Agriculture Reference
In-Depth Information
fractions. For example, the organic matter in the fraction larger than 2000
H
m decreased from 97
to 27% in the presence of the
m fraction after
446 days of incubation (Ziegler and Zech 1992). This litter-derived organic matter can serve as a
bonding agent or promote microbial activity that leads to the production of bonding agents (Guggen-
berger et al. 1996).
Beare et al. (1994) suggested that the incorporation of organic matter promotes the formation
of stable microaggregates within macroaggregates. Kladivko et al. (1986) found that, after drying,
aggregate stability was determined mainly by the type of plant remains, although the effect of
earthworms was still significant. Once incorporated into casts and if not subject to further distur-
bances, the organic matter can persist for many years (McInerney et al. 2001), with organic carbon
persistence and dynamics in earthworm casts dependent on complex interactions among soil texture,
temperature, and wetting cycles (McInerney and Bolger 2000). However, in some instances, cast
stabilization has been observed in the absence of a source of organic residue (Marinissen and Dexter
1990; Marinissen et al. 1996). Similarly, although Haynes and Fraser (1998) observed fragments
of decomposing organic material adhering to aggregate surfaces, they noted stabilization in the
absence of a source of organic residue. It is likely that the type and extent of bonding will depend
on properties of the soil materials and on the quality and the quantity of the ingested organic debris.
Thus, several physical, chemical, and biological mechanisms probably contribute to the stabi-
lization of aggregates within casts, and their relative importance can vary under different conditions
and with different earthworm species. The continued stability of these aggregates can be influenced
by wetting and drying cycles and whether other soil organisms disrupt them. Successive wetting
and drying cycles contribute to the stability of natural aggregates by creating bonds of different
nature between the contact points of soil particles over time (Dexter et al. 1988).
In casts, Marinissen and Dexter (1990) assumed that the effects of drying-rewetting would be
more persistent with time than the effects of fungal hyphae. In newly remolded aggregates, Utomo
and Dexter (1982) showed that wetting and drying increased the percentage of water-stable aggre-
gates two- to fourfold. Nevertheless, Hindell et al. (1997b) reported opposite results for initially
air-dried casts and uningested soil. Air-dried samples slaked severely when immersed in water, and
they speculated that surface casts are the most subject to slaking following sudden rain or irrigation.
In a laboratory microcosm study, Shaw and Pawluk (1986) noted that soil structure development
was maximized when anecic and endogeic earthworm species were allowed to interact. In a field
study in a tropical region, however, Blanchart et al. (1997) noted that small eudrilid earthworms
accelerated the destruction of aggregates created by larger earthworms. This prevented accumulation
of large casts at the soil surface and, in some cases, led to the formation of a compact and
impermeable layer and to negative effects on plant growth (Blanchart et al. 1999; Chauvel et al.
1999). In temperate region soils, Ge et al. (2001) noted that casts near the soil surface degraded
rapidly unless protected by a mulch cover, and Shuster et al. (2000) noted that foraging and midden
building by anecic earthworm species reduced residue cover and exposed more soil and casts to
raindrop impact.
E. fetida
, leading to a predominant 630- to 2000-
H
S
C
, S
E
,
N
T
URFACE
ASTING
OIL
ROSION
AND
UTRIENT
RANSPORT
Although Darwin (1881) speculated that earthworms contribute to soil erosion, it is now known
that the net effect of their activity on soil losses depends on a number of interacting factors. By
burrowing into the soil and creating macropores, earthworms can increase infiltration rates 2- to
15-fold, which should lead to a reduction in runoff (Ehlers 1975; Joschko et al. 1989; W.M. Edwards
et al. 1990; Kladivko and Timmenga 1990; Bouch and Al-Addan 1997; Willoughby et al. 1997).
This, in turn, should contribute to a reduction in soil loss. In addition, earthworms can increase
surface roughness by casting on the soil surface, and their burrowing activity can disrupt soil crusts,
which should further increase infiltration and reduce runoff (Kladivko et al. 1986).
