2016 IB Biology Curriculum

1. Essential idea: Soil cycles are subject to disruption.
C.6 The nitrogen and phosphorus cycles

2. Understandings, Applications and Skills
Statement Guidance
C.6 U.1 Nitrogen-fixing bacteria convert atmospheric nitrogen to
ammonia.
C.6 U.2 Rhizobium associates with roots in a mutualistic relationship.
C.6 U.3 In the absence of oxygen denitrifying bacteria reduce nitrate in the
soil.
C.6 U.4 Phosphorus can be added to the phosphorus cycle by application
of fertilizer or removed by the harvesting of agricultural crops.
C.6 U.5 The rate of turnover in the phosphorus cycle is much lower than
the nitrogen cycle.
C.6 U.6 Availability of phosphate may become limiting to agriculture in the
future.
C.6 U.7 Leaching of mineral nutrients from agricultural land into rivers
causes eutrophication and leads to increased biochemical oxygen
demand.
C.6 A.1 The impact of waterlogging on the nitrogen cycle.
C.6 A.2 Insectivorous plants as an adaptation for low nitrogen availability
in waterlogged soils.
C.6 S.1 Drawing and labelling a diagram of the nitrogen cycle.
C.6 S.2 Assess the nutrient content of a soil sample.

3. Nitrogen Cycle
Key Chemical Ingredient: amino acids/proteins
• Earth’s atmosphere 80% nitrogen; unavailable to plants; cannot assimilate
• Nitrogen available to plants as
1. Ammonia (NH3)
2. Ammonium (NH4
+
)
3. Nitrate (NO3)
• Bacteria are essential to the nitrogen cycle
• Nitrogen gas in the atmosphere is very abundant, but is such a stable
molecule that bacteria are needed to break it apart and this process
consumes much energy
• Nitrogen enters ecosystems by atmospheric deposition (5-10%) or
Nitrogen fixation
• NH4
+
& NO3added to soil: dissolved in rain or fine dust (particulates)

4. Steps in Nitrogen CycleSteps in Nitrogen Cycle
• Five steps are involved in the nitrogen cycle
1. Nitrogen fixation Nitrogen must be fixed in order to be used
by plants, its atmospheric form (Azotobacter).
2. Ammonification Ammonia (NH3) is made by decomposing
bacteria (Azotobacter).
3. Nitrification For those plants who refuse to settle with
ammonia, they undergo nitrification. Bacteria (Nitrobacter )
convert most of the ammonia in soil to nitrite ions (NO3
-
)
4. Assimilation This is when plants absorb the substances
dropped off by nitrogen fixation and nitrification.
5. Denitrification If the nitrate ions choose not to assimilate
they leave the soil and are converted by specialized anaerobic
bacteria (Paracoccus) in water-logged soil, swamps, lakes.

5. The Nitrogen Cycle (G.4.5)

6. C.6 U.1 Nitrogen-fixing bacteria convert atmospheric nitrogen to ammonia.
Nitrogen gas Ammonia (NH3)
Nitrites (NO2
-
)nitrates (NO3
-
)
Azotobacter
Nitrobacter*
*Bacteria can be chemoautotrophs deriving
energy (for carbon fixation) from the bonds
in the compounds they convert.
Nitrosomonas*
The roles of
bacteria in
nitrogen fixation
http://en.wikipedia.org/wiki/File:Azotobacter_cells.jpg
Plants cannot directly
absorb and assimilate
nitrogen. It must be
first converted to
compounds such as
nitrates and ammonia.
http://on.be.net/1arnCUH
Nitrogen Fixation/Ammonification
Nitrification is the process of
converting ammonia into nitrates
Assimilation

7. C.6 U.2 Rhizobium associates with roots in a mutualistic relationship.
http://commons.wikimedia.org/wiki/File:French_bean_plant_from_lalbagh_2336.JPG
• The Azotobacter
bacteria supply
ammonia (fixed from
atmospheric nitrogen) to
the legume.
• The legume requires
ammonia for the
synthesis of amino
acids.
• Removing nitrogen from
the air. Legume supplies
carbohydrates (glucose)
to the Azotobacter
bacteria. The bacteria
use the carbohydrates
for processes such as
respiration.
*Mutualism describes relationships between
organisms in which both organisms benefit.

8. C.6 U.2 Rhizobium associates with roots in a mutualistic relationship.
http://commons.wikimedia.org/wiki/File:Nitrogen-fixing_nodules_in_the_roots_of_legumes..JPG
• 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.
• Legumes and the
Rhizobium grow
together to form
nodules on the
roots of the
legume.

9. C.6 U.3 In the absence of oxygen denitrifying bacteria reduce
nitrate in the soil.
• 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.
Denitrification reduces the availability of nitrogen compounds to
plants.
Nitrate (NO3
-
) Nitrogen (N2)
A chemical reduction process
carried out by bacteria
e.g. Paracoccus
http://microbewiki.kenyon.edu/index.php/File:P._Cloroaphis.jpg

10. C.6 A.1 The impact of waterlogging on the nitrogen cycle.
http://www.hampshirecam.co.uk/feb909_2.html

11. http://soer.justice.tas.gov.au/2009/image/1076/lan/id1076-p-SoilDegradationWaterlo-l.Jpg
• 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. Paracoccus.
• 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.

12. C.6 A.2 Insectivorous plants as an adaptation for low nitrogen availability in
waterlogged soils.
http://botany.org/Carnivorous_Plants/
Drosera sp. - the Sundews
Find out more
• Modified leaves have evolved to trap insects.
• Enzymes are secreted to (extracellular) digest
the animal.
• The products of digestion are absorbed by the
modified leaves.
“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.”

