Principles of terrestrial ecosystem ecology.pdf

down may also function in the formation and
breakdown of soil humus.
Long-Term Storage of Soil
Organic Matter
In climates that are favorable for decomposition,
humus is the major long-term reservoir of
soil carbon. Up to this point, we have focused
primarily on the factors controlling the breakdown
and loss of soil organic matter. Equally
important are the processes that transform soil
organic matter into relatively recalcitrant
humus, allowing its accumulation in soils. Soil
humus decomposes slowly for several reasons.
As with lignin, its highly irregular structure is
not efficiently attacked by a single enzyme
system. Its large size and highly cross-linked
form make most of the structure inaccessible to
soil enzymes.
Its tendency to bind with soil minerals protects
it from enzymatic attack. Much of the
SOM in soil is therefore not good “food” for
microbes, despite its high nitrogen content.
Humus also constitutes a large reservoir of
nitrogen in many ecosystems. This nitrogen
turns over extremely slowly, except when disturbance
increases the rate of humus decomposition.
As the carbon in humus is respired
away, the nitrogen is released, providing an
important nutrient source to support ecosystem
recovery after disturbance (see Chapter 13).
The sensitivity of humus to breakdown following
disturbance makes ecosystems with large
humus accumulations, such as tropical forests
or grasslands, particularly vulnerable to carbon
loss after such changes.
The formation of humus by humification
occurs through a combination of biotic and
abiotic processes (Zech and Kogel-Knabner
1994). The following five steps have been implicated
in humus formation (Fig. 7.12), although
the relative importance of factors governing
these steps is poorly understood.
1. Selective preservation. Decomposition
selectively degrades labile compounds in detritus,
leaving behind recalcitrant materials like
waxes, cutins, suberin, lignin, chitin, and
Long-Term Storage of Soil Organic Matter 169
Labile
compounds
2
Microbial
biomass
Plant litter
Phenolics Lignin,
waxes, etc.
3 1
Poly-
3 phenols
4 1
1
Quinones
Amino
compounds
5
Humus
1 Selective preservation
2 Microbial transformation 4 Quinone formation
3 Polyphenol formation 5 Abiotic condensation
Figure 7.12. Principle pathways of humus formation.
See text for details.
microbial cell walls. Partial microbial breakdown
of these recalcitrant leftovers often
produces compounds with reactive groups and
side chains that are common reactants in the
nonspecific soil reactions that occur during
humification.
2. Microbial transformation. Enzymatic
breakdown of SOM produces low molecular
weight water-soluble products, some of which
participate in humus formation. Amino compounds,
such as amino acids from protein
breakdown and sugar amines from degradation
of microbial cell walls, are particularly important
in humification (see Step 5).
3. Polyphenol formation. Soluble phenolic
compounds are important reactants in humus
formation.They come from at least three sources
(Haynes 1986): microbial degradation of plant
lignin, the synthesis of phenolic polymers by soil
microbes from simple nonlignin plant precursors,
and polyphenols produced by plants as
defenses against herbivores and pathogens.
4. Quinone formation. The polyphenol
oxidase and peroxidase enzymes produced
by fungi to break down lignin and other
phenolic compounds also convert polyphenols
into highly reactive compounds called quinones
(Fig. 7.13).
5. Abiotic condensation. The quinones spontaneously
undergo condensation reactions with
many soil compounds, especially compounds