Summary: these notes on web

1
Summary (these notes on web) Roots use
photosynthate (CHO materials) to do what it
takes to obtain adequate water and nutrients for
growth, maintenance, and reproduction. In many
environments (ours included) light is in
excess relative to other resources. CHO
products from Ps are therefore relatively
cheap and traded for more limited commodities
2

• REVIEW
• Root uptake growth
• mass flow
• diffusion (plus active transport in
• root)
• direct contact (?)
• exoenzymes (phosphotase to get P)
• 2) Mycorrhizae pay-off
• pay fungus to do the job
• 3) Microbial mayhem
• dump photosynthate (CHO)
• and hope to get nutrients?

3
The evolution of superorganisms???
(arrows represent flow of nutrients)
plant
litter
plant
Heterotrophic fungi
Mycorrhizal fungi
A closed, external nutrient cycle
Mutualism (but net gain could be less than that
provided other plantaffecting competition
outcome???)
(is this a violation of individual selection)
4

• feed free-living heterotrophic bacteria.
• (THESE ARE NOT CONNECTED TO ROOTS)
• Plants exude CHO compounds into
• rhizosphere.
• 2. Bacteria grow, immobilize nutrients (non-
• CHO materials) from other sources,
  including
• decomposing substrates
• 3. Plants hope a predator comes by, eats the
• microbe, and excretes nutrients!
• (probably a high probability ???)
• 4. Plant roots uptake nutrients via diffusion
• and active uptake.

5
4. Hydraulic Lift This one is free
moon
at night water leaks out of roots in upper
dry zone.
Text book misses an opportunity hereits not
just water oozing into the soil!
nutrients
water
(no nutrients)
6
Hydraulic Lift
During day water is withdrawn. Net
result provides nutrients. (this result could be
a logical consequence of root herbivory?)
nutrients
water
(no nutrients)
(activity may simply be consequence of
activities outside of plants control
7
Nutrient uptake influence rhizosphere

• Reduces nutrient concentrations
• Enhances decomposition (probably)
• Changes pH
• Roots excretes H when absorb cations
• adds to already acidifying conditions
• caused by carbonic acid.

8
Nutrient ratios dont vary too much

• Means availability of growth-limiting nutrient
  governs uptake of all nutrients, BUT
• Ratios are somewhat variable
• Similarity of ratios reflects regulation of
  uptake

9
N limited more P than N (NP lt14) P limited
more N than P (NPgt16)
10
BiomassN ratio a community and production
characteristic.
(WHAT DOES THE GRAPH OF BIOMASS TO N LOOK LIKE)
11
Nutrient use efficiency (NUE)(two definitions)

• Physiological approach (plant level)
• NUE a t
• a nutrient productivity (photosynthesis/g N)
• t residence time of nutrient in plant
• Ecosystem approach (stand-level)
• NUE g biomass/g nutrient in litter

(THE ECOSYSTEM APPROACH IS GOOD ENOUGH FOR US!)
12
C conifers. D deciduous, M mediterranean,
13

• If nutrients are so valuable to plants,
• (limiting relative to energy), then plants
• should expend a lot of energy
• trying to conserve these?
• Defend with chemicals? (also defends
• other plant parts/functions)
• Recycle resorb .nutrients rather
• than discard in senescent litter.

14
All species are similar in resorption
efficiency No major difference in proportion of
nutrients lost
8.6
15
All species are similar in the proportion of
nutrients leached
8.7
(non)Clicker question What explains the
differences among elements?
16
Chapter 9Terrestrial Nutrient Cycling
HAVING NOW STUDIED HOW PLANTS GET NUTRIENTS, WE
MORE TO

• Principles of
• Terrestrial Ecosytem Ecology
• Chapin, Matson and Mooney

17

• Why Nitrogen???
• Controls biotic diversity species
• composition
• 2. Controls NPP
• 3. Causes major environmental problems
• when in excess.

18
Inputs. How does nitrogen get into ecosystems
in a form the plants can use?
_
N N
ecosystem
19
Inputs Nitrogen Fixation (biological, but
potentially under Phosphorus
control) Nitrogen deposition sea spray
particulates/ammonium/organics from other
terrestrial ecosystems lightning
Historically, nitrogen deposition was uniformly
low.
20
Nitrogen fixation
N2 16 ATP 8e- 8H -gt 2NH3 16 ADP 16 Pi
H2

• Free Living Nitrogen Fixing Bacteria
• obligate anaerobes, e.g. Clostridium pasteurianum
  
• b) facultative anaerobes, e.g., Klebsiella, a
  close relative
• of E. coli.
• c) photosynthetic bacteria, e.g. Rhodobacter
• d) many Cyanobacteria
• e) obligate aerobes such as Azotobacter
• f) some methanogens

21
Nitrogen fixation

• Conversion of atmospheric N2 to NH4
• Requires abundant energy and P
• Inhibited by free oxygen (O2)

22
Paradox of nitrogen limitation

• Nitrogen is the element that most frequently
  limits terrestrial NPP
• N2 is the most abundant component of the
  atmosphere
• Why doesnt nitrogen fixation occur almost
  everywhere???
• Why dont N fixers have competitive advantage
  until N becomes non-limiting?