THE NITROGEN CYCLENitrates are essential for plant growth

THE NITROGEN CYCLE

Nitrates are essential for plant growth Root uptake Nitrate NO3- © 2008 Paul Billiet ODWS

Nitrates are recycled via microbes Nitrification Nitrification Ammonium NH4+ Ammonification Nitrite NO2- Soil organic nitrogen Animal protein © 2008 Paul Billiet ODWS

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 © 2008 Paul Billiet ODWS

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 © 2008 Paul Billiet ODWS

Nitrogen from the atmosphere Biological fixation Out gassing Atmospheric Nitrogen 4 000 000 000 Gt © 2008 Paul Billiet ODWS

Atmospheric nitrogen fixation Electrical storms Lightning provides sufficient energy to split the nitrogen atoms of nitrogen gas, Forming oxides of nitrogen NOx and NO2 © 2008 Paul Billiet ODWS

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

University of Sydney Root nodules

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 © 2008 Paul Billiet ODWS

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 © 2008 Paul Billiet ODWS

The human impact Biological fixation Industrial fixation © 2008 Paul Billiet ODWS

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 © 2008 Paul Billiet ODWS

The different sources of fixed nitrogen © 2008 Paul Billiet ODWS

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 © 2008 Paul Billiet ODWS

Fertilisers washed into river or lake New limiting factor imposes itself Decomposers (bacteria) increase in numbers Dead leaves ALGAL BLOOM Rapid growth of algae Death of algae Sewage or other organic waste Eutrophication © 2008 Paul Billiet ODWS

Increased Biochemical Oxygen Demand (BOD) Hot water from industry (Thermal pollution) Pollution from oil or detergents Decomposers (bacteria) increase in numbers Reduction in dissolved O2 Making things worse! © 2008 Paul Billiet ODWS

The death of a lake Death/emigration of freshwater fauna Methaemoglobinaemia in infants Stomach cancer link (WHO limit for nitrates 10mg dm-3) Increased nitrite levels NO3-  NO2- ANAEROBIC CONDITIONS Reduction in dissolved O2 © 2008 Paul Billiet ODWS

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? © 2008 Paul Billiet ODWS

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 © 2008 Paul Billiet ODWS

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 © 2008 Paul Billiet ODWS

© 2008 Paul Billiet ODWS