Quantifying the Effects of Earthworms on Soil Aggregation and Porosity - Earthworm Ecology - page 181

POROSITY AND INFILTRATION

C B M

HARACTERIZATION OF URROW ORPHOLOGY

Because they burrow extensively into mineral soil, endogeic and anecic earthworms can substan-
tially alter soil porosity. Estimates of the number of burrows in temperate region soils range as
high as 100 to 800 m

(Lavelle 1988). Although these burrows usually account for a small fraction
of the soil volume, because of their continuity, stability, and relatively large size compared with
pores formed by most other mechanisms, these macropores have the potential to greatly affect the
movement of air, water, and solutes. Moreover, unlike cracks, earthworm burrows tend to remain
open and continue to function as preferential flow paths under wet soil conditions (Friend and Chan
1995) and are less likely to be closed by vehicle-induction compaction than other soil macropores
(Alakukku et al. 2002). Quantifying burrow numbers and morphology is difficult, however, and a
number of techniques have been used to accomplish this task.
A commonly used technique is to count the number of burrows open at various depths in the
soil and measure their diameters. These counts can be performed manually or by taking photographs
and using image analysis techniques (Shipitalo and Protz 1987; Edwards et al. 1988a, 1988b). If
accurate information on burrow continuity is to be obtained, the observations must be made at
relatively narrowly spaced vertical intervals. This can be accomplished by serially sectioning the
soil either

− 2

or with impregnated soil thin sections (Ehlers 1975; Ligthart et al. 1993; McKenzie
and Dexter 1993; Schrader 1993; Hirth et al. 1996; Ligthart 1997; Pitknen and Nuutinen 1997;
Sveistrup et al. 1997; Springett and Gray 1998). These techniques, however, are laborious and are
often only partially successful, particularly in the soil layers near the surface, because of loose soil
aggregates and interference by plant roots (Ligthart et al. 1993). For example, McKenzie and Dexter
(1993) were successful only 20% of the time when they used a grid coordinate system and manual
excavation to measure earthworm burrow geometry.
A modification of the excavation technique that reduces some of the difficulties encountered
in trying to track the continuity of individual earthworm burrows and that can result in more accurate

in situ

characterization of their morphology is through the use of replicas of burrows made in situ ( Figure
10.2 ) . These replicas can be made using materials such as molten lead (Teotia et al. 1950), plaster

(Bouma et al. 1982; Wang et al. 1994), wax (Smettem 1986), or fiberglass resin (Shipitalo and Butt
1999; Shipitalo and Gibbs 2000). Like excavation, this technique is not always successful because
of an inability to fill the burrows completely with the impregnating media. Additional drawbacks
are that the technique works best only on burrows of relatively large diameter (i.e., >5 mm), and
removal of the replicas is a tedious operation.
More recently, x-ray computed tomography has been used to characterize earthworm burrow
morphology (Golabi et al. 1995; Daniel et al. 1997; Perret et al. 1997; Capowiez et al. 1998; Jegou
et al. 1998; Langmaack et al. 1999). A major advantage of this technique, compared with procedures
involving excavation, is that the soil is not disturbed during the analysis. Consequently, the dynamics
of burrow construction can be investigated. Besides the limited availability and high expense of
this equipment, a major disadvantage is that the resolution of the current generation of equipment
is such that only the morphology of large diameter burrows can be accurately assessed. Another
concern is that the samples are usually obtained by incubating earthworms in columns of soil that

will fit within the instruments rather than examining burrows formed by earthworms in the field

under natural conditions. As Springett and Gray (1998) noted when they manually excavated
burrows, there can be major differences between those formed in laboratory columns and those
formed in the field because of restriction of the available space in columns.

E E B I

FFECTS OF ARTHWORM URROWS ON NFILTRATION

A variety of field and laboratory techniques have been used to determine the effects of earthworm
burrows on infiltration. These techniques include dye and tracer studies, studies in which movement