A student sets out four heaps of dead leaves in a garden…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student sets out four heaps of dead leaves in a garden.
Heap P: leaves rich in nitrogen and sugars, in a warm, moist spot.
Heap Q: leaves rich in lignin, in the same warm, moist spot.
Heap R: leaves rich in nitrogen and sugars, in a cold, waterlogged spot with no air.
Heap S: leaves rich in lignin, in the same cold, waterlogged spot.
- Which heap decomposes fastest, and which slowest?
- Compare P and Q. Which is faster, and why?
- Compare P and R. Which is faster, and why?
- What builds up in R and S over the years?
Need a reminder? The key ideaShow
The five steps of decomposition are fragmentation, leaching, catabolism, humification and mineralisation. Lignin and chitin slow decomposition. Nitrogen and sugars speed it up. Warmth, moisture and oxygen favour it. A model: , with half-life .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) P is the fastest. It has a rich, easy substrate and a good climate. S is the slowest. It has a tough substrate and a bad climate.
Step 2. (b) P is faster than Q. The leaves of P are rich in nitrogen and water-soluble sugars, and Q is rich in lignin, which decomposes slowly.
Step 3. (c) P is faster than R. R is cold, and it has anaerobiosis, because it has no oxygen. Low temperature and anaerobiosis slow decomposition.
Step 4. (d) Undecayed organic matter builds up, as in a bog.
Answer: (a) P fastest, S slowest; (b) P, because of nitrogen and sugars; (c) P, because R is cold and has no oxygen; (d) undecayed organic matter.
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