Agriculture Reference
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sieved, non-ingested control soil incubated under similar conditions (Figure IV.57).
After one year, the total amount of SOM mineralised in casts of
Millsonia anomala
in
a sandy alfisol was 30 % less than that of the control and mineralisation rate was highly
limited. Similar results have been obtained with European Lumbricidae, namely
the endogeic species
Aporrectodea caliginosa
and the anecic
Lumbricus terrestris
(McInerney and Bolger, 1996), Physical protection probably occurs in the massive
structure of compact casts and it is likely that SOM dynamics in the thin casts of
'de-compacting' species would be different. No data currently exist to test this hypothesis.
Longer-term effects.
At scales of several years to decades, the effects of earthworms
on SOM dynamics are determined by the relative importances of short-term acceleration
of mineralisation during gut transit, and further effects in ageing casts. There is some
evidence that earthworms accelerate the overall turnover of SOM while the total soil
stock remains smaller but is, on average, more accessible. In a 7-year experiment in
a continuous maize crop, inoculation of the endogeic earthworm
P. corethrurus
increased the loss of C due to cropping by and decreased the rate of
incorporation of new C from the crop. Earthworms significantly decreased the proportion
of C in coarse organic fractions and increased C in the smallest
fraction. In a similar experiment conducted on a sandy alfisol at Lamto (Côte d'Ivoire),
the rate of incorporation of new C from the crop residues was increased from 2.9 to
6.2 % (Villenave
et al
.,1999).
Experiments comparing the functioning of soil in the presence and absence of earth-
worms have never been conducted for periods of more than a few months to years.
Consequences of the long-term accumulation in soils of earthworm biogenic structures
can only be assessed through modeling. Two attempts have been made using the
CENTURY model of soil organic matter dynamics of Parton
et al.
(1983). In the first
