(a) In the global nitrogen cycle, bacterial denitrification is recognised as important process that converts fixed nitrogen to atmospheric nitrogen gas, .
In marine nitrogen cycle, anaerobic oxidation of ammonium coupled to nitrate reduction, contributes substantially to -production in marine sediments.
(a) Ammonia is the first stable compound produced in fixation of atmospheric nitrogen in leguminous plant.
A molecule of nitrogen is changed into two molecules of ammonia.
It requires , a source of electron donor ferredoxin, hydrogen donor NADPH or FMNH2 and dinitrogenase enzyme.
(d) Thiobacillus denitrificans causes denitrification i.e., conversion of oxides of nitrogen to free .
(d) Thiobacillus denitrificans, a type of denitrifiers, converts nitrates in soil to free atmospheric nitrogen, thus depleting soil fertility and reducing agricultural productivity.
Thiobacillus is a genus of Gram-negative Betaproteobacteria.
Thiobacillus oxidizes thiosulfate and elemental sulphur to sulphate, and Acidithiobacillus ferrooxidans oxidizes ferrous ions to the ferric form.
This diverse oxidizing ability allows A. ferrooxidans to tolerate high concentrations of many different ions, including iron, copper, cobalt, nickel, and zinc.
(c) Ammonia and Hydrogen.
The steps involved in the process of nitrogen fixation are as follows: Rhizobia multiply and colonise the surroundings of roots and get attached to epidermal and root hair cells.
The root-hairs curl and the bacteria invade the root-hair.
An infection thread is produced carrying the bacteria into the cortex of the root, where they initiate the nodule formation in the cortex of the root.
Then the bacteria are released from the thread into the cells which leads to the differentiation of specialised nitrogen fixing cells.
The nodule thus formed, establishes a direct vascular connection with the host for exchange of nutrients.
The nodule contains all the necessary biochemical components, such as the enzyme nitrogenase and leghaemoglobin.
The enzyme nitrogenase is a Mo-Fe protein and catalyses the conversion of atmospheric nitrogen to ammonia, the first stable product of nitrogen fixation.
The enzyme nitrogenase is highly sensitive to the molecular oxygen; it requires anaerobic conditions.
The nodules have adaptations that ensure that the enzyme is protected from oxygen.
To protect these enzymes, the nodule contains an oxygen scavenger called leg-haemoglobin.
(c) Deficiency of magnesium produces chlorosis due to breakdown of chlorophyll.
Chlorosis is a condition in which plant foliage produces insufficient chlorophyll.
When this happens, leaves do not have their normal green colour; they may be pale green, yellow, or yellow-white.
The affected plant has little or no ability to manufacture carbohydrates and may die unless the cause of its chlorophyll insufficiency is treated.
Specific nutrient deficiencies (often aggravated by high ) cause chlorosis, which may be corrected by supplemental feedings of iron, magnesium or nitrogen compounds in various combinations.
Some pesticides, particularly herbicides, may also cause chlorosis, both to target weeds and occasionally to the crop being treated.
(d) Grey spots of oat are caused by deficiency of Manganese.
In oats, deficiency produces a condition called grey speck which occurs in patches.
Oats become pale green and young leaves have spots or lesions of grey/brown necrotic tissue with orange margins.
These lesions will coalesce under severe Mn deficient conditions.
Plants are weak, stunted, floppy and pale green-yellow and appear water-stressed even when adequate soil moisture is available.
Close examination of the leaf may show slight interveinal chlorosis.
The distinction between green veins and yellow interveinal areas is poor.
Symptoms can be confused with red leather leaf, which is favoured by prevailing high humidity in high rainfall areas.
Symptoms can also be mistaken for take-all.
(a) The parts of the plants that show the deficiency symptoms also depend on the mobility of the element in the plant.
For element that are actively mobilised within the plants and exported to young developing tissues, the deficiency symptoms tend to appear first in the older tissues.
For example, nitrogen, potassium and magnesium are visible first in the senescent leaves.
(c) Potassium immobilisation is the conversion of water soluble potassium into an insoluble form.
Readily available potassium constitutes about of total potassium in soil, whereas slightly soluble potassium accounts for about .
(a) The best defined function of manganese is in the splitting of water to liberate oxygen during photosynthesis.
It is absorbed in the form of manganese ions .
It activates many enzymes involved in photosynthesis, respiration and nitrogen metabolism.
