Biogeochemical
Cycles
HL – Nitrogen and Phosphorous
The Nitrogen Cycle
http://cahnrs.wsu.edu/alumni/wp-content/uploads/sites/3/2011/10/nitrogen-cycle.jpg
Nitrogen Fixation
http://www.micropia.nl/
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

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.

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

The Phosphorous Cycle
http://reefkeeping.com/issues/2006-08/cj/images/image003LG.jpg
The Phosphorous Cycle

Short 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
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.
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
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