13. http://i.telegraph.co.uk/multimedia/archive/01464/plant-5_1464520i.jpg
Insectivorous plants cannot be truly considered carnivorous as only nitrogen compounds
are absorbed. The plant still obtains it’s energy from light via photosynthesis.

14. C.6 S.1 Drawing and labelling a diagram of the nitrogen cycle.

15. C.6 S.1 Drawing and labelling a diagram of the nitrogen cycle.
adapted from: http://commons.wikimedia.org/wiki/File:Nitrogen_Cycle.jpg#mediaviewer/File:Nitrogen_Cycle.svg
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.
Rhizobium
free-living
nitrogen-fixing
bacteria in the
soil
Azotobacter
Mutualistic nitrogen-fixing
bacteria in root nodules
Nitrification (x2)
Nitrosomonas
Nitrobacter
Uptake (by active transport)
and assimilation by plants
Natural nitrogen
fixation by lightning
Application of fertilizers
containing nitrogen (fixed
by the Haber process)
Transfer by
the food
chain
Denitrification
Pseudomonas
Death &
decomposition
Ammonification
Excretion

16. C.6 S.1 Drawing and labelling a diagram of the nitrogen cycle.
free-living nitrogen-
fixing bacteria in the soil
Azotobacter
Mutualistic
nitrogen-fixing
bacteria in
root nodules
Nitrification
Nitrobacter
Uptake (by active
transport) and
assimilation by
plants
Natural
nitrogen
fixation by
lightning
Application of
fertilizers
containing
nitrogen (fixed by
the Haber process)
Transfer by
the food
chain
Denitrification
Pseudomonas
Death &
decomposition
Ammonification
Excretion
Nitrification
Nitrosomonas
Rhizobium

17. Essential idea: Soil cycles are subject to disruption.
We consume phosphorus through food produced with fertilizers. The women above is spreading
phosphorus by hand in her rice paddy to increase production..
Phosphorus cycles
http://www.futureearth.org/blog/2014-oct-16/can-we-build-sustainable-phosphorus-governance

18. C.6 U.5 The rate of turnover in the phosphorus cycle is much lower than the
nitrogen cycle.
http://commons.wikimedia.org/wiki/File:Phosphorus_cycle.png

19. C.6 U.5 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.
•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 entering the food chains.
•The rate of turnover is relatively slow, compared with Nitrogen, as phosphate is only
slowly released to ecosystems by weathering.
•Organisms have a variety of uses for phosphate
 ATP
 DNA and RNA
 cell membranes
 skeletons in vertebrates

20. C.6 U.4 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 . The
processes remove phosphorus from the
cycle in one location and adds it to
another.
http://commons.wikimedia.org/wiki/File:Agriculture_in_Volgograd_Oblas
http://commons.wikimedia.org/wiki/File:Phosphate_Mine_Panorama.jpg

21. C.6 U.6 Availability of phosphate may become limiting to agriculture in the future.
• The demand for artificial fertilizer
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.
Impacts to agriculture of reduced
phosphate production are
potentially great.
• There are no sources of phosphate
fertilizer other than mining
minerals.
• There is no synthetic way of
creating phosphate fertilizers*,
though this may change in the
future.
*Yields per unit of farmland
would plummet without the
*Unlike ammonia which can be created by the
industrial conversion of plentiful supplies of
atmospheric nitrogen.
http://commons.wikimedia.org/wiki/File:Crop_spraying_near_
St_Mary_Bourne_-_geograph.org.uk_-_392462.jpg

22. http://commons.wikimedia.org/wiki/File:Phosphateproductionworldwide.svg
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.
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.
millions of
Metric tons
C.6 U.6 Availability of phosphate may become limiting to agriculture in
the future.

23. http://commons.wikimedia.org/wiki/File:Potomac_green_water.JPG
An increase in nutrients in aquatic ecosystems leads to eutrophication

24. C.6 U.7 Leaching of mineral nutrients from agricultural land into rivers causes
eutrophication and leads to increased biochemical oxygen demand.
http://nroc.mpls.k12.mn.us/Enviro
nmental%20Science/course
%20files/multimedia/lesson78/ani
mations/5a_Lake_Eutrophication.
html
• Rainfall leaches water-soluble
nutrients (e.g. phosphates,
ammonia and nitrates) from the
soil and carries them into rivers
and lakes.
• The nutrients can come either
from artificial fertilizers, natural
fertilizer such as manure or the
urine of livestock.
• Poorly drained, or waterlogged
soils encourages leaching of these
materials.
• An increase in nutrients in aquatic
ecosystems leads to
eutrophication a negative
environmental effect that could
include hypoxia, the depletion of
oxygen in the water, which may
cause death to aquatic animals.

25. 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’ also leading to an 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)
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

26. C.6 U.7 Leaching of mineral nutrients from agricultural land into rivers causes eutrophication
and leads to increased biochemical oxygen demand.
Red tide on Long Island has lead to eutrophication.

27. C.6 S.2 Assess the nutrient content of a soil sample.
• A soil test will assess the present levels of major plant nutrients, soil
pH, micronutrients and provide an estimate of total soil lead.
• Once complete, recommendations will include the amounts of
limestone and fertilizer, if necessary, to meet the requirements of the
specific plant or crop being grown. If elevated soil lead levels are
indicated, appropriate information will be included with your results
to address this problem.

28. Bibliography / Acknowledgments