How Earthworms Affect Plant Growth: Burrowing into the Mechanisms - Earthworm Ecology - page 27

FIGURE 2.6 Pasture root growth into burrows and casting of earthworms from native savannas and pastures
planted on highly weathered soils of the Colombian Eastern Plains. Note the two different types of structures:
globular ÑcompactÒ castings created by M. carimaguensis and their breakdown by smaller polyhumic endogeic
ÑdecompactingÒ earthworm species and mesofauna. (Photo P. Lavelle.)
The inner porosity of earthworm casts is also very variable depending on the earthworm species
producing them, particularly the earthwormÔs anterior and posterior internal morphology and
musculature (Lapied and Rossi 2000). A predominance of mesopores (10 to 20 Hm) was reported
in the casts of M. anomala (Blanchart et al. 1999), whereas pores in the casts of the compacting
species P. corethrurus were all smaller than 1 Hm (Chauvel et al. 1997). Thus, casts are much more
important for retaining plant-available water (fresh casts of many species have water contents above
70%) (Blanchart et al. 1999) and nutrients, whereas earthworm burrows are more important for
water by-pass flow, infiltration rates, gaseous exchanges, and root penetration and elongation.
Subsequently, several authors (Doube et al. 1997; Stockdill 1966; van Rhee 1969) reported increased
water use efficiency by crops in soils inoculated with earthworms in both pot and field experiments.
Earthworm casts, once they have undergone a stabilization process still not well understood
(Edwards and Shipitalo 1998), become water-stable aggregates, although their stability is very depen-
dent on the soil type, earthworm species, and earthworm feeding habits (Blanchart et al. 1999). Often,
an important part (5% or more) of the surface (A) horizon of soils passes annually through earthworm
intestines, particularly in tropical regions that are dominated by endogeic species (Lavelle 1988).
Under some circumstances, most of the topsoil may be composed of earthworm castings of different
ages, sometimes remaining long after the earthworms have disappeared (Buntley and Papendick 1960;
Graff 1971b; Pop and Postolache 1987; Lavelle 1988). Thus, because interaggregate spaces are
important in soil macroporosity, the physical arrangements of casts, particularly the larger casts
containing mostly water-stable macroaggregates (>2 mm diameter), can also have an important effect