1. C.6 The nitrogen and phosphorus cycles (AHL)
Essential idea: Soil cycles are subject to disruption.
Intensive agriculture relies heavily on the use of artificial fertilisers to maintain high crop yields. The manufacture and the application of fertilisers is a major disruption to nutrient cycles. One consequences of this is the unbalancing of natural ecosystems around agricultural areas.
By Chris Paine

2. Understandings, Applications and Skills
Statement
Guidance
C.6.U1
Nitrogen-fixing bacteria convert atmospheric nitrogen to ammonia.
C.6.U2
Rhizobium associates with roots in a mutualistic relationship.
C.6.U3
In the absence of oxygen denitrifying bacteria reduce nitrate in the soil.
C.6.U4
Phosphorus can be added to the phosphorus cycle by application of fertilizer or removed by the harvesting of agricultural crops.
C.6.U5
The rate of turnover in the phosphorus cycle is much lower than the nitrogen cycle.
C.6.U6
Availability of phosphate may become limiting to agriculture in the future.
C.6.U7
Leaching of mineral nutrients from agricultural land into rivers causes eutrophication and leads to increased biochemical oxygen demand.
C.6.A1
The impact of waterlogging on the nitrogen cycle.
C.6.A2
Insectivorous plants as an adaptation for low nitrogen availability in waterlogged soils.
C.6.S1
Drawing and labelling a diagram of the nitrogen cycle.
C.6.S2
Assess the nutrient content of a soil sample.

3. The roles of bacteria in nitrogen fixation
C.6.U1 Nitrogen-fixing bacteria convert atmospheric nitrogen to ammonia.
Rhizobium & Azotobacter
Nitrogen gas
Ammonia (NH3)
nitrogen fixation
The roles of bacteria in nitrogen fixation
Nitrosomonas*
Nitrification is the process of converting ammonia into nitrates
Nitrobacter*
nitrates (NO3-)
Nitrites (NO2-)
Plants cannot directly absorb and assimilate nitrogen. It must be first converted to compounds such as nitrates and ammonia.
*Bacteria can be chemoautotrophs deriving energy (for carbon fixation) from the bonds in the compounds they convert.

4. C.6.U2 Rhizobium associates with roots in a mutualistic relationship.
Azotobacter are free-living in the soil whereas bacteria of the genus Rhizobium are often not free-living but live in a close symbiotic association in the roots of plants such as the legume family.
The legume supplies carbohydrates to the bacteria. The bacteria use the carbohydrates for processes such as respiration.
Legumes and the Rhizobium grow together to form nodules on the roots of the legume.
The bacteria supply ammonia (fixed from atmospheric nitrogen) to the legume.
The legume requires ammonia for the synthesis of amino acids.
Mutualism describes relationships between organisms in which both organisms benefit.

5. A chemical reduction process carried out by bacteria
C.6.U3 In the absence of oxygen denitrifying bacteria reduce nitrate in the soil.
Denitrification reduces the availability of nitrogen compounds to plants.
A chemical reduction process carried out by bacteria
Nitrate (NO3-)
Nitrogen (N2)
e.g. Pseudomonas sp.
Electron transport is a key process in cellular respiration
Oxygen or nitrate can be used as an electron acceptor in electron transport.
Though oxygen is preferred in oxygen poor conditions nitrate is used and the process releases nitrogen gas a product.

6. C.6.A1 The impact of waterlogging on the nitrogen cycle.
Soil can become inundated by water, waterlogged, through flooding or irrigation with poor drainage.
Waterlogging reduces the oxygen availability in soils.
This encourages the process of denitrification by bacteria, e.g. Pseudomonas sp.
n.b. excess water in the soil also leads to greater leaching of nutrients, which leads to nutrient enrichment of rivers and lakes and therefore to eutrophication.

7. C.6.A2 Insectivorous plants as an adaptation for low nitrogen availability in waterlogged soils.
Dionaea muscipula - The Venus Flytrap
“Carnivorous plants have the most bizarre adaptations to low-nutrient environments. These plants obtain some nutrients by trapping and digesting various invertebrates, and occasionally even small frogs and mammals. Because insects are one of the most common prey items for most carnivorous plants, they are sometimes called insectivorous plants. It is not surprising that the most common habitat for these plants is in bogs and fens, where nutrient concentrations are low but water and sunshine seasonally abundant.”
Drosera sp. - the Sundews
Find out more

8. C.6.A2 Insectivorous plants as an adaptation for low nitrogen availability in waterlogged soils.
Modified leaves have evolved to trap insects. 
Enzymes are secreted to (extracellularly) digest the animal.
The products of digestion are absorbed by the modified leaves.
Insectivorous plants cannot be truly considered carnivorous as only nitrogen compounds are absorbed. The plant still obtains it’s energy from light via photosynthesis.

9. C.6.S1 Drawing and labelling a diagram of the nitrogen cycle.
On this diagram the pools (boxes) and fluxes (arrows) have been drawn on already. Add in the processes and state the bacteria related to the some of the processes.
Transfer by the food chain
Azotobacter
Nitrobacter
Denitrification
Nitrosomonas
Nitrification (x2)
Pseudomonas
Ammonification
Rhizobium
Death & decomposition
free-living nitrogen-fixing bacteria in the soil
Excretion
Natural nitrogen fixation by lightning
Application of fertilisers containing nitrogen (fixed by the Haber process)
Mutualistic nitrogen-fixing bacteria in root nodules
Uptake (by active transport) and assimilation by plants
adapted from:

10. C.6.S1 Drawing and labelling a diagram of the nitrogen cycle.
Denitrification
Pseudomonas
Transfer by the food chain
Uptake (by active transport) and assimilation by plants
Natural nitrogen fixation by lightning
Excretion
Application of fertilisers containing nitrogen (fixed by the Haber process)
Death & decomposition
Mutualistic nitrogen-fixing bacteria in root nodules
Rhizobium
Ammonification
Nitrification
Nitrobacter
free-living nitrogen-fixing bacteria in the soil
Nitrification
Nitrosomonas
Azotobacter
adapted from:

11. C.6.U5 The rate of turnover in the phosphorus cycle is much lower than the nitrogen cycle.
The phosphorous cycle shows the various different forms in which phosphorous can naturally be found.

