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GROUP:2
M.PHIL BOTANY
1ST SEMESTER
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Muhammad Shoaib Amjad Humaira Amin
Parveen Akhtar Shadana
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COURSE CODE: BIOL-735TOPIC: BIOLOGICAL
NITROGENFIXATION,PERSPECTIVES
AND LIMITATION
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DEFINITION
Biological nitrogen fixationis the process whereatmospheric nitrogen isreduced to amonia in thepresence of nitrogenaseenzyme with the help of
microorganisms
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Biological Nitrogen Fixation and SustainableAgriculture
Nitrogen fixing micro-organisms could be an importantcomponent of sustainable agricultural systems.
There are several significant reasons to seek alternativesto fertilisers that provide
chemically fixed nitrogen:
Environmental
EnergySustainability
Nutrition
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Environmental
Nitrogen fertilisers affect the balanceof the global nitrogen cycle, and may
pollute groundwater, increase the riskof chemical spills, and increaseatmospheric nitrous oxide (N2O), a
potent greenhouse gas.
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Energy
The primary energy source for themanufacture of nitrogen fertiliser is
natural gas together with petroleumand coal. On the contrary, the energyrequirements of BNF are met by
renewable sources such as plant-synthesised carbohydrates ratherthan from nonrenewable fossil fuels.
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Sustainability
Long- term sustainability of agriculturalsystems must rely on the use and
effective management of internalresources. The process of BNF offers aneconomically attractive and ecologically
sound means of reducing externalnitrogen input and improving the qualityand quantity of internal resources.
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Nutrition
It is estimated that about 20% of foodprotein worldwide is derived from
legumes. There are more than 13,000described species of legumes and for only3,000 species examined more than 90%were found to form root nodules. Because
few have been exploited for food, there isthe prospect that the utilisation oflegumes could be expanded substantially.
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Conti.
It is anticipated that increasingdemographic pressure and fooddemand will require theexploitation of BNF as a major
source nitrogen for plant proteinproduction.
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Factors, affecting BNF:Management decisions
Environmental factors affecting nitrogen fixation include:
Temperature
Moisture
acidity
several chemical components of the soil such asnitrogen, phosphorus, calcium and molybdenum content
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Conti..
It is often difficult to isolate the effect of theabove factors on inoculation success from theirinfluence on symbiosis and nitrogen fixation.
For example:acidity, as well as, calcium, aluminium and
manganese concentrations will interact andaffect both bacterial proliferation, root-hairinfection and plant growth.
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Conti.
In addition to the competitiveness of the
rhizobia in forming nodules and theeffectiveness of the rhizobium-host plantto fix N2, a series of edaphic, chemical
and biophysical factors exert a control onN2 fixation.
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Management practices like the intensity of tillageor intercropping practices will alter thoseedaphic, chemical and biophysical factors andtherefore influence BNF indirectly are:
Temperature & moistureEffect:
survival, of rhizobia in soil abilities to nodulate and fix nitrogen inhibition of nitrogen fixation
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Recommendations
Placement of inoculum in deeper soillayers when top soil temperature are high
The surface mulches may conservemoisture and reduce soil temperature
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Nitrogen FertilisationEffect:Generally combined N delays or inhibits
nodulation and nitrogen fixation. Because of thisadverse effect, N fertilisation usually is notrecommended for leguminous crops.
However there may be situations where N has to
be applied, such as to cereals in mixed croppingor rotations and then fertiliser may affectnitrogen fixation of the legume crop.
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Recommendations
The development of grain legumes which areless sensitive to mineral N should not be
pursued unless there is increase in N-uptake andan improvement in the overall use efficiency ofavailable N.
It is possible to apply small amounts of soil orfoliar N fertiliser, which may increase yieldwithout reducing the amount of nitrogen fixed.
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Pesticides, fungicides andinsecticides
Effects:
The compatibility of rhizobia with pesticides ispoorly understood except for Fungicides.
Insecticides have little adverse effect onnodulation when not directly applied on seed.
The effect of herbicides on rhizobial survival isunknown.
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Recommendations
Due to the variability of the effect it isrecommended to test the particularRhizobium inoculum and its behaviour in
respect to the product to be used, beforeapplication
The effect of pesticide on N fixationshould be minimised by separateplacement of rhizobia and pesticide
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Acid soils
Effects:
Acid soils constrain agricultural production and nitrogenfixation.
Recommendations:
Use of acid-tolerant legume cultivars and rhizobium
Soil liming should be limited to achieving a pH at whichavailable aluminium or manganese levels are no longertoxic.
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Tillage
Effects:
When tillage is minimised, lower rates of
mineralization and nitrification, coupledwith increased N immobilisation and ahigher potential for denitrification will lead
to a decrease in available N.
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Recommendations
Limiting tillage can stimulate N demandand N2 fixation.
Conservation and zero tillagemanagement practices will, therefore, leadto a stimulation of N2 fixation, at least
until a new equilibrium between residueinput and the rate of decomposition isreached.
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