Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 201

The amount of clay dispersed from soil depends both on the clay
dispersibility per unit of exposed surface area and on the exposed surface
areas of aggregates (Kay and Dexter, 1990). In order to focus on the
former, we measured clay dispersibility on 2-4 mm aggregates. Air-dried
aggregates (20 g) were immersed in 75 ml of de-ionized water and the vials
were shaken end-over-end for 1 h. Dispersible clay at this stage was issued
from aggregate disruption at scales > 0.2 mm. Dispersed clay was recov-
ered by three successive sedimentations and freeze dried. The experiments
were performed in triplicate for each soil. Particle size fractionation was also
performed on sub-samples of 2-4 mm aggregates.

Results

Characteristics of the < 2 m m fractions (Table 3.14.1)

The clay-sized fractions were composed of kaolinite, illite and chlorite, with
small amounts of quartz. The comparison of the particle size fractionation
with the classical mechanical analyses of the soils indicated that all clay
was not recovered in the forest soil and in the soil cultivated for 7 years:
very stable microaggregates could not be dispersed with ultrasound and
remained in the silt fractions. The C content of the < 2 m fractions

decreased with cultivation, but the contribution of C in forest < 2

µ

µ mto

soil C increased with cultivation.

Clay dispersion

Clay dispersibility at 1 h was calculated by dividing the amount of dispersed
clay by the total clay in 2-4 mm aggregates. It increased with time of
cultivation and was related linearly to the C content of dispersible clay

Table 3.14.1. Characteristics of the < 2 µ m fractions.

Dispersible
clay
(% F < 2 µ m)

C content
dispersible clay
(mg C g − 1 )

Mass
(g 100 g − 1 soil)

C content
(mg C g − 1 )

Time of
cultivation
(years)

C
(% soil
C)

C : N

Mean Mean Mean SD Mean Mean SD SD SD SD

0 14.7 1.3 117.8 5.2 15.2 0.2 29% 7.3 (0.4) 89.2 0.4

7 15.5 1.3 77.5 1 .2 13.2 0.2 38% 10.7 (0.2) 65.8 11.4

35 16.8 1.9 48.1 1.4 12.2 0.2 48% 15.3 (0.2) 38.7 2.5