Agriculture Reference
In-Depth Information
from the atmosphere and convert this into organic forms usable by plants.
Such biological nitrogen fixers (BNF) include members of the genus
Rhi-
zobium, Azospirillum
, and blue-green algae. Rhizobia are symbiotically
associated with legumes and nitrogen fixation occurs within root or stem
nodules where the bacterium resides [3]. The genus
Azospirillum
also has
several N-fixing species, which are rhizobacteria associated with mono-
cots and dicots such as grasses, wheat, maize and
Brassica chinensis
L.
[4,5].
Azospirillum
strains have been isolated from rice repeatedly, and
recently the strain
Azospirillum
sp. B510 has been sequenced [6,7]. Con-
siderable N fixation by
Azotobacter
spp. and
Azospirillum
spp. in the rice
crop rhizosphere was reported repeatedly [6,8], but others [9] questioned
such high amounts of non-symbiotic N fixation in agriculture. Instead, it
was hypothesized that the beneficial effect of
Azospirillum inoculums
may
not derive from its N-fixing properties but from its stimulating effect on
root development [2], probably often triggered by phytohormones [10].
This view was confirmed by [11], who concluded that the main effect of
Azospirillum
spp. is the stimulation of the density and length of root hairs,
the rate of appearance of lateral roots, and the root surface area. Phytohor-
mone production and a beneficial effect on plant growth were also shown
for a range of other microorganisms [12,13].
Another important genus for biofertilizer producers is
Trichoderma
,
a fungus present in nearly all soils.
Trichoderma
spp. thrive in the rhizo-
sphere and can also attack and parasitize other fungi.
Trichoderma
spp.
have been known for decades to increase plant growth and crop yield
[14,15,16], to improve crop nutrition and fertilizer uptake [16,17], to
speed up plant growth and enhance plant greenness [18], as well as to con-
trol numerous plant pathogens [19,20,21]. A part of these effects may also
be related to the fact that some
Trichoderma
spp. seem to hasten the miner-
alization of organic materials [22], thus probably releasing nutrients from
soil organic matter. Positive effects on plant nutrition were also described
for other organisms, and many soil bacteria may enhance the mineral up-
take of the plant, as for example by the increased solubility of phosphate
in the soil solution [23].
There is a wide range of reports on the effect of biofertilizer application
in crops grown in non-fl ooded soils (unlike lowland rice), and the technol-
ogy for
Rhizobium
inoculation of leguminose plants is well established.
