Fjölrit RALA - 15.06.2004, Page 67

Shrinkage and drainage in undisturbed soil cores and aggregates
from a range of European volcanic soils
F. Bartoli
Laboratoire Sols et Environnement INPL(ENSAIA)-INRA, Vandoeuvre-lés-Nancy France,
Andosols are characterized by their large retention capacities due to their large capillary
porosities (low bulk densities). Despite previous work, shrinkage of Andosols has not been
fully explored. Shrinkage of fine-grained soils on drying is caused by movements of
microaggregates as a result of pore-water tension developed by capillary menisci. Our
hypothesis was therefore as follows: the intensity of the shrinkage process occurring in
undisturbed Andosol horizons varies as a function of their initial capillary porosities. We
widely validated this hypothesis. We present the results of shrinkage and drainage on
controlled drying of either undisturbed soil cores or aggregate counterparts from four Ah-Bw
horizon couples and one H1 horizon from five of the COST 622 reference volcanic soils of
Europe (Soil Resources of European Volcanic Systems). The soils used were all Andosols:
one Fulvic Silic and one Pachic Hydric Aluandic from Azores Islands, Portugal, one Molli
Silic and one Pachic Fulvic Silic from Iceland, and one Silic from Tenerife Island, Spain,
according to the WRB classification. Undisturbed wet soil cores of 28.6 cm3 (2.7 cm
diameter, 5 cm height) were sampled. In the laboratory, these soil cores were submitted to
capillary rise for 48 h and to controlled 40°C drying kinetics for 39 to 51 hours thereafter,
with regular (mostly each hour) measurements of both total soil volume and volumetric soil
moisture. The main results are as follows.
Total volumetric shrinkage of the soils was clearly controlled by initial capillary porosity
(Fig. la), which was in its tum controlled by the proportion of organo-mineral clay (Fig. lb),
determined by the Na resin method (Bartoli et al., 1991) and which concentrated most of
allophane, ferrrihydrite, Al-humus and organic coatings, key microaggregative soil
constituents (Bartoli et al., unpublished results).
y = 297,16 x -179,69 r = 0,920, p < 0,001
Initial capillary porosity (cm ^.cnr3 )
y = 0,006 x + 0,437 r = 0,921, p < 0,001
Ah
Bw
PRT-IM5
Ah
Bw
PRT-N6
Ah
Bw
ISL-N8
Ah
Bw
ESP-NIO
H1 ISL-N9
Figure 1 Relationships between initial capillary porosity and total volumetric shrinkage (on
40°C controlled drying kinetics) (a) or clay content (b). N5, N6, N8 and N10 Andosols were
those from Faial, Pico (Azores Islands), Iceland and Tenerife Island, respectively. The N9 H1
horizon, rich in organic C (21.8 %), belongs to a Pachic Fulvic Silic Andosol from Iceland
Shrinkage was also more pronounced for the Bw horizons than for their topsoil counterparts
(Fig. la). Furthemore, our results show inter-relationships between weathering intensity (clay
content) in the Bw horizons, development of capillary porosity (microaggregation) and
sensibility to shrinkage, with a positive gradient from the Icelandic Andosol to the Azores
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