Soil as a Support of Biodiversity and Functions - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 142

services on which human well-being is de-
pendent. Soil is also one of the most import-
ant reservoirs of biological diversity on our
planet and, above all, one of the last bastions
of such biodiversity (Swift et al ., 1998). Since
most soil organisms are heterotrophs, SOM
is of major importance for the sustaining of
this biodiversity. One gram of soil is reported
to host up to 10 billion microorganisms and
thousands of different species (Torsvik and
Øvreås, 2002; Roesch et al ., 2007). Soil is also
the site of numerous biological and ecological
processes, and therefore performs a consider-
able number of ecosystem services resulting
from the complex taxonomic and functional
assemblages of the indigenous communities
and interactions between organisms (Cole-
man and Whitman, 2004). Pimentel et al .
(1997) estimated the economic profit derived
from soil biodiversity at US$1546 billion, al-
though the relative value of the associated
services remains to be determined (Huguenin
et al ., 2006).
Among the ecological, social and eco-
nomic services identified, the role of soil as
a reservoir of biodiversity has now been well
established, along with its role in surface-
water purification, recycling of mineral
elements (soil fertility) and carbon storage
(as a sink for atmospheric CO 2 , soil fertility),
the latter processes being related directly to
climatic changes and plant productivity
( Pimentel et al ., 1997). However, the studies
involving a characterization/quantification
of soil biodiversity and its translation into
biological functions are much fewer than
those dealing with the biodiversity of organ-
isms living on its surface (notably plants),
and therefore our knowledge of the below-
ground diversity and functioning remains
limited (Prosser et al ., 2007). This holds even
more for microbial communities, which are
still considered as being ubiquitous on the
basis of the statement made by Beijerinck
(1913) that 'everything is everywhere' and
showing a high functional redundancy. Thus,
soil microbial communities are regarded as
a functional 'black box', generating fluxes of
different intensities that are solely depend-
ent on abiotic factors such as temperature,
moisture and pH. The hypothesis that the
diversity and composition of microbial

communities, as well as trophic interactions
between populations, may play a functional
role has so far basically been excluded
(McGill, 1996; Gignoux et al ., 2001). This
has resulted in: (i) a limited effort in charac-
terizing soil microbial diversity, its spatial
distribution and contribution to soil func-
tioning (Ranjard et al ., 2010); and (ii) an in-
sufficient, even non-existent, consideration
of microbial diversity in the models cur-
rently used to quantify fluxes of matter and
energy in soils (Ingwersen et al ., 2008).
Technical difficulties account partly for
these scientific lacunae in microbial ecology.
Microorganisms, as their name implies, are of
microscopic size (in the order of one micro-
metre for bacteria). Their diversity in soils is
huge (Torsvik and Øvreås, 2002; Bates et al .,
2011), and the very large range of environ-
mental conditions (soil types, climatic zones,
land uses) is likely to enhance this diversity
even more. Most (more than 90%) of these
microorganisms cannot be cultured on avail-
able media (Schloss and Handelsman, 2003;
Rajendhran and Gunasekaran, 2008), which
has meant that, until recently, they could
not be studied. Soil microorganisms are also
hidden within the soil, which is a heteroge-
neous but structured matrix, and thus hin-
ders access to this biotic component. These
various difficulties have, for a long time,
permitted only a truncated vision of soil
biodiversity.
However, thanks to major advances in
molecular biology during the past 20 years,
techniques have been developed to investi-
gate and decipher the diversity of soil micro-
bial communities in situ and without a priori
knowledge (Ranjard et al ., 2001). In this sci-
entific context, this chapter aims at provid-
ing a better understanding of present and future
prospects in soil microbial ecology and func-
tioning, with specific attention on the C cycle.
It is divided into three main sections. The
first provides a survey of the develop-
ment of molecular tools and how they repre-
sent a unique opportunity to progress in our
knowledge of soil biodiversity and function-
ing. The second section focuses on the im-
portance of SOM as a driver of soil abundance
and biodiversity at different spatial scales.
The third section presents actual developments