Agriculture Reference
In-Depth Information
amounts of nutrients associated with the earthworm burrow walls can benefit root growth (Graff
1971a), and cast-filled earthworm burrows usually have large quantities of plant-available nutrients
stored in the casts (mechanism 7). The distribution of roots in soil is often related closely to the
zones of earthworm activity (Edwards and Lofty 1978, 1980), and root densities can be increased
significantly by earthworm activities. In newly reclaimed polders inoculated with earthworms and
planted with fruit trees in the Netherlands, van Rhee (1977) reported significantly greater root
densities in the earthworm-inoculated sites but no effects on fruit production. Conversely, in a pot
experiment in Mexico that compared pots inoculated with earthworms those with no earthworms,
Brown (1999) observed significantly greater root densities, as well as more root and shoot biomass,
but no increase in productivity of beans in the presence of
Polypheretima elongata
. The earthworm
burrows were commonly filled with roots, and the root distribution throughout these pots showed
a much more even (homogeneous) distribution, a factor considered to confer greater plant resistance
to environmental stresses (Smucker 1993).
The proportion of roots found in deep earthworm burrows (e.g., in the B horizons) compared
with those in the soil matrix can be very high (Kirkham 1981; Logsdon and Linden 1992), and
these roots may be important in maintaining plant water dynamics. However, estimates of the
proportion of roots in earthworm burrows may be exaggerated because roots in earthworm burrows
are more easily observed, whereas the rest of the root system may be concealed in the soil matrix
(Logsdon and Linden 1992; Kretzschmar 1998). A three-dimensional estimation of interactions
between roots and earthworm burrows is still not available (Kretzschmar 1998), and considerable
efforts need to be made to understand these interactions and the mechanisms that control them
(Tisdall and McKenzie 1995).
Thus, it is a combination of the composition (ecological category, species) of the earthworm
community present at a given location, the placement of their casts (surface, belowground, deep
in soil, near roots, etc.), the quantities of casts deposited and their age, and the amount, type, depth,
and openness of the earthworm burrows produced, the interaction of microorganisms with earth-
worm structures, the physicochemical soil environment, and land management that determine the
ultimate effects of earthworms on soil structure and the rooting environment.
7. C
HANGES
IN
N
UTRIENT
S
PATIOTEMPORAL
A
VAILABILITY
C
AUSED
BY
E
ARTHWORMS
The availability of many essential plant nutrients has been shown to increase in structures produced
by various earthworm species, especially in their casts (e.g., Mulongoy and Bedoret 1989; Barois
et al. 1999) and burrow walls. This greater nutrient availability is mainly a result of the selective
feeding of earthworms on regions of the soil rich in organic matter, clay, and nutrients (Barois et
al. 1999; Cortez and Hameed 2001), gut-associated processes, and cast-associated processes (
Figure
species; Devliegher and Verstraete 1997; Brussaard 1999). Such processes include the grinding
action of the gizzard, the priming of microbial activity in the gut, and the greater populations and
activity of microorganisms in the earthworm casts and burrows (Figure 2.7), that induce chemical
changes in earthworm-worked soil (e.g., Lee 1985; Edwards and Bohlen 1996).
These nutrient enrichment processes (Devliegher and Verstraete 1995; Brussaard 1999) differ
greatly according to the earthworm species involved, their ecological categories, and the feeding
habits, particularly the amounts of plant litter they ingest. The type and placement of the earthworm
casts are also important, affecting the spatiotemporal availability of the nutrients they contain
(Figure 2.7). Surface earthworm casts dry out much more quickly, harden, and, if compact, are
likely to limit root penetration, thereby reducing the ability of plant roots to obtain the nutrients
stored inside the casts (nutrient protection) until they are broken down (
Figure 2.6
and Figure 2.7).
Belowground earthworm casts remain fresh and moist for much longer periods of time and, if they
are of the decompact types (with more meso- and macropores and macroaggregates), allow roots
to penetrate more easily (Figure 2.6) and profit from the greater nutrient contents available to plants.
