Soil Formation - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 86

(Taylor et al ., 2009). These organisms are sym-
biotic to plants and play a key role in provid-
ing plants with inorganic and organic nutrients
that they might not otherwise be able to ob-
tain. Taylor et al . (2009) reviewed evidence
that the rise of land plants during the Devon-
ian and subsequent expansion of angiosperms
from the Cretaceous onwards, and the con-
comitant drops in atmospheric CO 2 , coincided
with the co-evolution of land plants, respect-
ively, with these fungi. Subsequent evidence
was provided through mechanistic mathem-
atical modelling of biological weathering pro-
cesses (Taylor et al ., 2011) and experimental
studies under differing modern analogues to
evolutionary tree physiology and mycorrhizal
associations (Quirk et al ., 2012). These stud-
ies demonstrated that these co-evolutionary
trends could explain the enhanced biological
weathering of the continents associated with
soil formation as the mechanism for atmos-
pheric CO 2 decline.

rest in significant part on the concepts intro-
duced by Brimhall and co-workers (Brim-
hall and Dietrich, 1987; Brimhall et al .,
1991). They defined soil strain ( ∈ ), which is
the change in volume relative to the initial
volume of parent material, by the equation:

r
r

C i

= pp
s

,

1

(6.1)

∈ − ,

C

i , s

where r p and r s are the bulk density of the
parent material and soil, respectively, and
C i ,p and C i ,s are the concentration (wt %) of
an immobile element i in the parent and
soil, respectively. Normalization to an index
element, for which immobility is assumed,
is required because of variable gains and
losses of both the major and minor elements
during weathering. The elements zirconium
(Zr) and titanium (Ti) are commonly em-
ployed as index elements, but niobium (Nb)
and tantalum (Ta) might be better choices in
strongly weathered soil (Kurtz et al ., 2000).
As noted above, positive strain (expan-
sion) has been shown to be characteristic
of the early stages of weathering processes.
For example, Egli et al . (2001) reported on
a chronosequence of glacial moraines in
the Swiss Alps ranging in age from 150 to
10,000 ybp. They found that expansion (posi-
tive strain) was a characteristic of young soils
due to the increased porosity of soil com-
pared to rock, as well as the incorporation of
low-density organic matter near the surface.
Likewise, initial expansion over times meas-
ured in thousands to tens of thousands of
years characterizes both a 240 ka chronose-
quence formed on beach terraces in California
(Merritts et al ., 1992), and a ~ 4 My chrono-
sequence formed on dated basalt flows in
Hawaii (Vitousek et al ., 1997). Thus, disag-
gregation of rock and increased porosity
characterizes the initial processes of soil for-
mation over a broad range of climatic condi-
tions. Biological processes largely drive this
expansion. The dominant biological pro-
cesses are associated with the accumulation
of roots and organic matter. Roots exert sig-
nificant force that can break apart the parent
rock, initially entering cracks as small as
100 μm and exerting pressures over 1 MPa
(Gabet et al ., 2003).

The role of time

The role of time in soil formation, with
insights into biotic influences, can be illus-
trated through consideration of soil chron-
osequences. Chronosequences are suites of
soils of different ages in topographically
stable environments evolved under similar
conditions of vegetation, climate and parent
material (see Birkland, 1999, for an over-
view). Chronosequences permit the isola-
tion of the variable time in weathering and
soil formation and are, in effect, the result of
a natural experiment. It also should be
noted that the relative stability of the state
factors of soil formation that define a chron-
osequence are not likely to be replicated for
the majority of soil weathering environments.
Nevertheless, the study of chronosequences
has proved to be a powerful tool to under-
stand critical zone processes and soil-forming
processes.
A key observation based on chronose-
quence studies is that the early stages of soil
formation are associated with volumetric
increase due to rock break-up, whereas the
later stages are characterized by volumetric
decrease due to mass loss. These conclusions