Soil Formation - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 92

physically transport this material through
burrowing and turbation.
Fresh organic material is gradually
transformed into humic material that binds
to mineral surfaces and is rendered less
bioavailable and less degradable, thus pre-
serving the remaining organic carbon. The
smallest mineral particles, clay minerals and
nanometric-sized oxides provide the great-
est specific surface area and associated ad-
sorption capacity by mineral mass. Sorption
of humic substances to these polar mineral
surfaces renders them more hydrophobic
and prone to aggregation due to the ionic
composition of pore water solutions. These
agglomerations, together with bound micro-
organisms and fragments of rock and decay-
ing biomass, form particles of micron size
and larger. These intermediate aggregates
can also be incorporated into the larger ac-
cretions. The sorption surface area of rock
fragments and minerals within aggregates
also provides sorption and ion exchange
surfaces that sequester ionic forms of min-
eral elements (PO 3− , K + ) as they are released
during the decomposition of biomass; hence
providing a reservoir of nutrients within the
soil profile. The formation of larger aggre-
gates also favours the drainage of pore
waters and ingress of atmospheric O 2 to
support root and microbial respiration in
the subsurface.
During soil formation, aggregate forma-
tion and the development of soil structure
are essential to establishing the full range
of vital soil functions, and are also an indi-
cator of the state of a soil and its capacity to
deliver ecosystem services of value. Central
to this are both the input of recently pro-
duced organic matter, to drive rapid de-
composition and nutrient release associated
with larger accretions of aggregates, and the
role of decomposed organic matter, to be
preserved as a surface-bound coating that
also contributes to the formation of larger
aggregates. All of these components and
characteristics of aggregates and their for-
mation contribute with the specific roles
described here to deliver the full range of
soil functions. Proactive management of
soil carbon provides a key point of human
intervention to enhance soil structure and

the associated delivery of soil functions
(Chapters 9-14, this volume).

The Role of Humans

The natural processes outlined above are
being impacted and in some cases over-
whelmed by anthropogenic ones. The rela-
tion between humans and the Earth has
changed markedly over evolutionary his-
tory, from a minor influence on ecosystems
to being major drivers of ecosystem change
on a global scale. The geologist, A.P. Pavlov
(1854-1929), was perhaps the first to recog-
nize the magnitude of human influence on
global ecosystems, referencing his contem-
porary time as the 'anthropogenic era' (see
Vernadsky, 2005); many years later, the 'An-
thropocene' has been proposed as a new
geologic epoch (Zalasiewicz, 2008).
One example of the role of humans is
the movement of sediment including soil
on Earth's surface. Wilkinson and McElroy
(2007) argue that geologic denudation of the
continents occurs dominantly at headwaters
(83% of global river flux is derived from the
most elevated 10% of Earth's surface). In
contrast, subarial erosion by humans (agri-
culture primarily) now dominantly occurs
at lower elevations. The mean denudation
over the past half-billion years of Earth his-
tory has lowered continental surfaces by a
few tens of metres per million years. In
comparison, construction and agricultural
activities currently result in the transport of
enough sediment and rock to lower all ice-
free continental surfaces by a few hundred
metres per million years. Mean cropland
soil loss from the USA (amounting to about
11% of the global land area) is equivalent to
a lowering of over 200 m per million years
(Wilkinson and McElroy, 2007). Cropland
erosion rates in Asia, Africa and South
America are higher yet (Pimentel et al .,
1995). As a result, humans are now an order
of magnitude more important at moving
sediment than the sum of all other natural
processes operating on the surface of the
planet (Wilkinson, 2005). Maps reconstruct-
ing the development of agriculture across
the USA between 1790 and 1997 (Waisanen