Agriculture Reference
In-Depth Information
After a short sketch of the evolution of
the soil carbon concept, illustrating the
interactions between scientific, industrial/
technical and societal/economic changes,
we show that sufficient techniques are avail-
able for the implementation of large-scale
SOC sequestration. Furthermore, we dis-
cuss the discrepancies between knowledge
of available techniques and implementa-
tion. An analysis of the bottlenecks to im-
plementation then identifies where decisions
have to be made in order to enable robust,
integrated, sustainable and overarching SOC
management strategies. One example is pro-
vided by the potential to restore SOC stocks
where they have been depleted on large
geographical scales, such as the cropland
soils of the USA (Ogle
et al
., 2003, 2006,
2010). The observed increase of SOC in
these soils can be attributed mainly to the
conversion of annual cropland to grassland,
a strategy that has been supported by the US
federally funded Conservation Reserve Pro-
gram (Ogle
et al
., 2010).
forestry research. Different 'humus hori-
zons' and their development and functions
for forest growth were analysed and de-
scribed. At that time, understanding of the
biological origins of humus began to be
based on observations of the different hu-
mification stages of organic remains (Man-
lay
et al
., 2007). The 'humus principle' gave
an even wider, more philosophical and hol-
istic perception of humus, defining humus
as a principle that signified the relationship
between 'the living earth and other organ-
isms' (Rusch, 1968).
Initial understanding of the role of
SOM in plant growth did not differentiate
between plant nutrients and carbon sources.
It was believed that plants relied on soil as a
source of carbon (carbon heterotrophy) and
nutrients. This led to the assumption that
soils should be tilled as frequently as pos-
sible to grind the soil particles as small as
possible to enhance plant root uptake. At
the end of the 18th century, some experi-
mentalists demonstrated the gaseous origin
of carbon and discovered the role of light in
photosynthesis. However, their findings were
highly disputed (Manlay
et al
., 2007). Dur-
ing that time, and independently of the on-
going dispute, Thaer developed the first
comprehensive fertilization scheme for sus-
tainable agriculture which was based on the
whole plant-soil system. Thaer's systematic
approach to assess soil fertility and to de-
rive a rational fertilizer scheme included
factors such as soil texture, lime and humus
content, crop species, organic fertilization
management and yield (Feller
et al
., 2003).
The concept of the cycling of organic
and mineral substances was central in Lieb-
ig's humus theory of soils. However, the in-
tegrative role of SOM moved into the back-
ground through the recognition of the role
of 'salts in soils' as plant nutrients. Liebig
was the first to synthesize this new know-
ledge into his
Theory of Mineral Nutrition
of Plants
(Liebig, 1840). Liebig's concepts
can be regarded as the basis of modern agri-
cultural science, accelerating the produc-
tion and use of mineral fertilizers. However,
the role of SOM as vital for plant growth
was not abandoned completely, and many
scientists at that time held an intermediate
A Short History of the
Soil Carbon Concept
A comprehensive and detailed review on
the history and evolution of soil organic
matter (SOM) concepts is given by Manlay
et al
. (2007). The concept of SOC (see Chap-
ter
14,
this volume, for the distinction be-
tween SOC and SOM) and its importance to
soil fertility and sustainability is now well
established in the scientific and the agro-
nomic community. However, this has not
always been the case. The recognition of
SOM as a concept is linked closely to the
understanding of its functions, ecologically
and in terms of sustainable and productive
cropping.
The term 'humus' was regarded as a
precursor for SOM. The word 'humus' was
used descriptively for different constituents
of the soil. Its meaning changed over time
and varied in different parts of Europe. In
the 19th century, a more structured ap-
proach to defining the meaning of 'humus'
was taken, with efforts coming mainly from
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