Agriculture Reference
In-Depth Information
to barley (
Hordeum vulgare
L.) under different irrigation (irrigation and no irrigation) and
tillage (tillage or and no till) management were isolated and identified according to the
procedure of Caesar-TonThat et al. (2007). The field study also included plots of grass and
alfalfa that have been undisturbed and minimally managed for more than 20 years. Using the
same sampling and analysis procedures as described above, bacteria were isolated from soil in
these treatments (irrigated and non-irrigated) for the purpose of providing control
comparisons. For the aggregation assays, suspensions consisting of soil particles (0.125g/ml,
< 0.05 mm aggregate sieved-fraction) and bacteria from individual species were prepared in
concentrations of 10
4
, 10
5
, and 10
6
cells /ml in 10 ml of sterile deionized water. The
soil/bacteria mixtures were consistently vortexed for 10 sec then soil particles were allowed
to settle for 5 min. Images of the liquid suspension were immediately captured with a
commercial digital camera using near infrared red settings (800-1000 nm), then images were
converted into grey scale images using Adobe Photoshop software (version 7.0) in order to
evaluate the reflectance of the controls which contain only soil particles without the bacteria.
Isolates possessing the aggregative ability are indicated with shading in Table 1. Among all
the identified species, it is obvious that some species aggregate soil better than others and
some do not aggregate at all; more gram-negative species from α-Proteobacteria
(
Brevundimonas
,
Sphingomonas
,
Sphingopyxis
,
Novosphingobium
spp.), β-Proteobacteria
(
Burkholderia
spp.), γ-Proteobacteria (
Flavimonas
,
Pseudomonas
,
Stenotrophomonas
spp.),
Flavobacteria (
Chryseobacterium, Flavobacterium
spp.), and Sphingobacteria classes
(
Sphingobacterium
spp.) were soil aggregators compared to the gram-positive species which
included mostly
Bacillus
and
Microbacterium
spp. Regardless of the irrigation practices, the
percentage of isolates (regardless of species) with ability to aggregate soil were higher under
no till than under till (under irrigation, 32.84 % vs. 21.13 %, respectively; under no-irrigation,
23.53% vs. 7.14 %, respectively) and a greater percentage of species among all the taxa
identified from each treatment were soil aggregators under no-till compared to till (under
irrigation 21.52 % vs. 13.92 %, respectively; under no irrigation 13.92 % vs. 5.06 %,
respectively), suggesting that tillage management has an impact on the growth and survival of
the dominant soil aggregating bacterial communities in the microaggregates. Furthermore, the
total number of the soil aggregating isolates and species in microaggregates of irrigated
treatments was higher than in non-irrigated treatments indicated that conditions under
irrigation favor the presence of these beneficial bacteria.
A no-till system is characterized by sowing of crops directly into the crop residues that
remain on the soil surface from the previous crop without soil preparation measures (e. g.,
plowing and disking), except for the narrow slit required for seed placement. Slow
decomposition in no-till systems due to the partial contact between crop residues and soil
results in higher organic content in the surface soil, promoting better structure, fertility, water
infiltration and water-holding capacity. In contrast, conventional tillage mainly accomplished
by a moldboard plow or disk, is characterized by a high degree of soil disturbance and loss of
organic matter; soil layers are inverted, crop residues are completely incorporated into the
plow layer leaving no organic residues on the surface. The long-term maintenance of a
significant amount of organic matter on the soil surface is important for microbial growth and
activity because it minimizes extreme temperature shifts, maintains adequate moisture and
contributes to aeration. More accumulation of crop-derived C in free microaggregates has
been observed in no tillage compared to conventional tillage (Six et al., 1999)
