Agriculture Reference
In-Depth Information
Earthworm removal
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FIGURE 8.5
Changes of carbon contents in soils of a grass savanna at Lamto (Cte dÔIvoire) in the presence
and absence of endogeic earthworms; results of a simulation using the CENTURY model. (From A. Martin
and W.J. Parton unpublished data, 1993.)
years, the largest proportion lost in the slow pool that includes physically protected organic matter
(Figure 8.5).
This suggests that slow decomposition rates of soil C may be influenced significantly by
earthworm activities. This pool would comprise organic matter that binds microaggregates into
macroaggregates (Elliott 1986), which is generally lost during cultivations. Earthworms may play
an important role in stabilizing SOM, hence maintaining the SOM stock and soil structure in
agroecosystems in the longer term.
E
A
S
P
ARTHWORM
CTIVITIES
AND
UCCESSIONAL
ROCESSES
Successional processes in vegetation dynamics such as those observed in natural forests may precede
or follow significant changes in the organic status of soils. Several examples indicate that earthworm
activities during these successions vary significantly (Miles 1985; Scheu 1992) and may be limited
to periods when the organic matter that they can digest is available. Sampling soil invertebrate
populations in a diachronic series of hevea plantations in the Cte dÔIvoire showed great changes
in the soil faunal communities as plantations aged (Gilot et al. 1995).
During the early stages of succession, soil invertebrate communities were dominated by
termites, especially xylophagous species. After a few years, the abundance of this group of
termites declined, and other groups of termites dominated the termite communities. After 20
years, earthworms became dominant; mesohumic and endogeic species categories prevailed.
Finally, in a 30-year plantation, soil invertebrate populations decreased steadily, as did the
productivity of the hevea.
It has been suggested that these changes could reflect successions in soil invertebrate commu-
nities after changes in the quality and quantity of organic matter. When the plantations were created,
woody material from the primary forest was left on the soil surface. Xylophagous termites were
the first invertebrate group that used this resource. They transformed decaying wood into fecal
pellets that may have been used later by other groups of termites and surface-living earthworms.
Eventually, fecal pellets of humivorous termites may have been incorporated into the soil and been
used as food by endogeic mesohumic and oligohumic species of earthworms. Once the organic
matter from the wood had passed through this food chain and lost most of its energy, which was
stored as carbon compounds, the food resource base for soil invertebrates was reduced to the plant
residues currently available in the hevea plantation, and invertebrate populations decreased drasti-
cally. Interestingly, this sharp decrease in invertebrate populations coincided with a reduction in
productivity of the hevea.
