Download Plants have evolved three separate strategies for photosynthesis

Answers to Second exam.... explained.
Plant characteristics
Low lignin leaves could describe leaves with high specific leaf area, but thin leaves are
particularly known to be high in nitrogen concentration.
Leaves that live longer than one year usually are high lignin, low nitrogen leaves.
Leaves with a low compensation point must have ‘low costs’ and be low in nitrogen.
In spite of the fact that C4 plants are better at using nitrogen, have a greater affinity for
the carbon dioxide molecule, and are most productive in hot, dry environments like
grasslands, they do not dominate. C3s are found in the arctic and tropics. CAM is
known only for its water use efficiency. The plants are not very productive.
Grasslands are the slackers in terms of light absorbance. It takes a lot of leaf area to
absorb light in a grassland, so it is the least efficient in terms of light absorbed per unit
area of leaves.
NPP of terrestrial ecosystems rarely exceeds 2% and usually averages closer to 1% of
incoming light.
When light exceeds the saturation level of a leaf, the usual response is heating. Only C3
plants, however, exhibit photorespiration.
NPP averages about half of GPP>
Tower carbon dioxide data can tell us net carbon flux. It can tell you if NEE (net
ecosystem exchange) is positive with respect to the ecosystem, zero (neither gaining or
losing) or negative. That’s all it can tell you. It tells you nothing about the amount of
carbon stored in the ecosystem.
Plant NPP fates...before consumption or use by something include: 1) aboveground plant
tissue allocation, 2) belowground plant tissue allocation, 3) allocation to mycorrhizae, 4)
losses to soils, and 5) volatilization to the atmosphere. (6) Disturbances can remove
chunks of NPP.
About 95% of NPP gets transferred to the decomposer food web in forests.
C3 plants are the most successful, and increased carbon dioxide and nitrogen from human
sources make them even more successful.
Soil animals help microbes a lot, but soil animals themselves rarely respire more than
10% of the carbon found in dead organic matter.
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Humus is lignin that’s been infused with a lot of fragments from microbes and
exoenzymes. Hence, it’s loaded with nitrogen. The nitrogen may not be very accessable
to organisms, but it’s there.
Exoenzymes cost the same amount of energy to build as enzymes, but any breakdown of
bonds (decomposition) that goes on externally cannot provide energy to the organism that
made them. A percentage of exoenzymes are likely lost without providing a ‘benefit’ to
the owner. There probably is an additional energy cost to transport the enzymes out of
the cell.
The size of molecules is the requirement for exoenzymes.
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A C4 plant can be viewed as ‘superior’ to the C3 plant:because its enzyme has a greater efficiency
for carbon dioxide. because the plant can have higher water use efficiencies, and because the
plant does not require as many Ribisco enzymes per unit of leaf to generate the same amount of
photosynthesis, which makes it more nitrogen efficient.
.An ecosystem stores an average of 10,000 g of carbon per sq. m in the soil and has an annual
decomposition rate of 10% per year will therefore have an NPP of (.1*10,000) = 1000 grams per
sq m per year.
If an ecosystem has heterotrophic respiration rates that match their NPP rate, then the
system has a zero value for net ecosystem production.
If a leaf moves its leaf angle to 60 degrees towards the sun, then light is “diluted” 50%
per unit leaf area. Hence, a leaf that saturates at 50% light when it’s flat will saturate at
100% when it’s at that angle.
Global NPP shows a pattern of high production at the equator to low production at the
poles. Carbon storage, in contrast, shows maximum values in cold temperate or cold
boreal systems. This difference is due to the fact that decomposition is much more
sensitive to temperature controls than NPP. Cold, wet conditions produce very low
decomposition but modest NPP, whereas hot, dry conditions produces low NPP but more
rapid decomposition. The lack of large soil animals or simple changes in the
decomposability of molecules like lignin in cold regions appear to be the causal
mechanisms.
Vegetables may have vitamins, but can be low in nutrients. To get them to decay we
would want to 1) control the microclimate...warm and moist is best, 2) we would add
starter populations of microbes and soil animals, and 3) we might supplement the
substrate to enhance the rate of decomposition. It’s all about substrate quality,
microclimate, and biota....
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The endpoint of decomposition (well, endpoint under some conditions) is called humus
and is very slow to decay. However, it’s loaded with nitrogen as a result of the
contribution of microbes to the original CHO molecules.
Soil animals can enhance the rates of microbial respiration (up to a point...then they
‘overgraze) by: removing senescent microbes from surfaces, allowing for recolonization
by healthy microbes, by creating new surface area for microbes to work over, and by
creating microclimates (fragmenting the litter...moving it into the soil) that make
microbes happier.
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