References

Role of Microorganisms in Carbon Cycling in Soils 145
Plant debris
1cm 1mm
minerals
5µm
bacteria
50 µm
Fig. 3. Hierarchy of soil aggregates (Oades and Waters 1991, modified). The graphs shows:
the bulk soil (above, left), a macroaggregate consisting of smaller aggregates including
plant debris (above, right), an intermediately sized aggregate composed of microaggregates
(below, right), a microaggegate showing minerals and bacteria associated with plant debris
(below, center)andamicroaggregatewithstabilizedsoilorganicmatter(below, left)
al. 1991; Schulten et al. 1993; Desjardins et al. 1994; Guggenberger et al. 1994)
point out that Corg content increases with diminishing particle size, whereas
the C/N ratio decreases. Jastrow et al. (1996) showed that organic matter
recently introduced into soil is predominantly located in larger aggregates.
Tisdall and Oades (1982) demonstrated that macroaggregates (> 250 µm)
can be destroyed by agricultural practices, whereas microaggregates cannot.
Figure 3 shows the concept of the aggregate hierarchy. (1) Plant residues
are introduced into the soil. This young organic matter comes into contact
with the mineral substances, e.g., through macrofaunal action, and
will be mainly contained in large aggregates several millimeters in size.
(2) During the decomposition of organic matter, the size of the organic
particles decreases, and the stability of the remaining Corg increases. This
Corg fraction is then located in smaller aggregates, which partly compose
larger ones. (3) Finally, we observe small mineral–organic matter–microbe
associations. After the death of the microbes, the mineral–organic matter
complexes, that physically protect the organic carbon against further decomposition,
remain. These associations are called microaggregates and
are highly stable.
Physical separation of particle size fractions revealed that much of the
soil microbial biomass is associated with the smaller fractions (fine silt and
clay); the highly variable amount of microbial biomass within the coarse
fractions strongly depends on the quantity and quality of the macroorganic
matter located there (Jocteur Monrozier et al. 1991). Investigations