THE NITROGEN CYCLE

By the end of this lesson I will: (k) describe the role of decomposers in the decomposition of organic material;   (l) describe how microorganisms recycle nitrogen within ecosystems. (Only Nitrosomonas, Nitrobacter and Rhizobium need to be identified by name).

Nitrates are essential for plant growth Plant protein Root uptake Nitrate NO3-

Nitrates are recycled via microbes Root uptake Nitrate NO3- Plant protein Animal protein Soil organic nitrogen Ammonification Ammonium NH4+ Nitrification Nitrite NO2- Nitrification

Amino acids + 11/2O2  CO2 + H2O + NH3 + 736kJ Ammonification Nitrogen enters the soil through the decomposition of protein in dead organic matter Amino acids + 11/2O2  CO2 + H2O + NH3 + 736kJ This process liberates a lot of energy which can be used by the saprotrophic microbes

Nitrification This involves two oxidation processes The ammonia produced by ammonification is an energy rich substrate for Nitrosomas bacteria They oxidise it to nitrite: NH3 + 11/2O2  NO2- + H2O + 276kJ This in turn provides a substrate for Nitrobacter bacteria oxidise the nitrite to nitrate: NO3- + 1/2O2  NO3- + 73 kJ This energy is the only source of energy for these prokaryotes They are chemoautotrophs

Nitrogen from the atmosphere Out gassing Atmospheric fixation Atmospheric Nitrogen 4 000 000 000 Gt Root uptake Nitrate NO3- Plant protein Soil organic nitrogen Biological fixation

Atmospheric nitrogen fixation Electrical storms Lightning provides sufficient energy to split the nitrogen atoms of nitrogen gas, Forming oxides of nitrogen NOx and NO2

Atmospheric Pollution This also happens inside the internal combustion engines of cars The exhaust emissions of cars contribute a lot to atmospheric pollution in the form of NOx These compounds form photochemical smogs They are green house gases They dissolve in rain to contribute to acid rain in the form of nitric acid The rain falling on soil and running into rivers They contribute to the eutrophication of water bodies © 2008 Paul Billiet ODWS

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 & non-sterile soil 0.7 16.4 Nitrate added & non-sterile soil 11.6 15.3

Conclusion Adding nitrate fertiliser clearly helps the growth of both plants The presence of microbes permits the peas to grow much better than the oats The peas grow better in the presence of the microbes than they do with nitrate fertiliser added 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 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 N2  2N - 669 kJ 2N + 8H+  NH3 + H2 + 54 kJ 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 Cyanobacteria are nitrogen fixers that also fix carbon (these are photosynthetic) Rhizobium bacteria are mutualistic with certain plant species e.g. Legumes They grow in root nodules Azotobacter are bacteria associated with the rooting zone (the rhizosphere) of plants in grasslands

The human impact Atmospheric Nitrogen Industrial fixation Nitrate NO3- Atmospheric fixation Out gassing Plant protein Atmospheric Nitrogen Ammonium NH4+ Soil organic nitrogen Industrial fixation Biological fixation

Industrial N-Fixation The Haber-Bosch Process N2 + 3H2  2NH3 - 92kJ The Haber process uses an iron catalyst High temperatures (500°C) High pressures (250 atmospheres) The energy require comes from burning fossil fuels (coal, gas or oil) Hydrogen is produced from natural gas (methane) or other hydrocarbon

The different sources of fixed nitrogen Production / M tonnes a-1 Biological 175 Industrial 50 Internal Combustion 20 Atmospheric 10

Eutrophication Nutrient enrichment of water bodies Nitrates and ammonia are very soluble in water They are easily washed (leached) from free draining soils These soils tend to be deficient in nitrogen When fertiliser is added to these soils it too will be washed out into water bodies There algae benefit from the extra nitrogen This leads to a serious form of water pollution

Fertilisers washed into river or lake Sewage or other organic waste Eutrophication Fertilisers washed into river or lake ALGAL BLOOM Rapid growth of algae Dead leaves New limiting factor imposes itself Sewage or other organic waste Death of algae Decomposers (bacteria) increase in numbers

Making things worse! Pollution from oil or detergents Decomposers (bacteria) increase in numbers Pollution from oil or detergents Hot water from industry (Thermal pollution) Increased Biochemical Oxygen Demand (BOD) Reduction in dissolved O2

The death of a lake Death/emigration of freshwater fauna Reduction in dissolved O2 ANAEROBIC CONDITIONS Increased nitrite levels NO3-  NO2- Death/emigration of freshwater fauna Methaemoglobinaemia in infants Stomach cancer link (WHO limit for nitrates 10mg dm-3)

The future of industrial nitrogen fixation Food production relies heavily upon synthetic fertilisers made by consuming a lot of fossil energy Food will become more expensive to produce Nitrogen fixing microbes, using an enzyme system, do the same process at standard temperatures and pressures essentially using solar energy Answer: Genetically engineered biological nitrogen fixation?

Making things better The need for synthetic fertilisers can be reduced by cultural practices Avoiding the use of soluble fertilisers in sandy (free draining soil) prevents leaching Rotating crops permits the soil to recover from nitrogen hungry crops (e.g. wheat) Adding a nitrogen fixing crop into the rotation cycle Ploughing aerates the soil and reduces denitrification Draining water logged soil also helps reduce denitrification

Return to the atmosphere: Denitrification Nitrates and nitrites can be used a source of oxygen for Pseudomonas bacteria Favourable conditions: Cold waterlogged (anaerobic) soils 2NO3-  3O2 + N2providing up to 2385kJ 2NO2-  2O2 + N2  The liberated oxygen is used as an electron acceptor in the processes that oxidise organic molecules, such as glucose These microbes are, therefore, heterotrophs

Soil organic nitrogen 9500 Gt Sediments 10 Gt Nitrification Root uptake Biological fixation Ammonium NH4+ Ammonification Nitrite NO2- Dissolved in water 6000 Gt Denitrification Leaching Nitrate NO3- Soil organic nitrogen 9500 Gt Atmospheric fixation Out gassing Industrial fixation Plant protein 3500 Gt Animal protein Atmospheric Nitrogen 4 000 000 000 Gt