Questions on decomposition (text, chpt 7 and class notes).
1. Essay question: Explain the three main factors that control the rate of decomposition
of plant material. (say, leaf litter).
2. What's the difference between decomposition and mineralization (use text)?
3. How is it possible for mass loss (decomposition) to occur, where substantial amounts
of carbon dioxide are released back to the atmosphere, but none of the nutrients in the
plant material are released as minerals to the environment?
4. Roughly how much of NPP is transferred directly from plants to the detritus pool in
a) a temperate zone forest.
b) a well-managed grassland.
5.Litterbag studies show that fauna (invertebrates that eat microbes and detritus) can
speed the rate of decay of litter by up to 50%. Yet these animals only respire away about
5% of the litter carbon themselves. What's going on here?
6. How does RESORPTION or RETRANSLOCATION of nutrients by plants affect the quality
of plant litter?
7. What's the term for the ability of microbes to retain nutrients from decomposing
litter?
8. If microbes are making a living of eating pure CHO (e.g., sugar) within the soil,
where are they getting their nutrients?
9. Why are small molecules of organic carbon of "higher quality" to consumers? Why
is the net energy gain less when you 'eat' (think like a microbe) large molecules?
10. What's an exoenzyme, and why is the production of exoenzymes considered to be
"risky business"?
11. AET is a great predictor of NPP, but AET is only a pretty good predictor of
plant decomposition rates. Why the difference?
12. Why is lignin so recalcitrant?
13. Lignin contains only CHO compounds. Yet, when this material is finally converted
into humus, it contains almost 1 nitrogen atom for every 10-11 carbon atoms.
How does this happen?
14. What would be an estimate of the percent decay rate of raw litter in a temperate
grassland or temperate forest? (see http://culter.colorado.edu/~tims/ecosystem/decay.jpg
and subsequent notes that explain the figure).
15. What would be a reasonable percent decay estimate (percent per year loss) for
the most recalcitrant stuff found in the prairie or forest?
16. Decay in the tropics is relatively faster with respect to plant production than
is decay in the boreal forest or tundra. (this means the ratio of live plant/dead
stuff changes). Suggest some hypotheses why this might happen.
(Note added in preparation. One additional hypothesis is that recalcitrant materials
are simply more decomposable under higher temperatures because temperature alters the
three dimensional structure of these molecules in ways that favor their breakdown
under warm conditions. Why would a change in three dimensional structure be important?)
17a. In a temperate forest, litterfall is estimated at 1000 grams per sq. meter per year,
and carbon storage in the soil is estimated at 10,000 grams per sq meter. What's the
average decomposition rate (as a % of the soil organic matter pool?).
17b. If the production of this forest was increased to 2000 grams per sq. meter per year,
How much larger would the soil carbon pool grow IF decomposition remained unchanged?
18. What is anaerobic decomposition? Who is involved? List the elements or ions or materials
that provide the electron acceptors in anaerobic decomposition (see Table 3.4 of old or new text
for a list of these). What electron acceptors are 'preferred' in decomposition and what determines
this preference? (i.e., if you lived in an anaerobic environment, what material would you 'breathe'
and why would you choose this?)
Also, check out Figure 3.4 (pg 65 of text). This really helps explain the patterns!