Soil Formation - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 88

the organic carbon content of soil has been
shown to increase with time in chronose-
quences. Typically, there is an initial rapid
increase in organic carbon during the early
stages of soil formation (e.g. Harden et al .,
1992). In a recent study of soil formation pro-
cesses in the forefield of the Damma Glacier
in central Switzerland, the progressive stages of
soil formation as the glacier has retreated
over the past 150 years demonstrate increases
in soil carbon to around 3% in the most de-
veloped soil profile studied at the site (Ber-
nasconi et al ., 2011). The C org accumulation in
soil may then continue for many millennia, but
at a much lower rate. In support of this con-
clusion, Schlesinger (1990) derived long-term
C org accumulation rate data from (unculti-
vated) soil chronosequences spanning a wide
range of ecosystem types. He documented
that this slow accumulation accounted for
<1% of terrestrial net primary production.
Understanding C org -related processes in
soil is confounded by the fact that there are
multiple timescales of organic accumula-
tion and decomposition. Characterizing the
rates of these processes has benefited sub-
stantially from the application of radiocar-
bon ( 14 C) dating techniques (see Trumbore,
2009, for an overview). This is because dif-
ferent classes of organic compounds have
vastly different turnover times. Soil litter has
a short residence time measured in years.
Longer-term stabilization of C org is tied to in-
organic soil evolution pathways that result
in sorption to mineral phases with high sur-
face area and surface charge. The resulting
mineral-stabilized C org may have residence
times measured in millennia. Soil C org may
also be protected by the formation of aggre-
gates (discussed in more detail below).

surface of the regolith (physical denudation)
driven by topography removes only mater-
ial from the uppermost layers. In addition to
the export of material, erosion can also re-
sult in downslope deposition, thus thicken-
ing the downslope profile and burying
material that was formerly at the surface.
The thickness of the regolith along topo-
graphic gradients is determined by the com-
petition between the kinetics of mineral
weathering (chemical denudation rate) and
the physical denudation rate. At one ex-
treme, where the physical denudation rate
is much more rapid than mineral weather-
ing kinetics, the regolith will be thin, and
relatively fresh material is then continu-
ously exposed to chemical weathering. In
effect, physical denudation has the effect of
moving less weathered material upward in
the profile (Fig. 6.1 ).
The balance between chemical and phys-
ical denudation has been the focus of consid-
erable research (e.g. Yoo and Mudd, 2008;
Gabet and Mudd, 2009; Hilley et al ., 2010). A
correlation between rates of physical denuda-
tion and chemical weathering has been recog-
nized. This correlation may result from the
fact that as the denudation rate increases,
fresher, more readily weathered material is
brought to the surface. One recent analysis
(West, 2012) concluded that the effective
thickness of the weathering zone varied rela-
tively little across several orders of magnitude
of denudation rate, but with increasing de-
nudation rate, the contribution of bedrock
weathering becomes increasingly significant.
A powerful advance in characterizing
the mechanisms of erosion has been the
analysis of cosmogenic nuclides such as
10 Be and 26 Al in river sediment, soil hori-
zons, profiles and hill slopes (see review by
von Blanckenburg, 2005). These data inte-
grate denudation history on timescales of
10 3 -10 5 years. Among the significant trends
identified by von Blanckenburg, rates of
weathering co-vary primarily with physical
erosion rates and much less with tempera-
ture or precipitation.
Erosion and soil organic carbon content
are linked. As summarized by Lal (2003),
erosion can have a number of effects on soil
carbon that may differ between the sites of

Role of topography

The low topographic gradient regolith-forming
regime associated with chronosequences is
modified significantly by the introduction
of a topographic gradient, which changes
the overall balance of weathering processes.
Whereas chemical denudation removes ma-
terial from throughout the weathering pro-
file, the physical removal of material from the