Agriculture Reference
In-Depth Information
CO
2
Intimate mixing of mineral
and organic particles
Litter comminution and decomposition
Microbial and faunal proliferation
External rumen
DRILOSPHERE COMPONENTS
MIDDEN
CO
2
N
2
BURROW SYSTEM
CASTS
N
2
O
G
as
es, wate
r
B
urrows open to soil surface
Soil surface
Roots growing in
burrow and into cast
Mostly below ground
Changes in microbial biomass,
activity, diversity and
successional processes
Changes in the concentration
of nutrients (C, N, some
macro and micro)
Microporosity
Aggregation (C protection)
EARTHWORM
SURFACE
Closed burrows
(cast-filled)
Nutrient
absorption, leaching
Mucus and other secretions
Respiration
Open burrows/macro-
porosity
Walls
(Changes in C, N, microbial
populations and activity)
Aestivation/diapause chamber
EARTHWORM
INTERNAL
PROCESSES
FOREGUT
MIDGUT
HINDGUT
Antifungal and
antibacterial
secretions
(antibiotics?)
External mucus
secretion (along
whole body
surface)
Digestion of fungal hyphae,
bacteria, trophozoites, algae
Intestinal mucus
(assimilable C and N)
Exonephridial
(external) N
excretion
Assoc. N
2
fixation
(e.g.,
Chlostridia
?)
Reassimilation of C
Food selection/
ingestion
Crop
C
Feces
egestion
(casts)
Bacterial stimulation,
enzyme production,
digestion of organic
compounds
Changes in P & K
solubility,
ammonification
(Org. N NH
3
)
Calciferous glands
(CaCO
3
secretion,
pH increase)
Metabolic
wastes
Gizzard
(grinding)
Tissue production
(nutrient
immobilization)
Enteronephridial
N excretion
(into the gut)
Figure 12.1
Schematic representation of the drilosphere components and their relationships with the external
and internal earthworm environment, microorganisms, and organic matter. (Modified from Brown et al. 2000.)
characteristics (parent material, soil type, climatic regime). These interactions occur in a number
of key biological spheres that are the foci of intense microbial activity associated with the decom-
position of organic residues, including those from the rhizosphere (roots), detritusphere (surface
detritus), drilosphere (earthworms), myrmecosphere (ants), and termitosphere (termites) (Lavelle
2002). Some of the functional interactions between microorganisms, earthworms, and the structures
in the drilosphere they generate (Figure 12.1) can affect soil organic matter (OM) dynamics
(production, decomposition, stabilization) at various spatiotemporal scales, from the earthworm gut
to its burrows and casts (Lavelle 1997; Brown et al. 2000; see
Chapter 8
, this volume).
The spatial scales at which soil organisms act are determined mainly by their size and mode
of operation. Three spatial scales can be recognized based on animal size (micro, meso, and macro;
Wallwork 1970), and the processes that occur at each scale are crucial to primary production,
decomposition, nutrient cycling, soil aggregation, and root health. At the microscale, there are algae
and bacteria (which are unable to move long distances except if carried by water or larger soil
organisms) and fungi (in which hyphal growth provides the capacity to colonize new soil).
Still at a microscale, but increasing in size and spatial influence, the micro-food web includes
the microfauna, such as nematodes, protozoa, rotifers, and other organisms that feed mainly on the
microflora and are important in regulating nutrient cycling, particularly in the rhizosphere (e.g.,
Clarholm 1985; Ingham et al. 1985).
At the mesoscale, larger organisms such as enchytraeids and micro- and mesoarthropods feed
on litter, microorganisms, and invertebrates and are important in accelerating nutrient cycling and
in the small-scale dispersal of microorganisms (Hassal et al. 1987).
Finally at the macroscale, there are larger invertebrates, such as earthworms, termites, mac-
roarthropods and ants; these are termed
(Lavelle et al. 1997). They disperse
microorganisms, produce important physical structures (mounds, burrows, casts, pellets) that may
occupy or modify a great portion of the soil volume (and the microbial communities found therein),
and therefore regulate many soil processes.
ecosystem engineers
