Agriculture Reference
In-Depth Information
allow for estimates of the amounts and types of C that need to be consumed by earthworms to
support their populations. Very little information is available on the C and N assimilation efficiencies
of earthworms, and there are no data for a wide range of common earthworm species. Carbon
assimilation efficiency is assumed to be quite low (e.g., 2 to 6%) (Bolton and Phillipson 1976),
but this remains to be confirmed for organic matter of different quality and for different earthworm
species.
There is also a need to determine the N assimilation efficiencies of earthworms, particularly
relating to their role in the N cycle (Binet and Trehen 1992). Although there is certainly enough
N in the soil to satisfy the demand by earthworms for N, much of it is considered unavailable.
Nevertheless, earthworms excrete copious amounts of N in their urine and mucus, and N turnover
appears to be quite rapid (Ferriere and Bouch 1985; Barois et al. 1987; Binet and Trehen 1992).
Therefore, earthworms do not appear to conserve N efficiently, which suggests that they either
ingest materials with high N content selectively or have very high N assimilation efficiencies. Using
a budget approach with
N, Binet and Trehen (1992) calculated an N assimilation efficiency of
27%, much higher than estimates of C assimilation efficiencies. A more detailed quantification of
earthworm assimilation efficiencies was done using two species of field-collected earthworms in
laboratory experiments with
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N-labeled soybean and rye litter (Whalen and Parmelee 1999). The
N assimilation efficiency determined by this method ranged from 10 to 26% for
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Aporrectodea
tuberculata
, which is very close to the assimilation efficiencies
determined by Binet and Trehen using a budgetary approach. As with C, assimilation rates of N
need to be determined for a wide range of earthworm species that occupy different ecological
niches and for food resources of different quality. It is likely that the absence of research on these
critical areas has been hampered by a lack of suitable methodologies, but the use of stable isotopes
has considerable promise for quantifying C and N assimilation efficiencies (Whalen and Janzen
2002, 2003; Schmidt et al. 2003).
The nitrogenous compounds that earthworms excrete in their urine and mucus may provide a
particularly labile source of N for soil microorganisms. Earthworm urine is composed primarily of
ammonium and urea, and the mucus is composed of mucoproteins with a low C:N ratio of 3.8
(Scheu 1991). There are few realistic estimates of rates of urine or mucus excretion by earthworms.
Early estimates of urine and mucus excretion rates were based on the method of Needham (1957),
who placed earthworms in a small volume of water and analyzed the water for N content after 24
hours. Whether such estimates are realistic remains to be confirmed. Nevertheless, it appears that
the amounts of N excreted in earthworm urine and mucus could be substantial (Dash and Patra
1979; Lee 1983).
In our chapter in the first edition of this topic (Parmelee et al. 1998), we suggested that
and 25 to 30% for
L. terrestris
N
would be a useful tool in tracer studies for quantifying earthworm N excretion rates; in fact, this
tool has been applied to N excretion studies since that time. Nitrogen excretion rates of 274 to 744
mg N g
15
N-labeled earth-
worms were incubated for 48 hours in unlabeled soil (Whalen et al. 2000). Combining these
excretion rates with data for earthworm population and biomass data from the field resulted in
estimates of total annual N excretion of 41.5 kg N ha
fresh earthworm tissues were estimated from experiments in which
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1
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in a corn agroecosystem in Ohio. That
estimate was equivalent to 22% of crop uptake and compared well with an estimate of 30 to 40
kg N ha
−
1
year
excreted by a population of
L. terrestris
in temperate woodland (Satchell 1963)
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1
−
1
and 29 to 36 kg N ha
N turnover in temperate grassland (Curry et al.
1995). Although results from labeling experiments must carefully consider assumptions used to
calculate N excretion and turnover (Schmidt et al. 2003; Whalen and Janzen 2003), this approach
still holds considerable promise.
Another particularly intriguing area of research is the microbiology of the earthworm gut.
Microbial processes in the earthworm gut may be important in increasing the assimilation efficien-
cies of C and N. Barois and Lavelle (1986) first proposed the hypothesis that earthworms and soil
microorganisms have developed a mutualistic relationship to exploit soil organic matter reserves
year
estimated from
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1
−
1
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