Principles of terrestrial ecosystem ecology.pdf

microbial attack by lignin-impregnated cell
walls. Fragmentation of litter greatly enhances
microbial decomposition by piercing these protective
barriers and by increasing the ratio of
litter surface area to mass.
Animals are the main agents of litter
fragmentation, although freeze–thaw and
wetting–drying cycles can also disrupt the cellular
structure of litter. Animals fragment litter
as a by-product of their feeding activities. Bears,
voles, and other mammals tear apart wood
or mix the soil as they search for insects,
plant roots, and other food. Soil invertebrates
fragment the litter to produce particles that
are small enough to ingest. Enzymes in animal
guts digest the microbial “jam” that coats the
surface of litter particles, providing energy
and nutrients to support animal growth and
reproduction.
Chemical Alteration
Fungi
Fungi are the main initial decomposers of terrestrial
dead plant material and, together with
bacteria, account for 80 to 90% of the total
decomposer biomass and respiration. Fungi
have networks of hyphae (i.e., filaments that
enable them to grow into new substrates and
transport materials through the soil over distances
of centimeters to meters). Hyphal networks
enable fungi to acquire their carbon in
one place and their nitrogen in another, much
as plants gain CO2 from the air and water and
nutrients from the soil. Fungi that decompose
litter on the forest floor, for example, may
acquire carbon from the litter and nitrogen
from the mineral soil. Fungi are the principal
decomposers of fresh plant litter, because they
secrete enzymes that enable them to penetrate
the cuticle of dead leaves or the suberized exterior
of roots to gain access to the interior of a
dead plant organ. Here they proliferate within
and between dead plant cells.At a smaller scale,
some fungi gain access to the nitrogen and
other labile constituents of dead cells by breaking
down the lignin in cell walls. This energy
investment in lignin-degrading enzymes serves
Chemical Alteration 153
primarily to gain access to the relatively labile
contents of the interior of cells.
Fungi produce hyphae with a dense concentration
of cytoplasm when there is adequate
substrate to support growth. The hyphae
contain more vacuoles (and proportionally less
cytoplasm) when resources are scarce.This flexible
growth strategy enables fungi to grow into
new areas to explore for substrate, even when
current substrates are exhausted. A substantial
proportion (perhaps 25%) of the carbon and
nitrogen used to support fungal growth are
transported from elsewhere in the hyphal
network, rather than being absorbed from the
immediate environment where the fungal
growth occurs (Mary et al. 1996).
Fungi have enzyme systems capable of
breaking down virtually all classes of plant
compounds.They have a competitive advantage
over bacteria in decomposing tissues with low
nutrient concentrations because of their ability
to import nitrogen and phosphorus. White-rot
fungi specialize on lignin degradation in logs,
whereas brown-rot fungi cleave some of the
side-chains of lignin but leave the phenol units
behind (giving the wood a brown color).Whiterot
fungi are generally outcompeted by more
rapidly growing microbes when nitrogen is
abundant, so nitrogen additions have little
effect (or sometimes a negative effect) on
white-rot fungal decomposition of wood.
Fungi account for 60 to 90% of the microbial
biomass in forest soils, where litter frequently
has a high lignin and low nitrogen concentration.
They have a competitive advantage at low
pH, which is also common in forest soils. Fungi
make up about half the microbial biomass in
grassland soils, where pH is higher and wood is
absent. Most fungi lack a capacity for anaerobic
metabolism and are therefore absent
from or dormant in anaerobic soils and aquatic
sediments.
Mycorrhizae are a symbiotic association
between plant roots and fungi in which the
plant gains nutrients from the fungus in return
for carbohydrates (see Chapter 8). Although
mycorrhizal fungi get most of their carbon from
plant roots, they can also play a role in decomposition
by breaking down proteins into amino
acids, which are absorbed; amino acids both