Soil Formation - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 87

With time, volumetric collapse replaces
expansion in many weathering sequences,
as chemical weathering processes lead to
elemental loss from the parent material. In
topographically stable environments, low
rates of physical denudation implies that
volumetric loss is caused by chemical de-
nudation - the removal of elements that are
transported hydrologically from the soil pro-
file as solutes (Brantley et al ., 2011). This
transition is in part facilitated by increased
access to water and the increased surface area
of mineral particles during the early expan-
sion phase. In the California beach terrace
and Hawaii chronosequence studies men-
tioned above, this transition from expansion
to collapse required times exceeding 40 Ky.
However, elemental gains may also occur, for
example by the addition of dust at the soil
surface (Derry and Chadwick, 2007). As is the
case for volumetric change, characterizing
element losses and gains requires normaliza-
tion to an index element according to the
equation (Brimhall and Dietrich, 1987):

are a fundamental control on soil formation
and reflect the influence of parent material.
Brantley (2005) calculated that the mean
lifetime of a 1 mm crystal at pH 5 might dif-
fer by over two orders of magnitude be-
tween fayalite (Fe 2 SiO 4 - 1900 years) and
K-feldspar (KAlSi 3 O 8 - 740,000 years); re-
moval of the mineral, quartz, might require
34,000,000 years under these conditions. In
the case of granodiorite referenced above,
Na and K removal in the early stages of
regolith formation reflects removal of
plagioclase (NaAlSi 3 O 8 ) and K-feldspar, re-
spectively. The rate and extent of element
removal is governed by a range of factors, as
summarized by Brantley (2005) and White
(2003). These differences in the weathering
rates of individual minerals are an import-
ant control that initial parent material can
exert on soil formation.
Time, as well as the nature of parent
material, also plays a significant role in the
availability of the important soil nutrients,
nitrogen (N) and phosphorus (P) (see review
by Vitousek et al ., 2010). Initially, soil P is
supplied predominantly from the parent
rock in the form of minerals such as apatite
((Ca 5 (PO 4 ) 3 (OH,F,Cl)). The P content of the
parent rock is thus a variable that provides
an initial condition on the P content of the
formed soil. With time, the initial endow-
ment of P is depleted by plant uptake, such
that very old soils tend to be P deficient. In
contrast, with the exception of some sedi-
mentary rocks, N is nearly absent in the par-
ent material. However, over time, symbiotic
microbial N fixers develop in the soil and
provide a mechanism to convert atmospheric
N to nutrient forms for biomass production.
As production occurs over time, the organic
N content of the soil accumulates with C org
and provides a sustained reservoir of nutri-
ent N. A sequence of soil N limitation fol-
lowed by P limitation may thus develop
with time (Lambers et al ., 2008).
Data from chronosequences have also
aided in the characterization of the evolu-
tion of organic carbon during soil forma-
tion. In the Egli et al . (2001) glacial moraine
study, soil organic carbon content was cor-
related linearly with the degree of initial ex-
pansion of the parent rock. More generally,

C
C
C
C

j , w

j , p

τ=
j

− 1 (6.2)

i , w

i , p

where t j is the mass transfer coefficient of
the j th element (t j = 0 means no mobiliza-
tion has occurred, − 1 reflects complete re-
moval and positive values reflect addition).
White et al . (2002) gave an example illus-
trating the application of Eqn (6.2) for
weathering of a kaolinitic soil overlying
porous saprolite formed from granodioritic
bedrock located in the state of Georgia,
USA. They found that t values for the major
rock-forming elements clustered near zero
in the deeper bedrock. However, calcium
(Ca) and sodium (Na) were removed pro-
gressively within the shallow bedrock (with
t values approaching − 1 within the bedrock
itself). Potassium and magnesium (Mg) were
mobilized within the overlying saprolite.
The depth dependency of these mass
transfer coefficients reflects the weathering
characteristics of specific minerals, par-
ticularly silicates that represent 90% of
Earth's crust. These weathering reactions