Soil as a Support of Biodiversity and Functions - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 148

support such a link (Griffiths et al ., 2000;
Wertz et al ., 2006), even if it has been sug-
gested in some studies (Zogg et al ., 1997;
Adrén et al ., 1999; Strickland et al ., 2009).
One reason often proposed is that, within the
soil microbial community, the decomposition
of organic matter in soil would be a highly
redundant function. However, analysis of
the enzymatic capacities required for SOM
degradation has highlighted the non-uniform
distribution of these capacities within the soil
microbial component. In particular, those
involved in the final degradation steps are
carried by only a small subset of the soil mi-
crobial community (Hu and van Bruggen,
1997; Schimel and Gulledge, 1998). This
could explain the successions of microbial
populations observed during the degradation
of plant residues added to soil (Bernard et al .,
2007; Nicolardot et al ., 2007; Pascault et al .,
2010), suggesting that different populations
are required for the different steps of SOM
decomposition.
In agreement with this hypothesis, Bell
et al . (2005) and Liebich et al . (2007) showed
that microbial diversity was playing a major
role in the degradation of plant residues.
However, these studies were based on the
use of consortia of microbial species that had
previously been isolated on culture media,
which therefore induced a strong selective
bias (only 1- 10% of soil microorganisms
can be cultured) and consequently precluded
the possibility of considering the resulting
microbial consortium as representative of
the indigenous communities (Griffiths et al .,
2001). To overcome this bias, Baumann et al .
(2012) developed an experimental strategy
relying on creating a diversity gradient by
inoculating sterile soil microcosms with
different dilutions of a soil suspension. This
strategy allowed the characterization of SOM
degradation along a microbial diversity
gradient under controlled laboratory condi-
tions. The microbial consortia that developed
in the corresponding microcosms contained
several hundreds of different populations,
even for the less diverse treatment. Results
obtained clearly demonstrated that micro-
bial diversity altered bulk chemical structure
and the decomposition of plant litter sugars,
and influenced the microbial oxidation of

particular lignin compounds, thus changing
SOM composition.
Regarding the importance of the C cycle
for ecosystem dynamics, and the contra-
dictory results of the insufficiently numer-
ous studies available in the literature, it
appears that further investigation and new
fundamental studies need to be carried out
to elucidate more the role of microbial di-
versity in C transformations in soil. In con-
trast to earlier investigations on this topic,
methodological developments offer a unique
opportunity to decipher relations between
the C cycle and active microbial populations.
As an example, the recently developed
DNA stable-isotope probing (DNA-SIP) method
allows specific characterization of the com-
munities actively involved in the decom-
position of C substrates labelled with stable
isotopes (e.g. 13 C) (Neufeld et al ., 2006; Bernard
et al ., 2007; Chen and Murrell, 2010). An-
other advantage of using labelled compounds
is the possibility to monitor not only mineral-
ization of the labelled C substrate but also that
of native SOM at the same time as the dynam-
ics of the degrading communities responsible
for the decomposition of each C pool. It thus
provides an opportunity to evaluate the im-
portance of microbial diversity in ecosystem
processes such as the priming effect that
may play an important role in soil carbon
balance (Fontaine et al ., 2003; Bernard et al .,
2007; Kuzyakov, 2010; Pascault et al ., 2013).
Use of such methods, including for manipu-
lating microbial diversity, may constitute a
decisive step towards experimental demon-
stration of the functional significance of micro-
bial diversity in C cycling in soil.
Nevertheless, extrapolation of the con-
clusions derived from investigations per-
formed under simplified controlled conditions
will obviously be limited, and results will
need to be generally applicable in order to ac-
quire a truly predictive dimension. A promis-
ing complementary strategy to achieve this
goal in future will be to combine the power-
ful and robust tools used to characterize mi-
crobial biodiversity (i.e. pyrosequencing of
ribosomal genes from soil samples) and
functioning, with extensive sampling on a
massive spatial scale (landscape, region,
country or continent). With such a strategy, it