1. Biogeochemical Cycles
HL – Nitrogen and Phosphorous

2. The Nitrogen Cycle

3. Nitrogen Fixation
While the atmosphere is 72% nitrogen gas (N2), plants cannot take up this form of the molecule.
Process of converting atmospheric nitrogen into ammonia (NH3) which can be absorbed by plants
Azotobacter and Rhizobium
Rhizobium has a mutualistic symbiotic relationship with the roots of certain plants (legumes)
Nitrogen is fixed by Rhizobium and the plant gives it carbohydrates

4. Nitrification Conversion of ammonia to nitrate
Nitrosomonas convert ammonia to nitrite (NO2-)
These are chemoautotrophs: they oxidize ammonia to produce energy resulting in nitrite. This energy is then used to fix carbon dioxide into organic carbon.
Nitrobacter converts nitrite to nitrate (NO3-)
Also chemoautotrophs, but they derive energy from nitrite for carbon fixation.
Nitrate produced is available to be used by plants.

5. Denitrification Conversion of nitrate to nitrogen (via reduction)
Pseudomonas denitrificans
Only occurs in the absence of oxygen because bacteria will use nitrate as the final electron acceptor when oxygen is not available
This process reduces the availability of nitrogen to living things
Waterlogging (flooding, overwatering, etc.) can lead to this process

6. The Phosphorous Cycle

7. The Phosphorous Cycle Short term cycle Long term cycle
Phosphorous is needed for many molecules (ATP, DNA, cell membranes) and is readily taken up into plants from the soils
Long term cycle
Much phosphorous is stored in rocks and is only slowly released into the environment through processes like weathering; thus, there is a relatively low turnover rate

8. Agriculture
While phosphorous is naturally found in soils, it is often a limiting factor
Farmers often add phosphorous through the use of fertilizers
Phosphate is mined for the purpose of creating fertilizers
Phosphate is a finite resource because most of it is found in our rock layer. This is of concern as it may limit our ability to continue to create the same crop yields.

9. Eutrophication Nutrient enrichment can lead to eutrophication
Extra nutrients in water lead to algal growth which blocks the sun for plants below.
Death of algae and plants leads to an increase in the amount of bacteria which are decomposing the dead material.
These bacteria have high BOD (biological oxygen demand) which leads to less oxygen being available in the water for other living things

10. Eutrophication
Waterlogging and poor irrigation leading to run-off events often cause this issue
Phosphates can dissolve into the water
Run-off takes phosphates to new locations
Fertilizers, animal wastes and release of sewage are major contributors