Agriculture Reference
In-Depth Information
Soil
Soil organic matter
0.1-10%
Mineral content
>90%
Living
15%
Nonliving
85%
Root biomass
5-15%
Faunal biomass
5-10%
Humus
70-85%
Inert carbon
5-20%
Microbial biomass
75-90%
Macro-organic matter
10-30%
FIGURE 12.2
Composition of soil organic matter in a typical fertile soil. (Adapted from Pankhurst et al.
1997.)
protozoa) in soil in temperate and subtropical regions occupies less than 1% of the surface soil
volume and is commonly on the order of 0.5 to 3 t ha
of biomass; that of the macrofauna is
commonly much lower (Brussard et al. 1990; Coleman et al. 1994), although higher values have
been recorded. For example, Doube et al.
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(1994a)
measured 1.15 t ha
biomass of the earthworm
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Aporrectodea trapezoides
in soil under a canola crop in Australia, and Barois et al. (1988) reported
up to 4 t ha
) in a tropical pasture in
Martinique. The amounts of carbon in the soil biota, nonetheless, are only a small percentage (<0.5
to 3.0%) of the total organic carbon in soil (Haines and Uren 1990; Gupta 1994; Sparling et al. 1994;
Lavelle and Spain 2001). The remaining organic carbon is bound in nonliving associations in soil.
The succession of microorganisms associated with the decomposition of each different type of
organic residue is similar in many respects but also has unique features that are dependent on their
origin (e.g., leaf tissue is buried with phyloplane species) and composition (e.g., all root exudates
leaked into the rhizosphere are readily assimilable) (Lavelle and Spain 2001). The microbial agents
responsible for decomposition of organic residues also vary with region, location, and management
practices. For example, at Lovinkhoeve in the Netherlands, Brussard et al. (1990) reported that
bacteria constituted by far the greatest portion of the microbial biomass (93 to 95%), with fungi
representing only about 5%. Similarly, in America at Horseshoe Bend in Georgia, Hendrix et al.
(1986, 1987) reported that bacteria predominated (60 to 76% biomass), with fungi making up only
17 to 22% of the microbial biomass. In contrast, Andren et al. (1988, 1990) stated that at
Kjetteslinge, Sweden, fungi dominated in a barley soil (64 to 69% of microbial biomass), and
bacteria made up only about 30% of the total biomass. In a forest soil in Canada, bacterial:fungal
ratios were 0.1 to 0.2 in the soil surface layers (Scheu and Parkinson 1994).
In the surface soils of agroecosystems (where most earthworms feed and grow), both bacteria
and fungi appear to be more abundant under reduced tillage than under conventional cultivation
(Hendrix et al. 1986; Brussard et al. 1990; Coleman et al. 1994). Absence of tillage appears to
increase the importance of fungi relative to bacteria as primary decomposers and hence as a source
for the food web. Consequently, fungivorous nematodes and earthworms are correspondingly more
abundant (Coleman et al. 1994). Such later increases in biomass appear to be in response to increases
in the biomass of a specific resource base (e.g., fungi), but the degree to which the composition of
the microbial community is influenced by the corresponding increase in predatory or bioturbating
organisms (e.g., earthworms) is unknown.
biomass of earthworms (mostly
Polypheretima elongata
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