Agriculture Reference
In-Depth Information
Because most of their activity is confined to surface soil horizons, however, endogeic earthworms
probably do not directly influence water movement deep into the profile (Ela et al. 1992). The fact
that portions of their burrows are often occluded with casts probably further limits their effectiveness
in water transport.
Thus, most research has centered on the effects of anecic earthworm species on infiltration and
on
L. terrestris
in particular. Burrows created by
L. terrestris
are normally single, nearly vertical
channels, up to 12 mm in diameter and 2.4 m deep (Edwards and Bohlen 1996). These burrows
can have several entrances directly underneath the midden, but these usually coalesce into a single
channel within the upper few centimeters of soil. Nevertheless, Shipitalo and Butt (1999) and
Shipitalo and Gibbs (2000) found that about 5% of the
L. terrestris
burrows they investigated were
Y shaped, with the two channels intersecting as deep as 69 cm below the soil surface.
One method that has been used to investigate water movement through natural
L. terrestris
burrows in the field involves placing surface-vented collection bottles beneath individual burrows
30 to 50 cm below the soil surface (Edwards et al. 1989; Shipitalo et al. 1994). Because the portions
of the burrows above the samplers are not disturbed, this technique can be used to investigate
infiltration into burrows with intact middens. Although middens would seem to inhibit entry of
water, these studies indicated that
L. terrestris
burrows could transmit substantial amounts of water.
In fact, Darwin (1881) did not consider middens to be a barrier to water movement. These studies
also indicated that the fraction of rainfall collected increased with rainfall intensity. With an intense
rainfall on a dry soil surface, Edwards et al. (1989) estimated that the monitored burrows collected
10% of the rainfall and an average of 13 times more water than expected based on the diameter
of the burrows at the soil surface.
Problems with the bottle sample technique include concern that interception of flow with the
samplers may allow more water to move through the burrows than would naturally occur because
infiltration characteristics of the soil surrounding the lower reaches of the burrow might limit
infiltration (Lee and Foster 1991; Golabi et al. 1995). In addition, after initially high rates of
infiltration, soil air pressure might restrict further water entry under field conditions (Linden and
Dixon 1976; Edwards et al. 1979; Baird 1997), a consequence precluded by the sampler design.
These concerns appear to be unfounded in most soils under most conditions because procedures
in which infiltration has been measured by introducing water directly into the openings of
individual
L. terrestris
burrows at the soil surface have demonstrated average infiltration rates
in the range of several hundred milliliters per minute, well in excess of the amounts measured
using the bottle sampler technique, for soils in Germany (Ehlers 1975), the Netherlands (Bouma
et al. 1982), Wisconsin (Wang et al. 1994), the U.K., and Ohio (Shipitalo and Butt 1999).
Moreover, the study by Shipitalo and Butt (1999) indicted that the presence of live
L. terrestris
in the burrows did not have detectable effects on infiltration. This addressed the concern of Lee
and Foster (1991) that anecic earthworms might tightly seal their burrows with their bodies and
limit infiltration. In fact, Shipitalo and Butt (1999) speculated that occupied burrows might be
more effective in transmitting water than are abandoned burrows because they are more likely
to maintain near-surface continuity.
The effects of earthworms on infiltration have also been investigated in the laboratory using
intact or repacked soil columns with resident or inoculated earthworms. Although these studies
have provided insight into mechanisms affecting infiltration, one concern, particularly with repacked
soil columns inoculated with earthworms, is that the burrows formed are not representative of those
constructed under more natural conditions (Springett and Gray 1998). Similarly, studies in which
artificially constructed macropores are used to investigate water movement through earthworm
burrows can have significant limitations and must be interpreted with caution (Joschko et al. 1989;
Roth and Joschko 1991; Ela et al. 1992; Li and Ghodrati 1995). In this case, an additional limitation
is that the artificial burrows lack the organic matter-rich lining or drilosphere, composed of earth-
worm excrement, mucus secretions, and plant remains, that can affect water and chemical movement
(Stehouwer et al. 1993, 1994).
