Agriculture Reference
In-Depth Information
Soil enzymes
. Soil organisms “digest” substrate molecules through the action
of enzymes, which can act inside or outside the cell. But there are also soil en-
zymes that exist independently of living organisms. They avoid denaturation and
degradation by being adsorbed on soil mineral or organic matter. The major types
present are hydrolases, transferases, oxidases, reductases, and decarboxylases. One
very common and stable soil enzyme is
urease
, which is important in the hydrol-
ysis of urea (section 5.4.1.2).
2.3.2.2
The Reducers
By far the most important reducers are the
arthropods
and
annelids
. Nematodes
and molluscs are also reducers.
Arthropods
. This group includes the wood lice, mites and springtails, insects
(adults and larvae), centipedes and millipedes. Many
mites
and
springtails
feed on
plant residues and fungi in the litter, especially where thick mats build up under
forests and undisturbed grassland. Their droppings appear as characteristic pellets
in the litter layer (fig. 2.13). Predatory adult mites and springtails feed on other
mites and nematodes, whereas the juveniles feed on bacteria and fungi. Mites can
be a pest of grapevines.
Many
beetles
and
insect larvae
live in the soil, some feeding saprophytically,
whereas others feed on living tissues and can be serious crop pests. Coprophagous
(or dung-eating) beetles are important in breaking down the dung of large herbi-
vores, especially in warmer climates. Their action greatly increases the rate at which
such residues are mixed with the soil.
Termites
are also important in tropical and
subtropical regions because they attack all forms of litter—tree trunks, branches,
and leaves—under forests and especially in grasslands that receive seasonal rain-
fall (savannas). In cool temperate regions, various species of ant are of some im-
portance in breaking down litter under pine forests and grassland.
Centipedes
are
carnivorous, feeding on other small animals, whereas
millipedes
feed on vegeta-
tion, much of which is in the form of living roots, bulbs, and tubers.
Annelids
.
Earthworms
are the most important group of annelids because of
their size and physical activity. Under favorable conditions, they are more im-
portant in consuming leaf litter than all the other invertebrates put together. The
Figure 2.13
Fecal pellets of mites in a
litter layer (White 1997).
Reproduced with
permission of Blackwell
Science Ltd.