Agriculture Reference
In-Depth Information
palatability (Edwards and Heath 1963; Satchell and Lowe 1967). The presence of toxins can also
have adverse effects; for example, the high mortality of juvenile
feeding in soil mixed
with fresh lucerne residues was attributed to the glucoside saponin (Bostrm and Lofs-Holmin
1986; Bostrm 1988). Fresh litter is usually not acceptable to earthworms but must undergo a
period of weathering before it is eaten. Factors involved include leaching of feeding inhibitors,
softening of hard tissues, and microbial degradation (Satchell and Lowe 1967; Wright 1972). The
role of the soil microflora is important not only in increasing palatability, but also in enhancing
nutrient content (Waters 1951; Wright 1972; Cooke and Luxton 1980). This is particularly important
in the case of lignified, nutrient-poor litter, such as straw, which undergoes a marked increase in
nitrogen content when incubated in the soil (Bostrm 1987; Curry and Byrne 1997). The importance
of microbial action in enabling endogeic earthworms to persist in tropical soils, which are low in
A. caliginosa
organic matter, through mutualistic associations with ingested soil microflora has also been estab-
lished (Lavelle et al. 1989).
Litter ingestion rates are sometimes used as a basis for assessing limitations likely to be
imposed on earthworm populations by the food supply; however, ingestion rates tend to be very
variable, reflecting differences in food quality and palatability. For this reason, rates of tissue
production per unit litter input may be more useful. Data from a number of studies, mostly
conducted in the laboratory under favorable conditions but including some field studies (Andersen
17 to 255 mg fresh mass per gram dry mass of ingested food in the case of
A. caliginosa
,
representing production-to-consumption ratios (P/C) of 0.3 to 4.3% on a dry mass basis. The
corresponding values for
L. terrestris
were 82 to 251 mg g
−
1
Bostrm and Lofs-Holmin (1986) included data for earthworm growth rates corrected for growth
in unamended soil; this was not necessary in the case of the Curry and Bolger (1984) and Boyle
(1990) studies because earthworms lost weight in the unamended peat or peat-mineral soil media
used. Earthworms grew faster on barley and lucerne residues that had been buried in the soil for
1 to 3 months prior to the start of the feeding trials (Bostrm 1987), but this pretreatment had
no effects on growth performance on milled
Festuca
species residues, which presumably were
acceptable with little or no weathering.
Table 6.3
summarizes data from a 3-year study of the earthworm populations in a winter cereal
field at Lyons, County Kildare, Ireland (Curry et al. 1995); the estimated food requirement of the
population is compared with the food supply in
Table 6.4
. There were 12 earthworm species
recorded; the most abundant were
A. chlorotica
(60 to 65% of adult numbers) and
A. caliginosa
(19 to 25% of adults).
TABLE 6.3
Tissue Production and Nitrogen Requirements of an
Earthworm Population in a Winter Cereal Field
1988
1989
1990
Mean population density (numbers m
Ï2
)
346
471
353
Mean biomass (gm
Ï2
)
61
57
59
Tissue production (gm
Ï2
)
68Ï156
87Ï210
89Ï137
Nitrogen requirement for production (gm
Ï2
)
1.2Ï2.8
1.5Ï3.8
1.6Ï2.5
Nitrogen loss via excretion and the like (gm
Ï2
)
3.6
3.3
3.0
Source
: Curry et al. 1995.
