Fjölrit RALA - 15.06.2004, Side 69

Physical and chemical study on irreversible changes of water retention
properties in an Azores Andisol.
C. Fernandez, F. van Oort and I. Lamy
INRA, Unité de Science du Sol, RD-10, 78026 Versailles, France
Introduction
Andisols are widely used for agronomic land-use in many parts of the world. However, they
are highly responsive to farming practices that do not sufficiently take into account their
specific physical and physico-chemical properties, in particular irreversible drying effects.
These effects have been for a long time attributed to a rearrangement of pore space (Maeda &
Warkentin, 1975) due to a micro-aggregation of elementary allophane particles (Kubota,
1972). On the other hand, Wada (1989) mentioned that negative charges could develop during
desiccation related to changes in coordination of some surface A1 atoms. In order to identify
mechanisms goveming irreversible drying effects of allophanic material, we studied both soil
water retention and surface charge characteristics on samples of an andisol with exceptional
model physical properties.
Material & Methods
Samples of a deep, 7-Snd horizon in an andisol located at the Azores Islands (Lagoa do
Caiado, Pico Island) were selected during the 2001 Cost 622 meeting. The soil was slightly
acid (pHwater 6.2), highly weathered, containing 24% allophane. It displayed a complete
isotropic character under optical polarising light. Physical study showed an extreme low bulk
density (0.18 Mg/m3) and a high water content (> 450%). Soil water retention was measured
during desiccation and rehydratation of centimetre-sized samples, initially equilibrated at
various water potentials. On the same samples, we determined the Zero Point of Charge
(ZPC) and total variable charges by NaOH - HCIO4 titration, with different ionic strenght.
Results & Discussion
During desiccation, the total water content decreased from 450 to 20% (air-dried), and normal
shrinkage was observed (Fig. 1) until drying to < 32.5 h.r. (< -150 Mpa). These results
indicated an absence of soil structure at a mm and pm scale in the studied soil material.
Rewetting experiments showed that notable loss of hydration capacity occurred for water
potentials > -320 kPa and < -33 MPa. For lower water potentials (-33 to - 234 MPa) only 7%
of the initial water content was reached (30g/100g soil). ZPC values strongly varied with the
state of drying of the samples. Calculated surface charges at different water potentials
revealed two distinct domains: > - 320 kPa and < - lMpa. The soil's rehydration capacity after
drying and the amounts of total negative surface charges were strongly correlated (Fig. 2).
Conclusions
This combined physical and chemical study suggested a two-step mechanism for irreversible
changes of water retention properties in andisols. On drying of a field moist sample, the first
step was mainly chemical and corresponded to coalescence of elementary allophane particles
resulting from water extraction with reduction of surface charges together with a slight
increase of bulk density. At higher water potentials, a second step was a mainly physical:
consolidation of the material with a strong increasing bulk density. Only minor losses of
surface charges were observed due to some interaction of chains of allophane particles. For
the studied model material, our findings highlighted a distinct threshold of irreversibility of
physico-chemical properties for water potential of approximately -500 kPa.
51