Summer 1994 (v6n3)
Robert L. Bugg
Article written for Sustainable
Agriculture/Technical Reviews. 1994
Earthworms are increasingly
recognized as indicators of agroecosystem health and as important tools
for ensuring soil improvement and efficient nutrient cycling. In the past,
SAREP has highlighted both historical and recent research on earthworms
(see Sustainable Agriculture News 3(1): 5,11 and Components
1(4):6-9). The literature has since proliferated rapidly; here we present
additional findings from more recent or underexposed research, and include
observations from an ongoing demonstration project, Biologically Integrated
Orchard Systems: BIOS (see Sustainable Agriculture 5(5):2).
Tillage Effects
Parmelee et al. (1990) conducted
a "piggyback" study in long-term research plots at Horseshoe
Bend in north-central Georgia. The long-term trial involved a sandy clay
loam soil planted to a soybean-cereal rye-sorghum rotation and managed
with vs. without tillage. Aporrectodea caliginosa was the dominant
annelid earthworm, and Lumbricus rubellus was also present. No-till
management led to a 1.42-fold increase in annelid density and biomass
over those observed with conventional tillage. Detailed sampling indicated
that densities of Enchytraeidae (a family of small earthworms) were higher
under no-till, which contradicted earlier preliminary sampling of the
same plots (Hendrix et al., 1986). When the pesticide carbofuran was used
on the long-term treatments, it resulted in a 47 percent increase in particulate
organic matter under the no-till regime.
Organic Matter and Nitrogen Cycling
Kretschmar and Ladd (1993)
conducted a laboratory study of the decomposition of subterranean clover
(Trifolium subterraneum) foliage incubated in columns of loamy
sand. Clover foliage was incorporated at varying depths and soil was compacted
at varying pressures. The earthworm Aporrectodea trapezoides was
then added to some columns, but not to others. Results suggested that
if herbage was deeply incorporated or the soil highly compacted, the earthworm
alleviated the problems of decreased oxidation rates, and thereby promoted
decomposition of the residues.
Ruz Jerez et al. (1988) conducted
a study on organic matter breakdown and nitrification as influenced by
the earthworms Lumbricus rubellus or Eisenia fetida. The
study was conducted in 2-liter laboratory glass incubation chambers containing
soil (fine sandy loam) and earthworms (10 per chamber). (Reviewer's
note: 5 earthworms per liter corresponds to high worm densities in the
field.) Dried wilted or senescing clover or grass residue was incorporated
into the upper 1 cm of soil in each chamber
Following an initial amount
of litter that would correspond to 700 kg dry matter (DM) per hectare,
additional litter was added at a rate of 350 kg DM per hectare per week
for 10 weeks thereafter. The total addition of clover or grass herbage
during the 11 weeks of the study was 4,200 kg DM per hectare. (Reviewer's
note: This is about the amount of organic matter that would result from
one fairly close mowing of a cover crop of annual grasses. clovers, or
medics in a Californian orchard.)
The researchers observed
an approximately 50 percent increase in mineral nitrogen after 11 weeks'
incubation with earthworms as compared to without; mineral nitrogen was
9 percent higher in chambers that were held at 22.5C than in those that
were held at 15C. When results were pooled over both temperatures, only
0.6 percent of the clover residue remained, whereas 9 percent of the grass
residue was recoverable; this difference is probably related to differences
in the residue's carbon-to-nitrogen ratio as well as its palatability
to earthworms. In chambers without earthworms, 11.3 percent of the clover
residue remained, and 13.7 percent of the grass residue. Microbial biomes
was reduced in chambers with earthworms. No information was presented
comparing results obtained for Lumbricus rubellus vs. Eisenia
fetida.
Test plants (ryegrass [Lolium
sp.]) grown in the various treatments following incubation suggested
a 25 percent increase in nitrogen uptake following incubation of herbage
and soil with, as opposed to without, earthworms. The authors suggested
that prior laboratory studies may have underestimated earthworm respiration
rates.
