Agriculture Reference
In-Depth Information
passage, both physically (by comminution, restructuring, movement from one location to another
in soil) and chemically (reduction in C:N ratio, change in quality).
As mentioned, when an earthworm ingests soil containing ÑdormantÒ microorganisms, these
microorganisms may become activated by a Ñpriming effectÒ (Lavelle et al. 1995) in the gut, which
often continues for a short time in the casts because of the abundance of soluble C and other
nutrient resources. Thus, in fresh casts of
A. caliginosa,
Scheu (1987) reported an increase of
approximately 90% in microbial respiration rates over a 4-week incubation period. Given the higher
microbial activity and biomass often observed in earthworm casts (Edwards and Bohlen 1996),
higher OM decomposition rates in casts would be expected. However, the OM may also be
ÑprotectedÒ from microbial attack in compact, water-stable castings of endogeic earthworm species,
leading to lower OM decomposition rates (Martin 1991; Ketterings et al. 1997; Marinissen and
Hillenaar 1997). Preferential assimilation by earthworms of the more labile soil OM fractions may
also help explain the lower mineralization rates in casts because the remaining (more recalcitrant)
OM in casts will have slower mineralization kinetics (McCartney et al. 1997). Thus, it is the balance
of the short-term priming effects and the longer-term protection effects induced by an earthworm
community in a given location that may ultimately determine whether earthworm activities are
enhancing the conservation or destruction of OM in the drilosphere (Brown et al. 2000).
Nevertheless, in the drilosphere, the short-term priming of microorganisms increases nutrient
mineralization rates, releasing more plant-available N and P (e.g., Edwards and Bohlen 1996; Barois
et al. 1999). Therefore, in terms of nutrients, earthworm casts and burrows (lined with carbon and
protein-rich mucus with a C:N ratio below 6.0; Cortez and Bouch 1987; Scheu 1991; Schmidt et
al. 1999) constitute very favorable microenvironments for microbial and invertebrate activity and
for plant root growth (Graff 1971; Tiunov and Scheu 1999; Jgou et al. 2001). Consequently,
populations and activity of microflora, microfauna, and other organisms may be higher in earthworm
burrows and casts than in surrounding soil (Brown 1995; Tiunov and Scheu 1999).
For instance, in a temperate grassland with a complex earthworm community, burrow walls
supported up to 42% of the total soil aerobic N
2
-fixing bacteria, 13% of anaerobic N
2
fixers, and
16% of denitrifying bacteria and had more ammonifiers, denitrifiers, free-living aerobic and anaer-
obic N
2
fixers, and proteolytic bacteria (Bhatnagar 1975). Earthworm casts had more cellulolytic
aerobes and hemicellulolytic, amylolytic, nitrifying, and denitrifying bacteria than the soil in which
they lived (Bhatnagar 1975; Loquet et al.
1977). Therefore, earthworm casts and burrows may be
important microsites for denitrification because they possess larger populations of denitrifiers,
higher levels of soluble C and NO
3
, and higher water contents than the surrounding soil (e.g.,
Elliott et al. 1990; Karsten and Drake 1997; Parkin and Berry 1999).
Several species of fungi can also grow rapidly in earthworm casts (Parle 1963a) from the
inocula remaining after passage through the gut (surviving spores and hyphal fragments) or from
the surrounding soil. Algae also appear to be able to take advantage of the high nutritional value
of earthworm casts. Of 19 species of algae found in casts of an unidentified earthworm species
(probably a lumbricid), 6 had higher growth rates in casts than in uningested soil (Shtina et al.
1981). Protozoa (live and encysted) that survive passage through the earthworm gut may also feed
on the increased numbers of bacteria and fungi found in earthworm casts and multiply rapidly, so
that their numbers become higher than in uningested soil (Shaw and Pawluk 1986).
Successional Processes within Casts
Passage through the earthworm intestine results in the removal of some of the active stages of
protozoans and some (less resistant) fungal spores, hyphae, and bacteria. The surviving, resistant
microorganisms, together with those found in the earthworm burrow walls, provide inocula for
colonization of the newly formed casts.
The microbial successional processes in casts have been little studied, and although a number
of authors have detailed the type and abundance of microorganisms found in casts (Brown 1995),
