Agriculture Reference
In-Depth Information
better. As soil enters the earthworm gut, moisture levels, pH, and water-soluble C content increase.
The soil is then mixed in the gizzard, followed by a significant increase in microbial activity. Water
and solutes are reabsorbed in the hindgut, and soil is egested with the C content reduced by as
much as 19%. The ÑprimingÒ of the microbial community with mucus leads to more complete and
efficient extraction of nutrients. The commonly observed burst of microbiological activity that
occurs in freshly deposited earthworm casts may merely be an after effect of the earthworm gut
processes. Although there is increasing evidence that this hypothesis is correct for both temperate
and tropical earthworms, research into earthworm gut microbiology will continue to provide
valuable information on how earthworms affect nutrient cycling processes. Studies of the micro-
biology, and nutrient dynamics of casts from different species of earthworms will be another fruitful
research area.
E
P
ARTHWORM
OPULATIONS
Earthworm population size, growth, reproduction, and mortality all have significant consequences
for C and N cycling, and estimates of population density and biomass in various habitats remain
an important area of research. Although there have been many surveys on earthworm populations
from a wide variety of ecosystems, many of these studies suffer from shortfalls such as inadequate
sampling frequency and reliance on qualitative sampling techniques. A problem that makes it
difficult to compare earthworm biomass estimates from different sites and studies is the inconsis-
tency with which earthworm population biomass is reported. The majority of population studies
report biomass as either fresh or dry weight, often with no indication of whether gut contents are
included in the reported values; this makes it difficult to compare data from different studies. It
would be helpful if all studies expressed earthworm population biomass as ash-free dry weight
(AFDW; earthworms ashed at 500
C for 4 hours) per unit area. It is also appropriate to express
biomass as grams C per unit area. As part of a continued effort to quantify earthworm biomass,
we also need to develop long-term data sets to evaluate temporal variability in earthworm population
A
size. And, as Lavelle et al. suggest (
Chapter 8
of this volume), greater attention needs to be paid
to the spatial or ÑpatchÒ dynamics of earthworm populations.
Earthworm population growth and reproduction rates are key to determining the amounts of C
and N that flow directly through earthworm tissues. Unfortunately, earthworm growth rates are
unknown for many species and conditions, and those that are available are often based on laboratory
experiments under different optimum environmental conditions. Growth rates need to be determined
under field conditions for a range of earthworm species. Similarly, very little is known about rates
of earthworm cocoon production in the field. Although cocoon biomass is only a small fraction of
the total earthworm biomass, knowledge of the rates of cocoon production can provide information
about overall earthworm reproductive rates and can indicate periods of high earthworm activity
and the potential rapid turnover of earthworm tissue.
Earthworms ingest organic matter with relatively wide C:N ratios and convert it to earthworm
tissues of lower C:N ratios (Syers and Springett 1984). In effect, this accelerates the cycling of
nutrients in soil, particularly N. There is evidence from field studies of selective feeding by
earthworms on organic materials with low C:N ratios, thereby leaving behind a pool of organic
material with a higher C:N ratio (Bohlen et al. 1997; Ketterings et al. 1997). Although some
earthworm N is excreted in urine and mucus, significant amounts are also returned to the soil in
the form of dead tissues. Because earthworm tissue is highly labile, dead earthworms can be an
important input of N into the soil. Satchell (1967) reported that over 70% of the N in dead earthworm
tissue was mineralized in less than 20 days, and that 60 to 70 kg of N ha
was returned to the soil
annually. Christensen (1988) also reported high seasonal inputs of N from dead earthworms.
Whalen et al. (1999) used
−
1
N-labeled earthworms in a microcosm experiment to trace the
movement of N from dead earthworms into the soil and its eventual uptake by plants and soil
microbes. Two days after earthworms were incorporated into the soil, 40% of earthworm N was
15
