Soil as a Support of Biodiversity and Functions - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 147

This distribution was again affected by soil
type (physico-chemical characteristics, espe-
cially pH) and land use, but SOM repre-
sented a minor driver on this scale. The
major influence of pH, on the distribution of
the structure and diversity of microbial com-
munities on large spatial scales, has also been
demonstrated by analogous studies in the UK
(Griffiths et al ., 2011) and USA (Fierer and
Jackson, 2006; Jones et al ., 2009).

these models as a functional 'black box',
with a very high level of functional redun-
dancy. As a result, only the size of the
microbiota (biomass), but not the diversity
or the composition of this pool, is integrated
into such models. Until recently, two main
reasons were put forward to justify this
pragmatic approach: (i) these 'black box'
models were able to provide accurate simu-
lations of SOM dynamics for a variety of
land uses with a minimal set of parameters;
and (ii) understanding of the diversity of the
microbial communities involved in ecosys-
tem processes had been restricted, mainly
because of methodological limitations.
Thanks to progress in modelling and
new methods in molecular microbial ecology
(see above), these reasons are no longer justi-
fied. Recent works on modelling the priming
effect (i.e. the increase in soil organic C min-
eralization following the input of a fresh or-
ganic C compound) suggest that the density
of microbial communities, together with
the competition between different microbial
functional groups, may control the rate of
mineralization of native SOM (Fontaine and
Barot, 2005; Neill and Gignoux, 2006). These
theoretical studies have confirmed that mi-
crobial diversity plays a crucial functional
role in organic matter turnover in soil, and
clearly suggest that this parameter must
be included in the models. They have also
evidenced the need for an empirical demon-
stration of the functional role of microbial
diversity in ecosystem processes.
Experiments performed on the grassland
systems of Jasper Ridge (California, USA) by
Horz et al . (2004) showed that modifications
of the diversity of the nitrating community in
soil in response to global changes (increased
atmospheric CO 2 , temperature, nitrogen (N)
deposition and soil moisture) could lead to an
increase in nitrification, thereby illustrating
that microbial diversity plays an important
role in the nitrogen cycle. This is in agreement
with Philippot et al . (2013), who recently
demonstrated that a reduction of the diver-
sity of the denitrifying community strongly
decreased N 2 O emission from soil.
In contrast, studies to date on the in-
volvement of microbial diversity in the carbon
cycle have not provided clear evidence to

Linking Soil Microbial Diversity
to Soil Processes

The question of the link between diversity
and ecosystem functioning, fundamental in
functional ecology, has, historically and up
to now, been addressed mainly by ecologists
studying macroorganisms, and particularly
plants, through experimentations involv-
ing manipulation of diversity (taxonomic
diversity or diversity of functional groups)
( Balvanera et al ., 2006). First studies were
reported in 1843, represented by the experi-
mentation set up on the English experimen-
tal station located at Rothamsted (Lawes
et al ., 1882). This long history explains the
abundant literature available on this topic
at present. These different works were able
to instigate a very rich theoretical and con-
ceptual framework (i.e. ecological insurance,
complementarities between niches, functional
redundancy), allowing a better understand-
ing of how biodiversity could influence eco-
system functioning (Loreau, 2000). In spite
of sometimes contradictory results, it is glo-
bally clear that plant diversity has positive
effects on ecosystem functioning, perform-
ance and stability, and thereby their cap-
acity to provide ecosystem services (Naeem
and Li, 1997; Tilman et al ., 1997).
Regarding microorganisms, this ques-
tion remains open, since very few studies
have really examined this aspect. As a
consequence, the microbial component of
soil is still roughly taken into account in
mathematical models designed to predict
the fate of major elements in the environ-
ment ( Ingwersen et al ., 2008). Indigenous
microbial communities are considered in