Agriculture Reference
In-Depth Information
for microorganisms characterized by vari-
ations in pH values, pore size and substrate
and nutrient, water and oxygen availabil-
ities, all impacting microbial survival and
development (Mummey et al ., 2006). This
has been highlighted by various studies based
on direct observations (Chenu et al ., 2001;
Nunan et al ., 2003), microbial biomass meas-
urements (Jocteur Monrozier et al ., 1991;
Gestel and Merckx, 1996) and community
structure and bacterial diversity character-
izations (Ranjard et al ., 2000; Mummey et al .,
2006; Kong et al ., 2011), which all evi-
denced a heterogeneous distribution of mi-
crobial communities.
At the scale of the microbial habitat,
carbon substrates represent major drivers of
the quantitative and qualitative distribu-
tion of microbial diversity, since they in-
duce the formation of ecological niches or
'hotspots' that correspond to zones of high
biological activity (Stotzky, 1997). Main
carbon inputs to soil are represented by:
(i)  the release of photosynthates by living
plants from the roots, called rhizodeposi-
tion (Nguyen, 2003); and (ii) plant biomass
resulting from its decay after its death. In
both cases, the provision of organic sub-
strates to heterotrophic soil microorgan-
isms in soils which are mostly oligotrophic
leads to a stimulation of the density and ac-
tivity of the microflora around the roots,
corresponding to the so-called 'rhizosphere'
hotspot (Hiltner, 1904), and around the
plant residues, which corresponds to the
'detritusphere' hotspot (Gaillard et al ., 1999).
The quality of these trophic niches depends
on plant genotypes which release different
amounts and types of rhizodeposits during
their development (Gransee and Witten-
mayer, 2000) and in their residues after
their death. In response to the resulting
variations of soil organic status, microbial
density, diversity and activity in the hot-
spots differ during plant development and
plant degradation compared to those in bulk
soil (Mougel et al ., 2006; Pascault et al ., 2010).
Recently, Davinic et  al . (2012) defined soil
aggregates as trophic niches of bacterial di-
versity, highlighting specific bacterial assem-
blages associated with the quality of organic
matter within these soil microhabitats,
supporting previous studies (Ranjard and
Richaume, 2001).
On larger scales, the distribution of mi-
crobial communities has also been demon-
strated to be heterogeneous, but structured.
The relative contribution of SOM to the
microbial distribution of abundance and
diversity varies according to the scale con-
sidered. Thus, SOM remains a major driver
of microbial communities' abundance and
diversity on the scale of a farm plot (Lejon
et al ., 2007), together with soil texture (John-
son et al ., 2003; Lejon et al ., 2007), pH (Bååth
and Anderson, 2003), land use (Nicolardot
et al ., 2007) and plant cover (Lejon et al .,
2005). However, on the landscape and up
to  the country scale, the physico-chemical
properties (mainly soil pH) and land use,
but not SOM, constitute the main drivers of
soil microbial communities (Dequiedt et al .,
2011). As an example, on the scale of France,
the molecular biomass, which represents
the microbial abundance, has been demon-
strated to vary from 5 to 15 μg of DNA g - 1 ,
depending on the soil. These variations were
associated with the major soil types, and in
particular their physico-chemical properties
and land uses. More precisely, the molecular
biomass values were linked positively to the
clay and calcium (Ca) contents, pH value and
cation exchange capacity of soils (Dequiedt
et  al ., 2011). The mean molecular biomass
values were highest in grassland and lowest
in orchards and vineyards, maybe because of
the poor plant diversity observed in or-
chards and vineyards due to perennial cul-
ture of the same plant genotypes and the
frequent absence of grass between the rows
in these crops. More precisely, low plant
diversity in orchards and vineyards may
lead to a low diversity of C substrates re-
leased into the soil, which in addition to
the low soil covering by plant cover in
these systems, may explain the low mean
molecular biomass values observed. In the
same way, analyses of the genetic struc-
ture of bacterial communities at the scale
of France, based on Automated Ribosomal
Intergenic Spacer Analysis (A-RISA) finger-
printing, showed that the distribution of
microbial diversity was also heterogeneous
but spatially structured (Dequiedt et al ., 2009).
Search WWH ::




Custom Search