Agriculture Reference
In-Depth Information
The soil-mixing activities of earthworms have been shown to promote the recovery of N and P
from organic residues, inorganic fertilizers, and rock phosphate in various pot and field trials using
the plant species
B. decumbens
,
Panicum maximum
, ryegrass, and maize (MacKay et al. 1982;
Mansell et al. 1981; Spain et al. 1992; Hameed et al. 1993, 1994a,b; Hu and Wu 1994; Gilot-
Villenave et al. 1996; G.G. Brown et al. 2000; Cortez et al. 2000).
However, earthworm activities can also sometimes lead to decreases in the availability of
nutrients to plant roots. Fresh earthworm casts rich in nutrients (especially N and P) that are
deposited on the soil surface may be eroded easily and are also hot spots of denitrification and
NH
3
volatilization (Elliott et al. 1990; Lensi et al. 1992; Karsten and Drake 1997). Furthermore,
earthworm burrows, especially those of anecic species, connected to the soil surface may promote
water bypass or mass flow, causing increased leaching of soluble nutrients (Anderson et al. 1983;
Hoogerkamp 1984; Knight et al. 1989; Haimi and Boucelham 1991; Edwards and Shipitalo 1998).
Nutrients from Earthworms (Death, Excretion)
Some authors have proposed that most N excretions and mucus secretions from earthworms may be
utilized rapidly by plants (e.g., Bouch and Ferrire 1986; Bouch et al. 1987; Hameed et al. 1994a,b;
Whalen et al. 1999, 2000). However, this contribution is probably not very large unless the earthworm
biomass is high and most of their activities are concentrated in the root zone. Further field research
on this topic, particularly using homogeneously labeled (
15
N,
32
P) earthworms, is needed.
The release of nutrients from dead earthworm tissues has often been believed to play important
role in plant productivity (Russell 1910; Satchell 1958; Barley 1961; Callaham and Hendrix 1998;
Whalen et al. 1999). However, although visual observations show that earthworm bodies decompose
very rapidly in soil, only a few reports have been published on the quantities of nutrients made
available from dead earthworm biomass (e.g., Satchell 1967; Christensen 1988; Martin 1990; Whalen
et al. 1999; Hodge et al. 2000). Furthermore, earthworm biomass is probably an important and
significant source of plant nutrients only in field and pot experiments, in which the inoculation rates
of earthworm biomass into soils is very high and when earthworm mortality or turnover rates are high.
For instance, the small soil volumes often used in pot experiments may be insufficient to
maintain earthworm populations inoculated into them to levels above the carrying capacity of the
soil. Under such conditions, many earthworms may die, liberating nutrients that, although in small
amounts relative to typical soil nutrient supplies, are enough to influence plant growth because of
the low soil:earthworm biomass ratio. This has occurred in many experiments, even in those that
based the rates of addition of earthworms to field populations but not to field biomass, thus adding
greater biomass than would normally occur in the field (e.g., Satchell 1958; Doube et al. 1994c;
Baker et al. 1996, 1997; Callaham and Hendrix 1998; Whalen et al. 1999). When larger soil volumes
were used, or more realistic earthworm populations and biomass were added, nutrients from dead
earthworms played a much smaller role in plant nutrition. This is probably the case even in field
situations with large earthworm biomass turnovers of up to 600 kg ha
−1
year
−1
(fresh mass), which
nevertheless can supply only a few kilograms per hectare per year of mineral N from decomposing
earthworm tissues (Brown et al., unpublished data).
CRAWLING FORWARD: THE CHALLENGE OF IDENTIFYING AND
QUANTIFYING THE POTENTIAL OF EARTHWORMS TO
INCREASE PLANT GROWTH
It is clear that a wide range of direct and indirect mechanisms of earthworm activities on plant
growth can be identified. However, to benefit from the potential of plant growth stimulation by
earthworms, promotion and decrease mechanisms in plant growth and their modes of action must
be understood within a given practical soil-plant-earthworm species and population context. There
