THE NITROGEN CYCLE
Nitrates are essential for plant growth Plant protein Root uptake Nitrate NO 3
Nitrates are recycled via microbes Animal protein Soil organic nitrogen Ammonification Ammonium NH 4 + Plant protein Root uptake Nitrification Nitrite NO 2 Nitrification Nitrate NO 3
Ammonification n Nitrogen enters the soil through the decomposition of protein in dead organic matter Amino acids + 1 1 / 2 O 2 CO 2 + H 2 O + NH 3 + 736kJ n This process liberates a lot of energy which can be used by the saprotrophic microbes
Nitrification n This involves two oxidation processes n The ammonia produced by ammonification is an energy rich substrate for Nitrosomas bacteria They oxidise it to nitrite: NH 3 + 1 1 / 2 O 2 NO 2 + H 2 O + 276kJ This in turn provides a substrate for Nitrobacter bacteria oxidise the nitrite to nitrate: NO 3 + 1 / 2 O 2 NO 3 + 73 kj n This energy is the only source of energy for these prokaryotes n They are chemoautotrophs
Nitrogen from the atmosphere Out gassin g Atmospheric fixation Atmospheric Nitrogen 4 000 000 000 Gt Biological fixation Soil organic nitrogen Plant protein Root uptake Nitrate NO 3
Atmospheric nitrogen fixation n Electrical storms n Lightning provides sufficient energy to split the nitrogen atoms of nitrogen gas, n Forming oxides of nitrogen NO x and NO 2
Atmospheric Pollution n This also happens inside the internal combustion engines of cars n The exhaust emissions of cars contribute a lot to atmospheric pollution in the form of NO x n These compounds form photochemical smogs n They are green house gases n They dissolve in rain to contribute to acid rain in the form of nitric acid n The rain falling on soil and running into rivers n They contribute to the eutrophication of water bodies
Biological nitrogen fixation Treatments Yield / g Oats Peas No nitrate & sterile soil 0.6 0.8 Nitrate added & sterile soil 12.0 12.9 No nitrate & nonsterile soil 0.7 16.4 Nitrate added & nonsterile soil 11.6 15.3
Conclusion n Adding nitrate fertiliser clearly helps the growth of both plants n The presence of microbes permits the peas to grow much better than the oats n The peas grow better in the presence of the microbes than they do with nitrate fertiliser added n The difference is due to the present of mutualistic nitrogen fixing bacteria which live in the pea roots.
Root nodules Alafalfa (Medicago sativa) USDA ARS University of Sydney
Only prokaryotes show nitrogen fixation n n n These organisms possess the nif gene complex which make the proteins, such as nitrogenase enzyme, used in nitrogen fixation Nitrogenase is a metalloprotein, protein subunits being combined with an iron, sulphur and molybdenum complex The reaction involves splitting nitrogen gas molecules and adding hydrogen to make ammonia N 2 2N 669 kj 2N + 8H + NH 3 + H 2 + 54 kj n n This is extremely energy expensive requiring 16 ATP molecules for each nitrogen molecule fixed The microbes that can fix nitrogen need a good supply of energy
The nitrogen fixers n Cyanobacteria are nitrogen fixers that also fix carbon (these are photosynthetic) n Rhizobium bacteria are mutualistic with certain plant species e.g. Legumes n They grow in root nodules n Azotobacter are bacteria associated with the rooting zone (the rhizosphere) of plants in grasslands
The human impact Atmospheric fixation Atmospheric Nitrogen Out gassin g Industrial fixation Biological fixation Soil organic nitrogen Ammonium NH 4 + Plant protein Nitrate NO 3
Industrial NFixation n The HaberBosch Process N 2 + 3H 2 2NH 3 92kJ n The Haber process uses an iron catalyst n High temperatures (500 C) n High pressures (250 atmospheres) n The energy require comes from burning fossil fuels (coal, gas or oil) n Hydrogen is produced from natural gas (methane) or other hydrocarbon
The different sources of fixed nitrogen Sources of fixed nitrogen Production / M tonnes a 1 Biological 175 Industrial 50 Internal Combustion 20 Atmospheric 10
Eutrophication n Nutrient enrichment of water bodies n Nitrates and ammonia are very soluble in water n They are easily washed (leached) from free draining soils n These soils tend to be deficient in nitrogen n When fertiliser is added to these soils it too will be washed out into water bodies n There algae benefit from the extra nitrogen n This leads to a serious form of water pollution
Eutrophication Fertilisers washed into river or lake Sewage or other organic waste New limiting factor imposes itself
Making things worse! Hot water from industry (Thermal pollution) Increased Biochemical Oxygen Demand (BOD) Pollution from oil or detergents Reduction in dissolved O 2
The death of a lake Reduction in dissolved O 2 Increased nitrite levels NO 3 NO 2 Death/emigration of freshwater fauna Methaemoglobinaemia in infants Stomach cancer link (WHO limit for nitrates 10mg dm 3 )
The future of industrial nitrogen fixation n Food production relies heavily upon synthetic fertilisers made by consuming a lot of fossil energy n Food will become more expensive to produce n Nitrogen fixing microbes, using an enzyme system, do the same process at standard temperatures and pressures essentially using solar energy n Answer: Genetically engineered biological nitrogen fixation?
Making things better n The need for synthetic fertilisers can be reduced by cultural practices n Avoiding the use of soluble fertilisers in sandy (free draining soil) prevents leaching n Rotating crops permits the soil to recover from nitrogen hungry crops (e.g. wheat) n Adding a nitrogen fixing crop into the rotation cycle n Ploughing aerates the soil and reduces denitrification n Draining water logged soil also helps reduce denitrification
Return to the atmosphere: Denitrification n Nitrates and nitrites can be used a source of oxygen for Pseudomonas bacteria n Favourable conditions: Cold waterlogged (anaerobic) soils 2NO 3 3O 2 + N 2 providing up to 2385kJ 2NO 2 2O 2 + N 2 n The liberated oxygen is used as an electron acceptor in the processes that oxidise organic molecules, such as glucose n These microbes are, therefore, heterotrophs
Atmospheric fixation Atmospheric Nitrogen 4 000 000 000 Gt Out gassin g Industrial fixation Animal protein Biological fixation Soil organic nitrogen 9500 Gt Ammonification Ammonium NH 4 + Plant protein 3500 Gt Root uptake Denitrification Dissolved in water 6000 Gt Nitrification Nitrite NO 2 Nitrification Nitrate NO 3 Leaching Sediments 10 Gt