Agriculture Reference
In-Depth Information
species are limited mainly to the plant-litter layer on the soil surface, decaying organic matter or
wood, and seldom penetrate soil more than superficially. The main role of these species seems to
be comminution of the organic matter into fine particles, which facilitates microbial activity.
Other species live just below the soil surface most of the year, except when the weather is very
cold or very dry; do not have permanent burrows; and ingest both organic matter and inorganic
materials. These species produce organically enriched soil materials in the form of casts, which
they deposit either randomly in the surface layers of soil or as distinct casts on the soil surface.
Finally, there are the truly soil-inhabiting species with permanent burrows that penetrate deep into
the soil. These species feed primarily on organic matter but also ingest considerable quantities of
inorganic materials and mix these thoroughly through the soil profile. These last species are of primary
importance in pedogenesis. All species depend on consuming organic matter in some form and play
an important role in the final stages of organic matter decomposition, which is humification into complex
amorphous colloids containing phenolic materials, probably by promoting microbial activity.
There is little doubt that, in many ecosystems, earthworms are the key organisms in the
breakdown of plant organic matter. Populations of earthworms usually expand in relation to the
availability of organic matter; in many temperate and even tropical forests, it seems that earthworms
have the capacity to consume the total annual litter fall. Such a total turnover has been calculated
for an English mixed woodland (Satchell 1967), an English apple orchard (Raw 1962), a tropical
forest in Nigeria (Madge 1965), and an oak forest in Japan (Sugi and Tanaka 1978); it seems likely
that similar calculations would be valid for other sites (Edwards and Bohlen 1996). There is current
speculation that invasions of lumbricids into North American forests are changing them dramatically
and having an impact on rates of organic matter turnover and soil cover (see
Chapters 5
,
8
,
9
, and
13
, this volume).
N
A
UTRIENT
VAILABILITY
During feeding by earthworms, the carbon:nitrogen ratio in the organic matter falls progressively;
moreover, most of the nitrogen is converted into the ammonium or nitrate form. At the same time,
the other nutrients, phosphorus and potassium, are converted into a form available to plants. Soils
that have poor populations of earthworms often develop a structure with a mat of decomposed
organic matter at the soil surface (Kubiena 1953); this can also occur in grassland and is common
on poor upland grasslands in temperate countries and in New Zealand in areas where earthworms
have not yet been introduced (Stockdill 1966) (see
Chapters 6
and
14
, this volume).
EFFECTS OF AGRICULTURE ON EARTHWORMS
Earthworm populations are affected greatly by many of the main agricultural practices; in particular,
cultivations, fertilizers, pesticides, and crop rotations exert major effects on earthworm activities
and communities.
have considerable effects on earthworm communities, particularly those species
with deep burrows. A single cultivation does not have any drastic effects on earthworm populations
other than by mechanical damage, destruction of permanent burrows, and exposure to bird predators.
However, repeated heavy cultivations progressively diminish earthworm populations. No till (direct
drill) and a variety of conservation tillage practices, such as ridge tillage and shallow plow, favor
the buildup of larger earthworm populations that are limited only by the availability of food
(Edwards and Lofty 1982a; Edwards and Bohlen 1996).
Cultivations
can be either organic or inorganic, including a broad range of organic manures from
sources such as cattle, pigs, poultry, sewage wastes, and wastes from industries such as those
involving a brewery, paper pulp, or frozen potatoes. These materials are major factors in the buildup
of large field earthworm populations; when such organic wastes are added to agricultural land,
earthworm populations may double or triple in a single season. Some liquid manures that have not
Fertilizers
