Wed, March 18

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Wed, March 18 Essay 7 assigned, but can be
turned in Monday following spring break.
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Why old ecosystems are limited in Phosphorus
(millions of years)
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• Phosphorus facts
• All derived from parent materials
• No gaseous forms
• c) Largely insoluble except under limited
• conditions
• d) Phosphotase enzymes used both by plants
• and microbes to obtain it from organic
  forms.
• e) Propensity to form insoluble compounds with
• iron and aluminum (under low pH) or
• calcium (under high pH).
• H2PO4-
• HPO4
• PO4

-
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Why we dont run out of phosphorus...
Disturbances (landslides, scour, volcano,
glaciers etc.)
(millions of years)
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Phosphorus can be limiting because its so
insoluble!!!! And, phosphorus availability
is very pH sensitive!
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Tropical forests seem to do well with nitrogen
(high solar inputs allows for epiphytic
N-fixation?)
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Early data from Bolivia Grasses are nitrogen
but not phosphorus sensitive
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Data from Bolivia Third year of study grasses
were phosphorus sensitive
1600 1200 800 400 0
Phosphorus
Nitrogen
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In Bolivia, the plots protected from grazing
went from N limitation the second year to
phosphorus limitation the third year. Whats
with that? Whats the major difference between
these ions in the soil????
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Bolivia Extensive grazing, plants initially
root limited Low N in system initially limited
NPP N fixation N acquisition from upstream
sources provide sufficient N to plants Lack of P
mobility limits P availability to
plants Sometimes you just have to make up a
story that explains the results.
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How could we test to see if N or P limits a)
decomposition or b) Microbial respiration
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Percent Of Mass remaining
nutrient
control
time
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Microbial respiration
nutrient
control
time
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In situ respiration rates following glutamate
additions
CO2
DOC
Cleveland et al. 2002. Ecosystems 5680
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Phosphorus availability constrains microbial
activity in topics.
Respiratory Response Following Carbon (Glutamate)
Addition
Cleveland et al. 2002, Ecosystems 5680
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P Availability Also Limits the Decomposition of
Native C
Cleveland et al. 2002. Ecosystems 5680
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Can phosphorus be immobilized during the
decomposition process.like nitrogen? Exoenzymes
(phosphotase) appear to harvest Phosphorus
from organic substrates better than they can
harvest amino acids. Probably due to different
bonds C-N vs C-O-P
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• Hawaiian Nitrogen
• Not a major constitutent of lava ? low in young
  volcanic soils
• Accumulates over time via deposition ( 0.5
  g/m2/yr) and asymbiotic N fixation ( 0.1
  g/m2/yr)
• N commonly more limiting in younger soils
  (sometimes colimiting w/ P) P commonly more
  limiting in older tropical ecosystems

? ?
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Changes in mean tree height across the Hawaiian
age sequence N P limitations strongest in
younger older stands
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When is an element not limiting? ---when growth
is limited by something else How can the
availability of one element affect the abundance
of another? There is evidence that phosphorus
availability can control nitrogen availability
by controlling the amount of nitrogen fixation.
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South campus study shows the P enrichment can
increase N availability via stimulation of N
fixation
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• Treatments
• Sugar phosphorus
• 2) Sugar gypsum (calcium sulfate)
• 3) Gypsum (calcium sulfate)
• 4) Control
• 5) Phosphorus fertilizer

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Phosphorus limitation generated by calcium
sulfate additionbarely
(fertilizing with P increases Pthats comforting)
S sugar, G gypsum, Ccontrol
P phosphorus
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Phosphorus fertilization increased soil
inorganic nitrogen!(?)
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C4 grasses win under low resource competition,
weeds lose.
Relative abundance
Use nutrient limitation as a management strategy
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Many invaders are nutrient loving speciesbut not
all
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N fixation by cryptogamic crusts critical to
soil development
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• can you draw a phosphorus cycle that
• Includes
• Plants
• Dead organic matter
• Inorganic (sorbed) P
• Inputs from parent material
• Export due to particulate losses

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An exception to phosphorus solubility Its
usually in rock form, but under anaerobic
conditions, phosphates can go into solution.
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If youve got sunlight and phosphorus, N fixers
can make available nitrogen But if theres no
available phosphorus theres no way to pump N
into the system and the system will become
strongly N (or NP) limited.
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Healthy ecosystems dont leach much
plant-available N or P. This means that aquatic
systems may be limited by one, by the other, or
both.