Principles of terrestrial ecosystem ecology.pdf

174 7. Terrestrial Decomposition
Methane emission from soils to the atmosphere
is of global concern. Methane is 20-fold
more effective in absorbing infrared radiation
than is CO2. Moreover, its concentration in the
atmosphere has risen dramatically in recent
decades, in part as a result of increased area of
rice paddies and reservoirs (see Fig. 15.3). Even
in wetlands, methane accounts for only 5 to
15% of the carbon released to the atmosphere
by decomposers. Methane is thus quantitatively
more important in its role as a greenhouse gas
than as a path of carbon loss from ecosystems
(see Fig. 6.8).
Methane is even more highly reduced than
are carbohydrates, so it is an effective energy
source for organisms that have access to
oxygen. Another group of bacteria (methanotrophs)
that occur in the surface soils of wetlands
use this methane as an energy source and
consume much of the methane before it diffuses
to the atmosphere. The enzyme system
that converts ammonium to nitrate also reacts
with methane, causing well-aerated soils to be
a net sink for methane. There are therefore
important transfers between methane producers
and consumers that occur both vertically
within poorly drained ecosystems and horizontally
from lowland to upland ecosystems.
Summary
Decomposition is the conversion of dead
organic matter into CO2 and inorganic nutrients
through the action of leaching, fragmentation,
and chemical alteration. Leaching
removes soluble materials from decomposing
organic matter. Fragmentation by soil animals
breaks large pieces of organic matter into
smaller ones that provide a food source for soil
animals and create fresh surfaces for microbial
colonization. Fragmentation also mixes the
decomposing organic matter into the soil.
Chemical alteration of dead organic matter is
primarily a consequence of the activity of bacteria
and fungi, although some chemical reactions
occur spontaneously in the soil without
microbial mediation.
Decomposition rate is regulated by physical
environment, substrate quality, and the compo-
sition of the microbial community (including
soil animals). Carbon chemistry is a strong
determinant of litter quality; labile substrates,
such as sugars and proteins, decompose
more rapidly than recalcitrant ones, such as
lignin and microbial cell walls. Nitrogen and
phosphorus supply can also constrain the
decomposition of labile carbon substrates, such
as agricultural residues and root exudates.
Plants in high-resource environments produce
litter with high litter quality and therefore rapid
decomposition rates. Decomposition rate
declines with time, as recalcitrant substrates are
depleted. Soil animals strongly influence
decomposition by fragmenting litter, consuming
soil microbes, and mixing the litter into
mineral soil. The environmental factors that
favor NPP (warm, moist, fertile soils) also
promote decomposition so there is no clear
relationship between the amount of carbon that
accumulates in soils with either NPP or decomposition
rate.
Review Questions
1. What is decomposition, and why is it important
to the functioning of ecosystems?
2. What are the three major processes that
give rise to decomposition? What are the
major controls over each of these
processes? Which of these processes is
directly responsible for most of the mass
loss from decomposing litter?
3. What are the major similarities and differences
between bacteria and fungi in the
ways in which they decompose dead
organic matter? How do these two groups
of decomposers differ in their response to
moisture and nutrients? Why?
4. What roles do soil animals play in decomposition?
How does this role differ between
protozoans and earthworms?
5. Why do decomposer organisms secrete
enzymes into the soil rather than breaking
down dead organic matter inside their
bodies?
6. What chemical traits determine the quality
of soil organic matter? How do carbon
quality and the C:N ratio differ between