Agriculture Reference
In-Depth Information
The amounts of unbound organic matter decreased when earthworms were abundant and
increased when earthworm populations were at low densities. This is evidence that during the cycle
of growth, maturation, and senescence of the forest, the humus type changed, with maximum
development of a moder after 60 years and a mull after 10 years. We hypothesize that a forest
accumulates organic matter as litter and raw humus during its early phases of growth, when primary
production is high. Then, earthworm populations start to develop at the expense of these organic
accumulations, and they progressively incorporate the nondigested part of this raw organic matter
into organomineral complexes (
Figure 8.7
)
.
This process results in the release of large quantities of nutrients and the creation of physical
soil structures (macroaggregates, macropores, and galleries) typical of a mull-type humus soil. This
is believed to be favorable for the establishment and growth of seedlings. Processes by which
earthworm populations establish and the reasons why earthworms become able to live in what was
previously an acid environment have not yet been identified.
Recent cases of invasion of exotic lumbricid earthworms in northeast United States and Canada
provide rather similar situations. There, earthworms are able to feed on organic matter accumulated
in these forests, decrease the overall organic matter content, and accelerate the turnover of C and
N, increasing microbial biomass and changing microarthropod communities (see
Chapter 5
this
volume).
DISCUSSION
The long-term effects of earthworms on SOM dynamics vary depending on the scale of time
considered. When earthworms are introduced artificially into an ecosystem, they use part of the C
resources for their activities. In the African savannahs of Lamto (Cte dÔIvoire), the amount of C
mineralized directly through earthworm respiration was estimated as 1.2 t C ha
years in a grass
savannah, which is equivalent to about 5% of overall primary production (Lavelle 1978). The annual
average population densities and biomass of earthworms were 202 individuals and 39.7 g m
−
1
,
−
2
respectively, and this population ingested up to 1000 to 1250 t dry soil ha
year
. As part of this
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process, nutrients were released and made available to plants or microorganisms.
In the same savannah, the overall amounts of assimilable N, released as ammonium in feces,
or labile organic N in dead earthworms and mucus were estimated at 21.1 to 38.6 kg ha
year
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that comprised 60% of the overall population
biomass. The overall production of mineral N for the earthworm community is therefore expected
to range from 30 to 50 kg ha
of NH
-N in a population of
Millsonia anomala
4
year
. In tropical pastures, with earthworm biomass of 1 to 3 t ha
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and soils with higher contents of organic nitrogen, the contribution of earthworms to N mineral-
ization may probably reach a few hundred kilograms mineral N ha
. In temperate pastures,
the flux of mineral N through earthworms may be estimated as a few hundred kilogram ha
year
−
1
−
l
year
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(Syers et al. 1979; Hameed et al. 1994). Similar processes have been observed relating to P turnover,
but no actual estimates of amounts of P released ha
have been produced (Sharpley and
Syers 1976; Lopez-Hernandez et al. 1993; Brossard et al. 1996). There is some evidence that plants
may accumulate these nutrients, but the exact proportion, especially on a yearly basis, is not known
(Spain et al. 1992; Hameed et al. 1994).
Increased nutrient turnover from earthworm activities usually results in increased plant growth.
Most experiments on the scale of one to six successive cropping cycles showed significant positive
effects of earthworm activities on plant productivity; these effects seem to be proportional to the
earthworm biomass, within a limited range of biomass (Lavelle 1997). Whether this increased
production is sustained in the long term is not known. On one hand, earthworms tend to feed on
existing stocks of almost undecomposed organic matter and accelerate their decomposition. Once
these stocks are depleted, earthworm activities may cease, and the system returns to lower levels
of plant production.
year
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