Agriculture Reference
In-Depth Information
Models have also been used to determine the impact of earthworm burrows and other
macropores on inÝltration (Ehlers 1975; W.M. Edwards et al. 1979; Smettem and Collis-George
1985; Smettem 1986; Wang et al. 1994, Li and Ghodrati 1995). Although this approach is useful
in investigating the factors affecting inÝltration in earthworm burrows, collection of the burrow
data needed to obtain input parameters (i.e., burrow depth, length, diameter, volume) for the models
is difÝcult. Moreover, although most of these models indicate that the aforementioned parameters
should affect inÝltration capacity, Shipitalo and Butt (1999) were unable to detect any signiÝcant
correlations between these geometrical properties and inÝltration rates through
L. terrestris
burrows.
In addition, not all earthworm burrows conduct water (Ela et al. 1992; Trojan and Linden 1992;
Shipitalo et al. 2000). In fact, Bouma et al. (1982) stated that theoretical models are unlikely to
predict inÝltration in earthworm burrows successfully given the complexity and variability of the
morphological factors affecting hydraulic performance.
E
FFECTS
OF
E
ARTHWORM
B
URROWS
ON
W
ATER
Q
UALITY
Increased inÝltration attributable to earthworm activity in soils is generally regarded as beneÝcial
because it can reduce surface runoff, thereby increasing plant-available water and reducing the
potential for overland transport of sediment, nutrients, and agrochemicals (Shipitalo et al. 2000).
Earthworm burrows can also increase the efÝciency of subsurface drainage systems (Urbnek and
Doleza
ø
1992) and may help restore the inÝltration capacity of clogged septic system leach beds
(Jones et al. 1993). However, this increased inÝltration can increase the quantity and rate of solute
movement through the soil proÝle. This is of particular concern with
L. terrestris
burrows because
they are often deep enough to penetrate the entire soil proÝle (
Figure 10.2
). Thus, solutes transported
through these burrows can rapidly bypass the upper reaches of the proÝle, where uptake is most
likely to occur and biological activity and the potential for degradation are greatest. In addition,
because the velocity at which water moves through these macropores is much greater than when
the entire soil matrix is involved in the Þow process, the amount of soil a solute encounters and
its contact time with the soil are reduced.
It is difÝcult, however, to quantify the effects of earthworm burrows on chemical transport
because, as just discussed, it is difÝcult to measure their effects on inÝltration. An additional
complication is that the burrow linings can serve as both a source and a sink for various solutes.
For example, Edwards et al. (1992b) found that when nitrate-free water was poured into
L. terrestris
burrows and immediately collected 45 cm below the soil surface, it contained as much as 40 mg
of nitrate-nitrogen per liter. They speculated that the nitrate originated from the decomposition of
the organic matter lining the burrows. This contention is supported by the work of Parkin and Berry
(1999), in which higher microbial populations as well as higher nitriÝcation and denitriÝcation
rates were noted in
L. terrestris
burrow linings than in bulk soil. Edwards et al. (1992b) also noted
a Ývefold reduction in the concentration of alachlor and a ninefold reduction in the concentration
of atrazine when solutions of these two herbicides were poured into burrows and collected at the
bottom. When these solutions were poured through man-made artiÝcial burrows, the concentrations
were reduced by only about half. In this instance, the decreased herbicide concentrations were
attributed to sorption of the herbicides by the organic matter-rich linings of the burrows, a contention
supported by the work of Stehouwer et al. (1993, 1994). For this reason, chemical tracers are often
used to investigate solute movement in earthworm burrows.
The results of a number of Ýeld and laboratory chemical transport and tracer studies suggested
that earthworm burrows can increase overall water movement through the soil and contribute to a
slight increase in the leaching of surface-applied agrochemicals, particularly when intense storms
occur shortly after application on residue-covered no-till soils (Germann et al. 1984; Bicki and
Guo 1991; Edwards et al. 1992a; Trojan and Linden 1992). The potential for this to occur is greatly
reduced with time (Edwards et al. 1993, 1997; Logsdon 1995) and low intensity intervening rainfalls