Marinissen and de Ruiter
(1993) assessed data on the cycling of nitrogen and organic matter from
a study in the Netherlands, and from the long-term study in Horseshoe
Bend, Georgia (described above under Parmelee et al., 1990). As in the
Horseshoe Bend study, the dominant annelid at the Netherlands site was
Aporrectodea caliginosa (constituting 92 percent of the wet biomass
of annelids). Other species observed at the Netherlands site were Lumbricus
rubellus (6%) and Aporrectodea rosea (296). The researchers
developed projections for nitrification based on both direct and indirect
effects of earthworms. Direct effects were calculated based on varying
assumptions concerning production rates of dead tissue, casts, urine,
and mucus. Other assumptions that were varied concerned the carbon:nitrogen
ratios of the earthworms themselves and of the organic matter being processed.
Indirect effects of earthworms were also evaluated, based on the possibilities
that: 1) increased grazing by earthworms on microbes stimulates microbial
regrowth, and 2) earthworm-induced improvement of soil structure promotes
microbial activity.
Projections from the Netherlands
data suggested that earthworms are directly or indirectly responsible
for nitrification of from 10 to 100 kg nitrogen per hectare per year Data
from Horseshoe Bend, where earthworm densities were higher, suggested
corresponding figures of from 82 to 364 kg nitrogen per hectare per year.
These widely varying projections reflect a need for more precise assessment
of the parameters employed in the models.
Soil Structural Changes
Lee and Foster (1991) composed
a review article suggesting that earthworm burrows are important for water
infiltration only when irrigation or rainfall exceeds the soil capacity
for capillary uptake. Moreover, anecic earthworms (those that make deep,
permanent, vertical tunnels) may block burrow entrances with soil or plant
material, or position their bodies to obstruct flow down the burrows.
Any of these phenomena make earthworm burrows less effective in promoting
water infiltration.
Other research has indicated
that earthworm casts are frequently more stable than aggregates composed
of clay-organic matter complexes. The authors point out that this is not
always the case, but they provided no explanation for the discrepancy.
Zhang and Schrader (1993)
conducted laboratory studies on the aggregate stability of "natural,"
worm-induced, and pressure-induced aggregates. Worm-induced aggregates
from castings and burrow linings were less stable than "natural""
aggregates, but more so than those formed by human agency through mere
compression. The authors considered it unlikely that earthworms rupture
mineral particles by compression, but did suggest that the chemical bonds
of natural aggregates may be ruptured during ingestion by earthworms.
The tensile strength (resistance to crushing) of aggregates formed by
the three species of earthworms assessed was as follows: Lumbricus
terrestris>Aporrectodea longa>Aporrectodea caliginosa. Tensile
strength was positively correlated with organic matter content in the
worm-formed aggregates.
Toxicology
Martin (1986) conducted a
toxicological study that indicated that Aporrectodea caliginosa
is as sensitive or more so to pesticides than are other agriculturally
important earthworms. The author suggested that this species would be
a logical choice for screening pesticides intended for use in pasture
crops or crops grown in rotation with pasture.
Fertilizers
Ma et al. (1990) assessed
the effects of turfgrass fertilization with six types of nitrogenous fertilizers,
including mineral ammonium sulfate, nitrochalk (ammonium nitrate with
lime), sulfur-coated urea, organic-coatedurea, isobutylidenediurea, and
ureaformaldehyde. There were three rates of application for each of the
six fertilizers, corresponding to 60, 120, and 180 kg nitrogen per hectare
per year The trial was carried out in a loamy sand soil in Haren, Netherlands,
on a turf that included various annual and perennial grasses. Plots were
2.5 x 3.0 m and arrayed in a randomized complete block with two replications
for each of 18 treatments.
Results suggested profound
reductions caused by ammonium sulfate and by sulfur-coated urea in the
endogeic earthworms Aporrectodea caliginosa caliginosa and Aporrectodea
rosea. By contrast, the endogeic earthworm Aporrectodea caliginosa
tuberculata and the epigeic Lumbricus rubellus showed
less reduction. The observed reductions were believed by the authors to
have been caused by acidification. (Editor's note: Endogeic species forage
below the soil surface in horizontal, branching burrows; epigeic species
live in the superficial soil layers and feed on undecomposed plant litter.)
Aporrectodea caliginosa tuberculata and Lumbricus rubellus
have in the past been noted as tolerant of acid soils, whereas the types
of worms showing reductions have been regarded as doing best near neutral
pH. Nitrochalk had little effect on earthworm densities, and the other
fertilizers had intermediate effects.
