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Nitrogen and Phosphorus Cycles
Martha E. Rosemeyer
IES
January 29, 2003
One of three dynamic processes
of ecosystems
 Energy flow
 Nutrient cycles
 Succession
Energy flows from the sun to entropy
through a functioning food web
Two types of nutrient cycles
 Volatile


atmospheric reservior
N, S
 Non-volatile


No atmospheric reservior
P, K, Ca, Mg all micronutrients
Nitrogen cycle
N2O
N2
2
Nitrification
3
Gliessman,
1998
modified
N Pool Size (Stocks) are
relatively big
Atmosphere
g N 1021
Terrestrial biomass g N 1015
Soil organic matter
g N 1015
Flows small
Rate limiting steps with respect to
getting N into terrestrial system:
 Via lightning (very small)
 Via Biological N Fixation


natural
legume crops
 Via Industrial fixation of fertilizer
 Via Fossil fuel burning
What is biological nitrogen fixation
(BNF)?
 Second most important biochemical
reaction of ecosystem after photosynthesis
to life on earth
 Involves the reduction of N from molecular
state N2 to NH3 in which form it can be
incorporated into an organic (C containing)
molecular structure
 Involves microbes
BNF
 Regardless of organism uses nitrogenase
enzyme for to fix N2 to ammonia NH3
 NH3 + org. acids amino acids proteins
White clover, Trifolium repens
“Trebol blanco”
Temperate zone green manure crop
seed
Human-caused NF = or greater
Natural terrestrial BNF
BNF from crops
40 x 1012 g N fixed/yr
Fertilizer industry
80
Fossil fuel burning
20
Total human-caused
144
BNF terrestrial ecosystems 100
Microbes: The Ancient Ones
 Responsible for all major processes on
earth, including decomposition and
photosynthesis and nutrient cycling
 Major cycles of Earth could continue
without plants and animals
 Most microbes are beneficial!
Most bacteria 99% cannot be cultured
 Therefore we know little about them!
A ton of microscopic bacteria may
be active in each acre of soil.
Bacteria dot the surface of strands
of fungal hyphae.
--Soil Biology Primer
Life on earth: starting with
microbes
 First N fixed by lightning and meteor
impact
 Formation of first amino acids and protein
 N-fixation early because N-limited
environment in water
 Terrestrial system: P limited environment
 Mycorrhizal fungi have allowed plants to
become terrestrial
Nitrogen cycle:mediated by microbes
= Microbial mediation
2
3
Gliessman 1998
modified
P Cycle
 P is an important plant nutrient, anion
 Released from soil, taken up by plants and
released to soil
 Green Rev ag is totally dependent on P rock
 P rock is basically a valuable, nonrenewable resource for agriculture

some deposits contaminated with Cd, Pb, As
 10% of the P which is applied to land is
flushed way causing eutrophication of lakes
Dead zone in Gulf of Mexico due
to nutrients from Mississippi
The phosphorus cycle
= Microbial mediation
mycorrhizae
Gliessman 1998
P cycle
Brady, 1999
Roots of Common Bean (Phaseolus vulgaris)
from Costa Rica
Mycorrhizal fungus
stained blue
Non mycorrhizal
Components of association
 fungus + root = mycor + rhiza
 They are the rule, not the exception:
Most of world’s vascular plants have
association, except Brassicaceae (cabbage
family) and a few other plant families
 Fungus: takes up water and P, receives CHO
from plant
 Enables plants to live in low P, low water
environments
Mycorrhizae aid soil aggregation
Mycorrhizal fungi link root cells to soil particles. In this
photo, sand grains are bound to a root by hyphae from
endophytes (fungi similar to mycorrhizae), and by
polysaccharides secreted by the plant and the fungi.
Credit: Jerry Barrow, USDA-ARS Jornada Experimental Range,
Las Cruces, NM.
Closing nutrient cycles
 Making the cycle circular
 The nutrient comes back to the “stock” or
“pool” that it started with-- none is lost
 In a human time frame
 This is an important component of
sustainability
Chinese have used “night soil” for
centuries to close nutrient cycle