ICA 6 NUTRIENT CYCLES
1. What are the major macronutrients? CHONPS
What are the major micronutrients? Ca, P, Fe, Mg, K, Na
2. For what are these elements (nutrients) used in organisms?
CHO – organic compounds (carbos, lipids, proteins, nucleic acids) + water
N, P, S – proteins (amino acids), nucleic acids
Ca, P – bones, exoskeletons, cell membranes
Fe, Mg – pigments, enzymes; hemoglobin (Fe); chlorophyll (Mg)
K, Na – ionic balance; neural transmission
Explain how physiological and ecosystem ecology are linked. Processes of all
individuals’ physiology accumulate to affect entire ecosystem. Ecosystem organic
pools and fluxes = sum of what element transformations occur during physiology at
the individual level.
3. Figure 1. What does it mean to say “Chemicals cycle in the ecosystem”?
Elements move from biotic to abiotic back to biotic pools; they are never lost, but
always recyled (eventually).
4. Figure 2. (Thoroughly digest this figure…)
What are the four compartments (pools) of our global ecosystem?
A. Atmosphere B. Biosphere (all organisms) C. Lithosphere (soil, rock, minerals)
D. Hydrosphere (water)
Which elements have gaseous pools? C, N, O
Which elements have sedimentary pools? P, S
Why are these called ‘bio-geo-chemical ‘ cycles? (BGC) Cycles involve chemicals
moving between biological and geological pools.
5. How are fluxes related to pools? A flux is the movement of elements between pools.
What is the unit of a flux? amount of element moved/time/area or volume
What is a sink pool? pool with input/output increasing
What is a source pool? pool wth input/output decreasing
In which direction between pool types does a flux occur? from source to sink pool
What is residence time? pool size/flux = how long element stays in pool
6. WATER CYCLE Figure 3.
A. Trace the cycle of a water molecule from a rain cloud back to a rain cloud.
B. Account for the numbers on the figure; do they add up?
C. What does it mean: “The water cycle is physically, not chemically, based.”
Water is not chemically altered; it merely changes phases during the cycle.
7. CARBON CYCLE
A. Why is this cycle closely linked to global energy flux? Carbon compounds
contains chemical bond energy fixed by photosynthesis. Any transformation of
carbon compounds to release energy also alters the form of carbon, results in a
flux, and changes the pool in which the carbon is stored.
B. What are the principal classes of processes in the C-cycle?
1. assimilation/dissimilation processes in plants/decomposers
2. exchange of CO2 between atmosphere and oceans
3. sedimentation of carbonates
C. Process
Inorg-Org
Use/Get Energy
Electron Donor/Acceptor
Assimilation
I to O
Uses (reduction)
reducer = electron donor
Dissimilation
O to I
Gets (oxidation)
oxidizer = electron acceptor
Figure 4. What is the principal assimilation process in the C-cycle? photosynthesis
What is the principal dissimilation process? respiration
D. What process ‘kicks in’ under anaerobic conditions? methanogenesis
What is its end product? methane gas (CH4)
E. What are two principal greenhouse gases generated by the C-cycle? CO2 + CH4
F. Where is the major pool of C on earth? sedimentary rock How did it get there?
CO2 dissolves in water + H20  H2CO3  H+ + carbonate ions
Carbonate ions + Ca2+  CaCO3 (calcium carbonate); low solubility,
precipitates, and forms sediments that eventually are transformed to sedimentary
rock (e.g. limestone).
G. Figure 5. What are 2 new fluxes generating more CO2 due to human activities?
Fossil fuel burning and deforestation Which is greater? fossil fuel burning
H. Figure 6. What 2 sink pools are being altered greatly by these activities?
Atmosphere and ocean. What is meant by ‘the missing C sink’? CO2 sinks are
absorbing less CO2 than are being generated by sources. Where is the excess
CO2 going?
I. Figure 7. Describe the methodology of a FACE experiment. Control +
treatment rings of pipes; air is blown into areas of control; CO2 blown into
treatment so plants experience elevated levels of CO2 there.
Generate a hypothesis/prediction to answer the ?: Is plant productivity CO2-limited?
If…plant photosynthesis + ecosystem NPP are CO2-limited,
then…photosynthetic rate + NPP will be higher in areas with elevated CO2 than
control areas.
Figure 8. Why are there 3, not 2, treatments? The extra treatment is the same as elevated
treatment (with enclosure) but at ambient levels of CO2; it controls for any effect the
enclosure itself might have on plant growth.
What are the results? Production was greater within enclosures + ambient CO2 than
controls, but even greater when enclosure had evlated CO2.
What is the conclusion? Plant productivity is CO-2 limited; plants provided extra C02
respond with greater productivity, relative to those at ambient levels.