Collections From Merced County Almond Orchards
As part of the BIOS project
in Merced County, California, seven growers collected earthworms from
their orchards. These worms were identified by specialists Matthew Werner
of the UC Santa Cruz Agroecology Program and Sam James of Maharishi International
University, Iowa.
The endogeic earthworm Aporrectodea
caliginosa was the most widely-distributed species, having been collected
in all the orchards sampled. This nominal species is now regarded as a
complex of three closely related species (Matthew Werner, pers. comm.).
As noted in the above synopses, Aporrectodea caliginosa is frequently
encountered in other farming systems. It is regarded as having agricultural
importance, and diverse feeding habits, including feeding on soft tissue
of plant litter at the soil surface or on dead roots below the soil surface
(reviewed by Lee, 1985).
Werner noted that specimens
of Aporrectodea caliginosa from three of the orchards were unpigmented,
indicating a strictly subterranean existence. One of these farms also
included Aporrectodea turgida and was the only orchard at which
the epigeic species Lumbricus rubellus could be found. Specimens
of Aporrectodea caliginosa from the remaining four farms had moderate
to heavy pigmentation, suggesting at least occasional above-ground feeding,
casting, or travel. On one of these farms, the endogeic species Amynthas
diffringens and Microscolex dubius were also collected.
No anecic earthworms were
found, nor have obvious middens been seen at any of the BIOS farms. In
light of the apparent lack of anecics, Werner has suggested inoculative
release of the anecic earthworms Lumbricus terrestris or Aporrectodea
longa to promote more rapid litter incorporation. It is striking that
three of the four species collected recently at BIOS farms (Aporrectodea
caliginosa, Lumbricus rubellus, and Microscolex dubius)
are renowned as "peregrine or "wandering" earthworms, because
they have frequently been transported by humans to new locations (Lee,
1985).
References
Hendrix, R.E., R.W. Parmelee,
D.A. Crossley Jr., D.C. Coleman, E.R. Odum and R.M. Groffman. 1986. Detritus
food webs in conventional and no-tillage agroecosystems. Bioscience 36:374-380.
Kretzschmar, A. and J.N.
Ladd. 1993. Decomposition of 14C-labelled plant material In soll: The
Influence of substrate location, soil compaction and earthworm numbers.
Soil Biology and Biochemistry 25:803-809.
Lee, K.E. 1985. Peregrine
species of earthworms. In: Pagliai, A.M. Bonvicini and R Omodeo (eds.)
On Earthworms. Selected Symposia and Monographs, 2 Collani U.Z.I.
Mucchi Editore, Modena, Italy. pp. 315-327.
Lee, K.E. and R.C. Foster.
l991. Soil fauna and soil structure. Australian Journal of Soil Research
29:745-775.
Ma, Wel-Chun, L. Brussard
and J.A. De Ridder. 1990. Long-term effects of nitrogenous fertilizers
on grassland earthworms (Oligochaeta: Lumbricidae): Their relation to
soil acidification. Agriculture, Ecosystems and Environment 30:71-80.
Marinissen, J.C.Y. and RC.
de Ruiter. 1993. Contribution of earthworms to carbon and nitrogen cycling
in agro-ecosystems. Agriculture, Ecosystems and Environment 47:59-74.
Martin, N.A.1986.Toxicidty
of pesticides to Allolobophora caliginosa (Oligochaeta: Lubriddae).
New Zealand Journal of Agricultural Research 29:699-706.
Parmelee, R.W., M.H. Beare,
W. Cheng, P.F. Hendrix, S.J. Rider, D.A. Crossley Jr., and D.C. Coleman.
1990. Earthworms and enchytraeids in conventional and no-tillage agroecosystems:
A biocide approach to assess their role In organic matter breakdown. Biology
and Fertility of Soils 10:1-10.
Ruz Jerez, B.E., R.R. Bail,
and RW. Tillman. 1988. The role of earthworms in nitrogen release from
herbage residues. In: Jenkinson, D.S. and K.A. Smith (eds.) Nitrogen
Efficiency in Agricultural Soils. Elsevier Applied Science, New York,
NY. pp. 355-370.
Zhang, H. and S. Schrader.
1993. Earthworm effects on selected physical and chemical properties of
soil aggregates. Biology and Fertility of Soils 15:229234.
For more information write
to: R.L. Bugg, Information Group, SAREP. University of California, Davis,
CA 95616.
(DEC.521 ) Contributed by
Robert Bugg
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