Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 202

(Table 3.14.1, r 2 = 0.998). Dispersible clay had a lower C content than the
total < 2 µ m fraction.

Microstructure

Different forms of organic matter were observed in the < 2 m fractions:

micrometre cell wall or plant cell residues (Fig. 3.14.1a), microorganisms
or their remnants (Fig. 3.14.1b) and shapeless and structureless organic
matter (Fig. 3.14.1c). This organic matter was either free, i.e. not associ-
ated with minerals (Fig. 3.14.1a and b), or bound to clay particles.
Microaggregates enclosing plant debris were observed (Fig. 3.14.1d) as
well as complex clay aggregates in which the OM occurred as thin layers
between small stacks of clay sheets (Fig. 3.14.1e). Organic constituents
were much more abundant in thin sections from the forest soil than in those
from the soils cultivated for 7 or 35 years. Preliminary results from image
analyses showed that organomineral aggregates and free organic particles
were more abundant in the forest sample than in < 2

µ

µ m fractions from the

cultivated soils (Table 3.14.2).

Discussion and Conclusions

Cultivation of the humic loamy soils of this area led in a few years to
decreased aggregate stability and erosion problems (Le Bissonnais and
Arrouays, 1997). The present results show that decreases in aggregate
stability on the millimetre scale are accompanied by changes on the
micro-scale: clay-organic matter microaggregates were less abundant and
clay was more easily dispersed. Increases in clay dispersibility were also
found by Curtin et al . (1994) and by Fuller et al . (1995). In a previous study
(Chenu et al ., 1998, 2000), we measured contact angles of water on the
<2

m fractions from the same soils. The wettability of the fraction
decreased with the organic matter content on the fraction and with cultiva-
tion. Changes in macroaggregate stability in these soils with cultivation
were then partly due to modifications of the composition of the clay-sized
fractions and hence of their cohesion and wettability.
Balesdent et al . (1998) found that soil OM turnover was eight times
slower under forest than in cultivated soils. Our electron microscopy
observations showed that OM was largely associated with clay minerals
in the < 2

µ

m fraction of the forest soil, both as coatings in clay particles
and entrapped within very stable microaggregates. With cultivation,
organomineral microaggregates were much less abundant. We suggest that
OM was physically protected in the forest soil by virtue of its association

µ