Dust Production Mechanisms - Mineral Dust: A Key Player in the Earth System - page 99

particles will differ from that of the parent soil and, moreover, will change as a
function of U * .As U * increases, it becomes possible to mobilise larger and smaller
particles and thus to broaden the size distribution of the mobilised soil particles.
Because the dependence of the threshold on particle size is non-linear, a precise
description of the soil size distribution is required. The various forces that define the
erosion threshold act on individual particles in their natural state of aggregation. As
a result, what must be documented is the size distribution of in situ particles. This
distribution cannot be determined based on techniques that lead to disaggregation of
soil particles, as classically used to determine soil texture. Soil texture determination
provides the size distribution of the elementary particles composing a soil but does
not necessarily reflect the available in situ soil size distribution except if in situ soil
particles are individual elementary particles. Such a situation can be encountered,
for example, for sandy soils composed of individual quartz grains, but for loamy
and clayey soils, the two distributions are very different. Here, the undisturbed
distribution is mainly composed of particles in the range of 50-100 m, while fully
disturbed size distributions exhibit large fractions of particles in the single-micron
range (Shao 2000 ). In fact, fine dust particles are generally not present as loose
individual particles in soils. They are present in super-micron soil aggregates or
they are attached to larger solid soil grains (e.g. quartz grains). It must be noted that
there is no standard technique to measure the in situ soil size distribution and that
these soil characteristics are typically not documented in soil datasets. In addition,
there is no simple relationship to convert soil texture into in situ or undisturbed, soil
size distributions (Laurent et al. 2006 ). The in situ size distribution of the soil may
be affected by its composition. As an example, salts or organic matter can increase
the level of aggregation of soils.
Practically speaking, soils from arid or semi-arid regions usually do not lack
particles in the optimum size range (see the soil size distribution in Chatenet
et al. 1996 ; Laurent et al. 2006 ). As a result, in the field, the erosion threshold is
often controlled by the influence of other factors such as soil moisture and surface
roughness.

5.3.2 Influence of Soil Moisture

Soil moisture generally increases U t (Chepil 1956 ; Bisal and Hsieh 1966 ; Saleh
and Fryrear 1995 ). McKenna-Neuman and Nickling ( 1989 ) show that sand grains
are held together by the capillary effect of soil moisture. Fécan et al. ( 1999 ) notice
that the minimal soil moisture, for which an increase of U t is measured, varies as a
function of the soil texture. This can be explained by the capability of soil particles
to retain soil water. Soil water usually takes the form of a water film around the
grains, and the amount of water contained in this film increases with the soil clay
content because clay minerals can absorb water in the interlayer positions. When
the water absorption capacity of mineral grains is reached, the excess water can
accumulate to form interstitial water between the soil grains where then capillarity