Figure 9. Qualify your conclusion. Not all dominant grasses responded to elevated CO2;
the greater biomass with elevated CO2 was due to a response by only S. comata (needle
grass).
Some additional mechanisms to explain:
A. Needle grass under elevated CO2 was less digestible by grazers than grass
under ambient CO2.
What’s the ‘take-home’ message about future plant productivity and food
available to cattle and other grazers? Elevated CO2 has potentially wide-ranging
effects; while it may increase productivity, the plants produced may be less digestible to
grazers.
B. Needle grass had greater productivity because plots with elevated CO2 had
more soil water.
Create a scenario that accounts for the increase in soil moisture.
Include: elevated CO2, accelerated CO2 assimilation, stomates,
transpiration, WUE, withdrawal of water from soil
Because elevated CO2 led to accelerated CO2 assimilation, plnts closed stomates at
higher water potentials, thereby reducing transpiration (increasing WUE) and
reducing withdrawal of water from soil.
J. Figure 10.How does the amount of atmospheric CO2 today compare to the past?
Today’s levels are substantially lower than most of the past 600 million years.
What caused it to drop greatly between 500 to 300 million years ago? The first
forests on earth developed, thus fixing abundant C in their biomass.
What caused the great increases in coal deposits in the Carboniferous? These
forests were in swamping areas with anaerobic conditions causing incomplete
decomposition. Sedimentation of this biomass caused conversion to coal.
Predict how the earth’s temperature changed from peak to dip in CO2.
Decreased because less temperature-trapping CO2 in atmosphere.
What is the concern about elevated CO2 today, given that the concentration is
less than in the past. The speed with which it is happening is causing rapid
temperature rise. The rate will not allow much evolutionary response; so much
loss of species is expected unless migration is possible.
8. NITROGEN CYCLE Figure 11.
A. Trace a N atom from the atmosphere into a plant and back to the atmosphere.
What chemically happens during:
Nitrogen fixation: N2  NH4 (ammonia)
Nitrification x2: NH4  NO2 (nitrite)  NO3 (nitrate)
Plant assimilation: NO3 organic nitrogen (e.g. amino acids, nucleic acids)
Denitrification: NO3  NO2  NO  N2
Which of these processes are aerobic (A)? anaerobic (ANA)? Denitrification
and N-fixation are anaerobic; all others are aerobic
Which of these processes are microbial-dependent? (M) all except assimilation
and ammonification.
B. Figure 12. Describe the chemical interaction between plants and N-fixing
organisms. Rhizobium and blue-green algae have enzyme nitrogenase that
can break triple bond of N2 and form NH4, thus providing a form of nitrogen
that plants can transform into organic compounds. The plants provide the
microbes with an energy source in carbohydrates.
9. PHOSPORUS CYCLE Figure 13.
A. What is the dominant form of this element? phosphate Does it change chemically?
No Where is the major pool? rock Why is this called a ‘geologic cycle’? Bringing
P from geologic form (rock) into form that plants can absorb requires geological
processes, taking geologic time (e.g. uplift of sediments; rock breakdown \
(weathering)
How fast does the cycle occur? Very slowly If an atom goes from land to water,
how soon will it be available to organisms on land? Geologic eons.
B. What are mycorrhizae? fungi living on/in plant roots that aid in P uptake
Figure 14. Describe how they work to aid the plant. penetrate large volume of
soil; secrete enzymes/acids extracellularly; increase solubility of nutrients,
especially P, bring in much P for plant to use.
What does the plant give to the fungus? An energy source (carbohydrates)
vitamins; amino acids
C. Figure 15. What is one basic hypothesis/prediction being tested?
If…plant growth is dependent on availability of phosphorus
then,..plant dry weight of foliage will increase with increasing P concentration;
especially when mycorhhizae are present.than when absent.
Do the data support the hypothesis? Yes.
10. SULFUR CYCLE Figure 16.
Figure 17. Into what compounds is the sulfate assimilated by a plant incorporated?
amino acids
By what process is sulfate converted to H2S, FeS? desulhydration; dissimilatory
sulfur reduction
Is this an aerobic or anaerobic process? anaerobic
Where do we smell these products? in mucky soils; swamps; water-logged
anaerobic soils and sediments
How did sulfur get incorporated into coal? When non-decomposed plants got
buried in swamps, allowing these anaerobic processes to proceed.
Of what consequence is its presence? When we strip-mine for this coal,
sulfuric acid gets into streams, When we burn high-S coal, we increase the
amount of acid rain  both lower pH, lower Ca in soils, lower forest
productivity. Also lower pH in lakes disrupts aquatic community.