12. C.6.U5 The rate of turnover in the phosphorus cycle is much lower than the nitrogen cycle.
The phosphorous cycle shows the various different forms in which phosphorous can naturally be found.
Organisms have a variety of uses for phosphate
ATP
DNA and RNA
cell membranes
skeletons in vertebrates
Certain rocks, e.g. Phosphorite, contains high levels of phosphate minerals. Weathering of these rocks releases phosphates into the soil. Phosphates are a form of phosphorus that can is easily be absorbed by plants and hence enter food chains.
The rate of turnover (the speed of movement of phosphorous from one pool/sink to another) is relatively slow, compared with Nitrogen, as phosphate is only slowly released to ecosystems by weathering.

13. C.6.U4 Phosphorus can be added to the phosphorus cycle by application of fertilizer or removed by the harvesting of agricultural crops.
Phosphate is mined and converted to phosphate-based fertilizer – this increase the rate of turnover.
The fertilizer is then (transported great distances and) applied to crops *
Phosphorus in the biomass of crops is transferred from fields in one area to markets in other areas *
*these processes remove phosphorus from the cycle in one location and adds it to another.

14. C.6.U6 Availability of phosphate may become limiting to agriculture in the future.
The demand for artificial fertiliser in modern intensive farming is very high.
Consequently phosphate mining is being carried out at a much faster rate than the rocks can be naturally formed and hence replenished.
Therefore phosphate minerals are classified as a non-renewable resource.

15. C.6.U6 Availability of phosphate may become limiting to agriculture in the future.
The graph is based on US Geological Survey data and shows world phosphate production from mining.
World production has varied greatly, but overall there have been smaller increases to production after than before 1980.
millions of Metric tons
As the reserves of phosphate rock are depleted the production of phosphorous is likely to peak and then decline. Though some sources the peak is likely to occur in in the next 30 years it is difficult to judge particularly due to the fact new phosphate mineral deposits are still being discovered.

16. C.6.U6 Availability of phosphate may become limiting to agriculture in the future.
Impacts to agriculture of reduced phosphate production are potentially great.
Yields per unit of farmland would plummet without the addition of fertilizer.
There are no sources of phosphate fertiliser other than mining minerals.
There is no synthetic way of creating phosphate fertilisers*, though this may change in the future.
*Unlike ammonia which can be created by the industrial conversion of plentiful supplies of atmospheric nitrogen.

17. Poorly drained, waterlogged soils encourages leaching.
C.6.U7 Leaching of mineral nutrients from agricultural land into rivers causes eutrophication and leads to increased biochemical oxygen demand.
Rainfall leaches water-soluble nutrients (e.g. phosphates, ammonia and nitrates) from soil and carries them into rivers and lakes.
The nutrients can come either from artificial fertilisers, natural fertiliser such as manure or the urine of livestock.
Poorly drained, waterlogged soils encourages leaching.
An increase in nutrients in aquatic ecosystems leads to eutrophication.

18. C.6.U7 Leaching of mineral nutrients from agricultural land into rivers causes eutrophication and leads to increased biochemical oxygen demand.
An increase in nutrients in aquatic ecosystems leads to eutrophication.
Use the animations to learn about the process and the consequences:

19. C.6.U7 Leaching of mineral nutrients from agricultural land into rivers causes eutrophication and leads to increased biochemical oxygen demand.
An increase in nutrients in aquatic ecosystems leads to eutrophication.
In summary:
Algal growth is normally limited by the availability of nutrients such as nitrates and phosphates
Rapid growth in the algal populations occurs, these increases are called ‘algal blooms’
as the population of algae increase so naturally does the numbers of dead algae
the numbers of (saprotrophic) bacteria and microbes that feed on the dead algae also increase
an increase in biochemical oxygen demand (BOD) by the saprotrophic bacteria results in deoxygenation of the water supply (reduced dissolved O2)

20. The consequences to organisms of low levels of dissolved oxygen:
C.6.U7 Leaching of mineral nutrients from agricultural land into rivers causes eutrophication and leads to increased biochemical oxygen demand.
An increase in nutrients in aquatic ecosystems leads to eutrophication.
The consequences to organisms of low levels of dissolved oxygen:
death or emigration of oxygen sensitive organisms (e.g. fish)
proliferation of low dissolved O2 tolerant organisms
reduction of biodiversity
decrease in water transparency, i.e. an increase in turbidity stresses photosynthetic organisms …
… this in turn will affect the whole food chain
increased levels of toxins and greater numbers of pathogens means affected water is no longer suitable for bathing or drinking

21. C.6.S2 Assess the nutrient content of a soil sample.
Garden supply companies commonly sell soil quality assessment kits. The kits involve adding a chemical to a sample of soil that reacts with the nutrient in question, if present. A colour is produced that can be visually compared to a key.
 An example kit from Urban Farmer:
Guidance on proper use of tests and limitations of simple home test kits:

22. Bibliography / Acknowledgments
Jason de Nys