Agriculture Reference
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directly cycled through earthworms and the contained nutrients released. Estimates
range from a few kilograms to released in temperate soils (equivalent
to 20 % of the total amount recycled annually). Differences may be due to variation in
earthworm abundance, but can also result from inaccuracies in the assumptions made
in estimating the sizes of those compartments that are difficult to measure. These include
such factors as the amounts of N released in urine and cutaneous mucus (Syers et al.,
1979; Keogh, 1979; Scheu, 1987; James, 1991). Most of the nutrients thus released have
originated from casts which have high concentrations of assimilable nutrients.
Turnover of C and N in earthworm biomass
The rapid turnover of C and N in the earthworm biomass is a striking feature of the biology
of these animals. Under laboratory conditions, the C and N of anecic earthworm bodies
may be turned over in a few months (Bouché and Ferrière, 1986; Cortez et al., 1989; Binet,
1993). Part of this N is released in casts as ammonium, but more may directly flow into
the soil, through nephridiopores as urine (Needham, 1957), readily assimilable cutaneous
mucus or decaying bodies. Laverack (1963) assumed that the amount of N contained in
urine as ammonium and urea represents about half of that excreted through the intestinal
wall and the tegument; the amount of N secreted in urine may therefore be greater than
that in cutaneous mucus. Few estimates of the influence of earthworm populations on soil
nitrogen cycling include such effects which may lead to the release of 10-20 kg mineral
together with a further similar amount released from decomposing dead
bodies. In particular, the chemical nature and flow of cutaneous mucus are still poorly
known (see Cortez and Bouché, 1987; Scheu, 1991) although they certainly play a major
role in the enhanced microbial activity observed, for example, on burrow walls.
Transfers of C and nutrients in the soil system
Anecic activity results in a rapid and complete mixing of litter with the mineral soil and
an accelerated organic matter turnover. Litter does not accumulate at the soil surface and
the humus formed is a neutral mull. In the presence of anecic earthworms, released
into the atmosphere during nuclear tests and incorporated in litter, has been thoroughly
mixed with the soil down to a depth of 18 cm. In the absence of earthworms no
was found below 10 cm (Stout and Goh, 1980). Field and laboratory experiments using
artificially-labelled grass litter deposited at the soil surface have confirmed the ability
of earthworms to rapidly incorporate litter into the soil (Binet and Curmi, 1992; Hameed
et al ., 1993). The half-time of apparent decomposition of litter of perennial
ryegrass ( Lolium perenne) deposited at the surface of a temperate soil was 5.7 weeks in
the presence of earthworms and 20 weeks in their absence (Dietz and Bottner, 1981).
Organic matter removed from the litter system through feeding by anecic earthworms
accumulates in drilosphere structures. Microcosm experiments have shown that, in the
presence of Lumbricus terrestris, three times more N from litter was incorporated into
the soil in comparison with a treatment without earthworms. Seventy-two per cent of
this earthworm-incorporated nitrogen was located within drilosphere structures
(Figure IV.49) (Binet, 1993). Another important effect was the favouring of bacterial
over fungal proliferation.
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