Soil Formation - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 89

erosion and deposition. Locally, soil organic
carbon may be decreased by preferential
removal of the organic-rich surface layer, by
enhanced exposure of residual organic matter
to near-surface oxidative conditions, by in-
creased oxidation due to changes in soil tem-
perature and moisture, and by breakdown of
carbon protective aggregates (discussed fur-
ther below). Conversely, revegetation may
replenish organic matter at the erosional
site. The organic matter transported to a de-
positional site may be oxidized during trans-
port, or protected by burial sequestration
and reformation of soil aggregates. As discussed
below in the section on the role of humans,
the importance of the soil carbon-erosion
linkage has been increased significantly by
greatly enhanced erosion due to anthropo-
genic activities. The overall complexity of
these erosion-linked processes has led to a
variety of estimates on their net effect on the
global carbon budget that range from a glo-
bal source to a global sink (see, for example,
van Oost et al ., 2007). The latter authors cal-
culated that erosion was a net sink of carbon
globally (see also Chapters 3 and 19, respect-
ively, this volume).
A holistic view of near-surface pro-
cesses has recently emerged that can be con-
ceptualized as the operation of 'the critical
zone reactor' (Anderson et al ., 2007). In this
interpretation, solid material enters through
the lower regolith parent material boundary,
the reactor is stirred at the top by biological
and physical processes, sedimentary prod-
ucts are removed from the surface by ero-
sion and dissolved products leave by fluid
flow. Operating over Earth's entire land sur-
face, this critical zone reactor constitutes
our planet's weathering engine (Fig. 6.1 ).

et al ., 2011). The availability of water to pene-
trate the subsurface and to transport weather-
ing products is reflected in the water balance
at a site. Water availability for weathering is
the difference between infiltration and evapo-
transpiration; with surplus infiltration pro-
viding groundwater recharge and subsurface
discharge to surface waters. Greater subsur-
face flow volume removes dissolved weather-
ing products from the locale of source minerals;
preventing the back-reaction of kinetically
inhibiting solutes and maintaining pore
waters in a less saturated chemical state with
respect to the dissolution and alteration of
source minerals and the precipitation of
secondary minerals. Temperature effects occur
through the temperature dependence of the
thermodynamic solubility equilibrium of
mineral phases, and through the temperature
dependence of the rates of mineral dissol-
ution and precipitation reactions. In broad
terms, greater flushing of the subsurface and
warmer subsubsurface conditions should co-
incide with greater regolith development and
soil formation rates.
This picture of rates of weathering and
soil formation from regolith is confounded
by the fact that physical erosion of the land
surface occurs simultaneously with soil for-
mation. Net rates of soil formation are re-
flected in this flux balance of soil formation
and removal. The total flux of dissolved
and solid material from the regolith is the
denudation, expressed as the mass of ma-
terial per area per time. This is often ex-
pressed as an equivalent rate of lowering of
the land surface in units of depth per unit
time. In areas of high relief, greater topog-
raphy-driven water penetration and flush-
ing of the regolith and physical erosion
dominate. In this case, regolith alteration is
limited by the rate of chemical alteration;
i.e. weathering limited. In areas with low
rates of physical erosion, soil accumulates
and denudation is transport limited. Wilkin-
son and McElroy (2007) estimate globally
averaged rates of 0.062 mm year − 1 total land
surface denudation. This compares with areas
of rapid uplift that are estimated to be much
higher; 10- 20 mm year − 1 (cf. Burbank, 2002;
Montgomery and Brandon, 2002; Montgom-
ery, 2007), compared with erosion rates for

The role of climate

Water availability and temperature are con-
sidered to be among the primary factors in
regolith development and soil formation. The
conceptual underpinning and a synthesis of
data from weathering processes in granite ter-
rain across a range of climatic conditions il-
lustrate how these factors affect regolith
development and soil formation (Rasmussen