Agriculture Reference
In-Depth Information
uptake of nutrients by plants in response to earthworm activity have often led to greater transfer
of C and N from soils to shoots and more shoot biomass relative to that of roots (Wolters and
Stickan 1991; Klebsch et al. 1995). The actual benefits derived by the plants from earthworms
depend on the ability of the plant to extract these nutrients from the drilosphere and the soil solution.
If nutrients are released and made available to plants during drier or colder periods when plants
are dormant or not growing as actively or when the field is bare, then they may be lost (e.g., by
leaching) and will provide very few benefits to the plants. Thus, the synchrony of nutrient availability
(especially N) with the needs of the plants is critical, and earthworms may play an important role
in this process (Fragoso et al. 1997).
The increased amounts of plant-available nutrients in earthworm casts and burrows can promote
root growth considerably (Darwin 1881; Ehlers 1975; Spiers et al. 1986; see
Figure 2.6
)
, and their
importance to plant nutrition increases proportionally to the differences in nutrient status between
the earthworm casts the surrounding soils, the quantities of casts produced and their synchronization
and synlocalization with root growth needs. Thus, in deeper and possibly poorer soil zones,
earthworm casts and burrows may serve as hot spots of nutrient availability to plant roots (Mouat
and Keogh 1987) and promote fine root growth. Conversely, when the plants are growing in nutrient-
rich soils, the relative nutrient bioavailability stimulation in response to earthworm activities may
be less than in poor soils, and expected plant growth increases may also be less (e.g., Atlavinyte
and Vanagas 1973; Brown et al. 1999; Buse 1990; Doube et al. 1997) because the plants can obtain
most of their required nutrients without the earthworms.
Many experiments have demonstrated the importance of plant-available nutrients and PGR
substances in earthworm casts to plant responses (e.g., Dash and Das 1989; Kang and Ojo 1996;
Kang et al. 1994; Nijhawan and Kanwar 1952; Reddy et al. 1994; Tomati et al. 1987; Norgrove
and Hauser 1999; Kollmannsperger 1980). The plant response is usually proportional and related
positively to the quantity of earthworm casts applied or to the ratio of casts to soil or other substrates
used. However, experiments of this nature have the disadvantage of unrealistic experimental con-
ditions compared with the field and the vastly different chemical, physical, and biological properties
of the casts, depending on their source and age.
The effects of earthworms on nutrient mineralization are especially evident in sites newly
invaded by earthworms. For example, lumbricid or pheretimoid earthworm invasions into the forests
of North America have sometimes resulted in dramatic changes in the chemical status of soil,
transforming humus types from mor to mull (Langmaid 1964; Nielson and Hole 1964; see
Chapter
5
this volume). Corresponding C and N losses and increased nutrient turnover rates resulting from
earthworm invasion of new sites may be on the order tens to hundreds of kilograms per hectare
(OÔBrien and Stout 1978; Alban and Berry 1994; Scheu and Parkinson 1994a; Burtelow et al.
1998). Some plants may benefit from the increased amounts of available nutrients (Scheu and
Parkinson 1994b), but in general little is known of the effect of such large nutrient fluxes on overall
plant growth and their potential effects on plant communities (e.g., species composition and
biodiversity). Eutrophic and opportunistic plants may profit preferentially from earthworm presence
during the nutrient release phase (which may last several years) until a new ÑequilibriumÒ is reached,
when the lower total nutrient stocks and more rapid rates of turnover may exert relatively larger
and different selection pressures on the plant communities present.
Litter burial, ingestion, and digestion by earthworms, particularly anecic and epigeic earthworm
species, accelerates rates of decomposition, whereas endogeic earthworm species tend to promote
mineralization of the particular light and coarse organic matter fractions in soils (McCartney et al.
1997; Lavelle et al. 1998; Parmelee et al. 1998). Thus, the incorporation of organic matter by an
anecic earthworm species such as
M. carimaguensis
in the savannas of Colombia has been asso-
ciated with reduced Al saturation (through binding with organic matter) and reduced Al limitation
to grass growth (Decans et al. 1999). In New Zealand pastures, earthworms have been shown to
accelerate the incorporation of lime, fertilizers, and insecticides such as DDT (for grass grub control)
(Stockdill 1966, 1982; MacKay et al. 1982; Springett 1985), thereby promoting grass productivity.
