Reading 2
Compared to the atmosphere, soil is a place where temperature fluctuations are small and slow.
Consequently, soil animals are generally intolerant to sudden temperature changes and may not function
well over a very wide range. That’s why leaving bare earth exposed to the hot summer sun often slows plant
growth and why many thoughtful composters either put down a thin mulch in summer or try to rapidly
establish a cooling leaf canopy to shade raised beds. Except for a few microorganisms, soil animals breathe
oxygen just like other living things and so are dependent on an adequate air supply. Where soil is airless
due to compaction, poor drainage, or large proportions of very fine clay, soil animals are few in number.
The soil environment is generally quite moist, and even when the soil seems dry, the relative humidity
of soil air usually approaches 100 percent. Soil animals consequently have not developed the ability to
conserve their body moisture and are speedily killed by dry conditions. When faced with desiccation, they
retreat deeper into the soil if there is oxygen and pore spaces large enough to move about. So we see
another reason why a thin mulch that preserves surface moisture can greatly increase the beneficial
population of soil animals. Some single-cell animals and roundworms are capable of surviving stress by
encysting themselves, forming a little “seed” that preserves their genetic material and enough food to
reactivate it, coming back to life when conditions improve. These cysts may endure long periods of severe
freezing and sometimes temperatures of over 150 degrees Fahrenheit. Inhabitants of leaf litter reside close to the surface and so must be able to experience exposure to dryer
air and light for short times without damage. These are called primary decomposers. They spend most of
their time chewing on the thick reserve of moist leaves contacting the forest floor. Primary decomposers are
unable to digest the entire leaf. They extract only the easily-assimilated substances from their food:
proteins, sugars and other simple carbohydrates and fats. Cellulose and lignin are the two substances that
make up the hard, permanent, and woody parts of plants； these materials cannot be digested by most soil
animals. Interestingly, there are a few larvae whose digestive tract contains cellulose-decomposing bacteria
but these larvae have little overall effect.
By the time the primary decomposers are finished, the leaves have been mechanically disintegrated
and thoroughly moistened, worked over, chewed to tiny pieces and converted into minuscule bits of moist
excrement still containing active digestive enzymes. Many of the bacteria and fungi that were present on the
leaf surfaces have passed through this initial digestion process alive or as spores waiting and ready to
activate. Digestive wastes of primary decomposers are thoroughly inoculated with microorganisms that can
consume cellulose and lignin. Even though it looks broken down, it has not yet fully decomposed. It does
have a water-retentive, granular structure that facilitates the presence of air and moisture throughout the
mass creating perfect conditions for microbial digestion to proceed. Both secondary and primary
decomposers are necessary to complete the composting process.
【題組】36. According to the reading, which of the following is true about primary decomposers?
(A) Most of them are unable to digest cellulose and lignin.
(B) Most have a digestive tract that contains cellulose-decomposing bacteria.
(C) They can encyst themselves to camouflage.
(D) They desiccate leaves, creating mulch.
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