Agriculture Reference
In-Depth Information
These interactions must be observed and considered not only at different scales, but also in a
number of directions. Typically, studies of interactions have focused on one-way processes, com-
monly disregarding the possibility of multifaceted interactions. For instance, the one-way effects
of earthworms on plant growth have been investigated intensively (see
Chapter 2
, this volume), but
there may be other interactions, such as between earthworms and root pathogens or beneficial soil
microorganisms (Doube et al. 1994d,e), that also affect plant growth. Further, the corresponding
effects of plants and microbial processes on earthworms are rarely considered in the same studies.
This chapter primarily concerns interactions among the soil biota and their effects on plants in
temperate and subtropical agricultural systems, with an emphasis on cropped soils. We consider
interactions between earthworms, soil microorganisms (bacteria, fungi, algae, and protozoa), and
plants (roots, seeds, and aboveground parts), and examine them at three different spatial scales:
micro (gut, burrows, and casts), meso (whole soil), and macro (field), although there are few data
on some topics, particularly on interactions at larger scales, and comparative data concerning the
different life strategies of earthworms are lacking. At each scale, we address the two-way interac-
tions between earthworms and other organisms or processes and attempt to reach some conclusions
about the functional significance of these interactions.
The theme is that the composition and activity of the microbial communities responsible for
the decomposition of organic substrates in soil can be affected significantly by the activity of soil
macroinvertebrates, especially earthworms, which therefore can play a vital role in regulating the
microbial processes that maintain the biological health of soils.
ORGANIC MATTER AND MICROBIAL COMMUNITIES
In this section, we consider the amounts, types, and distribution of organic materials entering soil
systems and the succession of microorganisms that promote OM decomposition. These inputs and
microbial processes are the basis of the soil food webs that support and are modified by earthworms
(Killham 1994; Brown 1995; Lavelle and Spain 2001).
A
O
I
D
P
NNUAL
RGANIC
NPUTS
AND
ECOMPOSITION
ROCESSES
Dead organic residues comprise the major part of soil OM (
Figure 12.2
)
. These are derived mainly
from dead animals and three plant sources: surface plant residues or litter (which may be incorpo-
rated into soils by tillage or biotic processes), dead roots and sloughed cells, and exudates leaked
into the rhizosphere from living roots (Lynch and Whipps 1990). Living OM makes up only a
relatively small proportion of the total OM in soils (Figure 12.2), and soil microorganisms comprise
the major part of the living biomass. Nevertheless, interactions between the three living soil
components (roots, macrofauna, and microorganisms) have major influences on soil processes.
Annual inputs of surface organic residues into cropping systems can be as high as 40 t ha
−
1
(e.g., in sugar cane), although the mass of surface residues is commonly much lower (e.g., ca. 2
to 6 t ha
in temperate cereal production). In agricultural systems, aboveground and subsurface
plant residue inputs are commonly of a similar order of magnitude, with roughly half (range 35 to
80%) of the net photosynthate transferred to the roots of plants and entering the soil as roots or
root exudates (Zwartz et al. 1994). During the first year in the soil, between 60 and 75% of the
introduced carbon is respired and therefore lost to the pool of soil OM (Jenkinson and Ladd 1981),
with the remainder persisting in organic-soil associations and living tissues (Figure 12.2). The living
tissues, which are part of the soil food web, include autotrophic algae and a vast array of het-
erotrophic primary and secondary decomposers.
For instance, in temperate cereal fields, the primary decomposers consist mainly of bacteria (30
to 90% by weight) and fungi (5 to 70% by weight), which are grazed by a variety of secondary
decomposers, such as protozoans (0.6 to 6.0% of the total biomass) and earthworms (0 to 14% of
the total biomass) (Brussard et al. 1990). The microbial biomass (bacteria, fungi, algae, rotifers,
−
1
