Agriculture Reference
In-Depth Information
V
C
G
, P
,
Q
R
S
ERTICAL
ARBON
RADIENTS
ATCHINESS
AND
UALITY
OF
ESIDUES
IN
OIL
There are strong vertical gradients in the concentrations of OM and microbial biomass in the upper
soil layers. These gradients are strongest where there is no tillage, since tillage redistributes organic
residues through the soil profile. Some species of earthworms bury surface plant residues and are
more common in direct drilled (DD) or no-till (NT) fields than in conventionally cultivated (CC)
fields (Chan 2001). Despite this, soil carbon gradients are commonly, but not always (e.g., Brussard
et al. 1990), steeper in DD/NT than in CC cereal fields (Hendrix et al. 1986; Haines and Uren
1990), suggesting that tillage is more effective than biological processes in redistributing surface
OM residues through the soil profile.
The patchiness of the distribution of OM residues through the topsoil varies with the spatial
scale examined (Robertson 1994). Patchiness is extremely high at a small spatial scale (e.g., that
of a root, a fragment of buried OM, or an earthworm cast) but decreases as the spatial scale increases
and the microsite differences become integrated into a relatively homogeneous pattern of OM
distribution (e.g., at the paddock or field scale). The degree to which earthworm activity alters this
pattern is understood poorly, but fresh earthworm casts are clearly the foci of new and high microbial
activity.
The quality of organic inputs into soil, which vary with type of vegetation and management
practices, has an important bearing on the composition and functioning of microbial communities.
For example, the leaves and stalks of cereal crops contain large amounts of cellulose, hemicellulose,
and lignin and have high C:N ratios, whereas in the rhizosphere, the inputs from roots are soluble
carbohydrates, amino acids, and the like, with few recalcitrant residues (Bowen and Harper 1989;
Moody et al. 1995; Lynch and Whipps 1990).
S
M
D
P
UCCESSIONS
OF
ICROORGANISMS
DURING
THE
ECOMPOSITION
ROCESSES
There is a clear succession of microorganisms associated with the decomposition of fragments of
organic residues in litter and soil (Ponge 1991; Robinson et al. 1993; Moody et al. 1995) and in
the rhizosphere (Zwartz et al. 1994). The species composition of the microbial communities is
determined by the nature of their substrates and the colonists (which include phyloplane species,
i.e., those introduced to soil with plant residues) and soil residents found near the organic residues.
The initial colonists of decomposing organic residues (e.g., cereal straw) are microbial species
that can exploit soluble organic compounds (carbohydrates, organic acids, amino acids) and are
dominated by fungi such as
species (Harper and Lynch 1985;
Bowen and Harper 1989; Moody et al. 1995) and bacteria such as
Mucor,
Pythium,
and
Penicillium
species. The
second successional phase in the decomposition of organic residues is dominated by fungi such as
Pseudomonas
Trichoderma, Fusarium,
species, which
have the capacity to digest cellulose and hemicellulose (Garnett 1981; Moody et al. 1995), although
much still remains undecomposed at the end of this phase. The third phase of decomposition is
associated with the slow metabolism of recalcitrant organic residues with high lignin or polyphenol
contents. Fungi such as white rot fungi (Lavelle et al. 1993) and basidiomycetes commonly dominate
this phase of the successional process, although cellulolytic fungi that can decompose delignified
polysaccharides (e.g.,
and
Chaetomium
species and bacteria such as
Bacillus
Fusarium
and
Trichoderma
spp.) may also occur at this stage (Bowen 1990;
Bowen and Harper 1990; Moody et al. 1995).
This flush of primary decomposers induces corresponding increases in the abundance of sec-
ondary decomposers (protozoa, bacterivorous and fungivorous nematodes) (Anderson 1994; Zwarz
et al. 1994). In the rhizosphere, similar biological processes occur in response to root exudation
and sloughed dead root cells (Zwarz et al. 1994). Rhizosphere populations of bacteria and proto-
zoans may be many times higher than in surrounding soil, but fungi commonly show a proportion-
ately smaller increase or no increase in relation to bulk soil (Newman 1985; Gilbert et al. 1993;
Gupta 1994; Zwarz et al. 1994).
