Agriculture Reference
In-Depth Information
fobacterales, Desulfovibrionales, Syntrophobacterales
or
Clostridia
had
signifi cantly lower abundances in the alfalfa soils compared to scrubland
soils. By and large, bacteria belonging to these taxonomic groups are strict
anaerobes [64] and play an important role in anaerobic carbon cycling
in wet terrestrial ecosystems such as rice fi elds and wet lands [65,66).
Furthermore, tilling soils also affects soil aeration [67] and can elevate
the activity of aerobic microbes [14]. Notably, in the present study, a sig-
nifi cant loss of organic matter also was observed in the agricultural soils
and as soil organic matter is an important binding agent of soil particles
into aggregates [68], presumably lowering gas diffusion, this may have
contributed to lower sulphate-reducing populations in these desert agri-
cultural soils.
High proportions of OTUs belonging to the
Bacteroidetes
or the
Al-
pha-, Beta-
and
Gammaproteobacteria
as well as several families of
Ac-
tinobacteria
were more abundant in alfalfa soils. The higher abundance
of
Pseudomonas
(50% of the OTUs with signifi cantly higher signal in-
tensities) in the agricultural soils was also confi rmed by the detection
of
Pseudomonas
-specifi c gacA genes [46]. Amplicons of the gacA gene
fragment were obtained only for TC DNA from alfalfa soils, not from
scrubland soil (data not shown).
Betaproteobacteria
and
Bacteroides
were
considered to contain copiotrophic taxa as their abundance has been posi-
tively correlated with carbon mineralization rate and carbon availability
[69]. Several studies reported enrichments of
Alphaproteobacteria
[69],
Gammaproteobacteria
[46,70) or
Actinobacteria
[29,43) in the rhizo-
sphere where carbon availability is increased due to root exudates. The
number of OTUs detected by PhyloChip analysis was signifi cantly lower
in the soil from scrubland than alfalfa fi eld. In general, this fi nding is in
agreement with other studies, in which agriculture or low plant diversity
(often a direct result of converting natural land into agricultural use) did
not necessarily lead to a reduction of the bacterial diversity detected [48],
[71]-[73]. However, the effects of agricultural practices on soil bacterial
richness remain to be explored more fully, perhaps at a fi ner resolution
than was obtainable in this study. Firstly, total bacterial species richness
in soils is diffi cult to assess as many populations occur at low abundance
[74], [75], [76]. Secondly, a high proportion of OTUs belonging to
Acido-
bacteria, Chlorofl exi, Spirochaetes, Verrucomicrobia, Gemmatimonadetes
