram negative, rod-shaped, nitrogen-fixing bacterium which develops a symbiosis with the soybean plant the Glycine max. This bacterium belongs to the family of Rhizobiaceae which includes other nitrogen-fixing bacteria that develops symbiosis with the legumes (Wikipedia, 2006). They live in symbiotic relationship with the legumes. Once they encounter a root of legume they enter the root hairs and travel down a tube to the center of the root hair cell. This species are slower-growing, alkaline producing bacteria which is associated with soybeans. Later on, we will tackle the symbiotic relationship that is present between the soybean and the Bradyrhizobium japonicum.

Biologically, lighting gives way for nitrogen fixation to produced ammonia but only small amount can be produced. And lighting is natural method of nature to break nitrogen that present and abundant in the atmosphere. Industrially, the process of Haber-Bosch produced ammonia which uses iron-based catalyst, high pressure, and relatively amount of temperature.  Dinitrogen fixation is a process of nitrogen fixation using microorganisms in conversion of nitrogen to ammonia. Nowadays, it is a source of major conversion of nitrogen into ammonia.   The arid environment conditions deprived the nitrogen fixation. In arid regions, the major source of nitrogen input in agricultural soil is thru the process of biologic nitrogen fixation. Nitrogen has taken from its relatively inert molecular form which is present in the atmosphere, thus converting into nitrogen compounds that are useful for other chemical processes needed in plants (H. Zahran, 1999).

N2 + 8H + 8e- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi represents the biological equation of nitrogen fixation. From this equation, two moles of ammonia are produced from one mole of nitrogen gas. Ammonia that produced here quickly ionized to ammonium ions. The earth atmosphere contains abundant nitrogen in the form of nitrogen gas. The triple bond between nitrogen atoms makes the molecules inability to react. In order plants to used nitrogen for growth is to fixed nitrogen into ammonium (NH4) and nitrate ions (NO3) forms. Hence nitrogen gas makes biologically useful to plant organisms.

Organisms belongs to the group of prokaryotes has an ability to convert nitrogen to stable nitrogen gas. With nitrogenase complex enzymes, these organisms reduced ammonia from dinitrogen. Nitrogenase complex enzymes, on the other hand, are very sensitive to oxygen and caused them inactive when exposed because of the reactions done between iron proteins. The enzyme consists of two irons proteins and molybdenum. The bounding of nitrogen gas (N2) to the nitrogenase results the reactions of enzymes. By the contribution of electron in ferredoxin, the iron (Fe) protein reduced and then binds to ATP. ATP reduced the molybdenum-iron protein that cause electron of N2, N2 produced HN=NH.  In the further process of ferredoxin requiring two molecules of electron, HN=HN reduced to H2N=H2N resulting to 2NH3 (J. Deacon, The Nitrogen Cycle and Nitrogen Fixation).

There are two examples of nitrogen-fixing organisms. These are (1) free-living nitrogen-fixing organisms which live independently, (2) and other is symbiotic organisms. Such a familiar examples of symbiotic relationship regarding nitrogen-fixation are the root nodule of legumes of leguminous species. The leguminous plants have nodules which bound N-fixing bacteria and easily to multiply. But some plants swelled roots due to infections of these bacteria.  From gaseous nitrogen these bacteria absorbs air from the soil to convert ammonia. With this association of organisms, they are mutually benefited.

Plants give energy to bacteria for enables them to break-down gaseous nitrogen. Afterwards that gaseous nitrogen will be passed in plants to produce proteins and the cycle is called symbiotic nitrogen fixation. However not all legumes has an ability to fix or breaks nitrogen and most of them are different to quantity. There are some factors affecting to them such as (1) kind of legumes, (2) the ability of N-fixing bacteria, (3) pH and fertilizers present in the soil as well as the soil conditions itself, and (4) availability of plant’s food necessities. Plant necessities are carbohydrates, phosphorus, potassium, magnesium, calcium, iron, molybdenum, copper, and boron. But the soil is well-supplied of fertilizers it may be not required N-fixations because of nitrogen availability in the soil. Thus plants no need to encourage bacteria to do the tasks. This in turn will yield in greater nitrogen fixation in the soils that are low in available nitrogen. However, can also increase the root and shoot size of the soybeans.

Nodulation of Soybeans under normal circumstances, don’t need nitrogen fertilizer because of a symbiotic relationship of soybeans with the Rhizobium bacteria which is already found in the soil. We had already discussed that Rhizobium fixes nitrogen from the atmosphere which can be converted to ammonia for consumption of soybean plant. However, despite the presence of nodules in plants they still suffering from nitrogen deficiency. Considering that Rhizobium is living organisms still can be affected naturally by the weather or any physical disturbances. Many liquid and dry nitrogen sources in the environment contain urea which can easily volatilize. Some experts consider ammonium nitrate as a better choice since it won’t be volatilize. However, another problem is those ammonium nitrates are not widely available. Rainfalls still the necessary medium for soil to accumulate the nitrogen which then can be utilized by soybeans.

Inoculation is the process of applications specific amount of nitrogen-fixing bacteria in the soil or seeds before planting. This process provides sufficiently high number of viable nitrogen-fixing bacteria for effective nodulation of the legumes. Inoculations will be effective in four weeks with visual signs of nodules in the plant roots. The color and number of nodules will indicate the stable status of nitrogen fixation legumes in plants. The indications of leghemoglobin or the reddish pink in the nodules are related to the plant capacity of nitrogen fixing.  The nodules in the roots are influenced by mineral nitrogen present in the soil, where the plants are grown.  The process of nodulations affects the amount of available nitrate or ammonia, and level of concentrations in the soil which results mostly in depress nodulations.  Another the N2 fixation in active, growing, and nodulated legumes rate are suppressed by NO3 ions. Generally, soil with high level of nitrogen (applied or residual) affects the nodulations and N2 fixation. In effect the increased in nitrogen application will increase plant growth and grain yield of soybeans, thus, root and shoot size of soybeans also increases.

REFERNCES:

Adjei, M.B., K.H. Quesenberry and C.G. Chambliss. 1999. Nitrogen Fixation and Inoculation of Forage Legumes. University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS).Florida, USA.

Andy King C. and Larry C. Purcell. 2001. Soybean Nodule Size and Relationship to Nitrogen Fixation Response to Water Deficit. Crop Science 41:1099-1107 Hamdi

Deacon, Jim; The Microbial World: the Nitrogen Cycle and Nitrogen Fixation;

http://helios.bto.ed.ac.uk/bto/microbes/nitrogen.htm

Zahran, Hussein;1999; Rhizobium-Legume Symbiosis and Nitrogen Fixation under Severe Conditions and in an Arid Climate, Journal List; Microbiology and Molecular Biology Reviews; V.63(4);  968-989 http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=98982

Wikipedia; Soybeans; November 27, 2006; 17:42;  http://en.wikipedia.org/wiki/Soybean