Download 5# SUMMARY Biological N2 fixation as a major means of

5#
SUMMARY
Biological N2 fixation as a major means of contributing
to the nitrogen economy of agricultural soils is well recognised
Ecologically important flooded rice soils serve as an important
medium for nitrogen fixation*
There is increasing evidence
to suggest that hate rot r ophic free-living and associative
microorganisms contribute significantly to the nitrogen
economy of paddy soils# particularly under tropical conditions*
However* information on the effect of increasingly used
pesticides and fertilisers and their interaction on I$2 fixation
and N2-fixing microorganisms in rice soils is scanty*
Experiments were conducted to determine the s~
influenca of organic matter like Sesbania sp. and rice
straw on the nitrogenase activity of different rice soils*
influence of pesticides applied singly and/or in combination
on the populations of H2-fixing Agospirillum, Azotobacter
and anaerobic H2 fixers and their nitrogenase activity in
laboratory and field samples*
*
effects of water regime, fertilisers and organic matter
on nitrogenase activity. H2 fixation and the populations
of H2-fixing bacteria associated with different rice soils
under the impact of commonly used pesticides*
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*
effect, of changes in salinity condition on Ng fixation#
nitrogensse activity and Mg-fixing microbial population
of different saline soils with varying salinity and salt
tolerance of Agosplrillum isolates isolated from various
saline soils
*
influence of carbofuran# a carbamate pesticide widely
employed in rice cultivation, on Ng fixation and growth
factor (Xaa) production by pure cultures of Asospiriilum'
isolated from rice roots and soils#
*
yield responses due to root inoculation with Agosplrillum
under field conditions*
Employing sensitive gas chromatographic acetylene reduction
assay and micro^Kjeldahl method Mg fixation was studied in
soil and microbial systems#
Application of cellulose# fresh
and dry leaves of two species of Sesbania# significantly
stimulated the nitrogenase activity under both water regimes
in alluvial# laterite and acid sulphate saline Pokkali soils*
The stimulation was higher when dry leaves of sesbania were
applied compared to fresh leaves in all the soils.
Application
of S.aculeate resulted in a slightly higher stimulation of
the nitrogensse activity in most of the soils#
Commonly applied herbicides tbiobancarb and butachlor
at 2 and 4 jig/g soil level inhibited the populations of
anaerobic Mg fixers and Agosplri Hum in alluvial soil under
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nonflooded conditions#
Th© Inhibition was, however, more
drastic with regard to anaerobic •
fixers, at higher levels
of herbicide application (4 pg/g soil)•
Butachlor stimulated
the Agosplrillum population in flooded alluvial soil particularly
at lower concentration (2 fig/g).
In contrast, these herbicides
at both the levels stimulated the population of anaerobic Ng
fixers and Agofcobacter and inhibited the population of
Agospirillum in an acid sulphate PoMssli soil.
At lower
levels of the herbicide application the population of 'anaerobic
fixers and Agosplrillum were stimulated in a lat©rite sukinda
soil*
Results indicate differential behaviour of the herbicides
on the select groups of bacteria depending oh the soil type
and water regime*
Carbary 1, an insecticide commonly used in rice, had
almost no effect on soil nitrogenase activity under submerged
conditions while a stimulation of nitrogensse occurred due to
carbary 1 addition in a nohflooded soil at I© fig/g soil level;
but to a less extent at So pg/g soil level*.
Application of
the herbicide butachlor and the insecticide carbofuran at
2 pg/g level individually reduced the population of Agosplrillum
and anaerobic
fixers In a' nonflooded alluvial soli#
In
contrast, butachlor stimulated the' population of anaerobic
t?2 fixers in an acid saline Polskali soil under a similar
water regime#
Further, an increased level of carbofuran
(4 pg/g soil) stimulated tbs -population of Asospirllluin,
anaerobic
fixers and Agotobacfcer in all the three soils
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studied*
This stimulatory effect occurred, though not to the
same extent* In the presence of butaehlor* which when applied
alone was inhibitory to the former two groups in an alluvial
soil*
Moreover* an increase in the level of carbofuran
(4 jug/g soil), stimulated the'population of nitrogen fixing
microorganisms and nitrogenase activity in all the three soils*
irrespective of the water regime*
Evidence was also provided
for the stimulation of Asosnlrilium population upon the
increased level of carbofuran application*
Thus* the effect
of pesticide combinations in relation to specific group of
&2•fixing microorganisms and nitrogenase activity varied with
the soil type and water regime.
Application of lower levels of nitrogen fertiliser
(20 pg/g urea-N) along with pesticides stimulated soil
nitrogenas® irrespective of the soil used while higher nitrogen
levels (80 pg/g urea-K) decreased the activity.
Nitrogenase
activity'was stimulated in the presence of low level of ur©a«N
and carbofuran.
how level of fertiliser N also stimulated the
N2-fixing bacterial population irrespective of the soil type.
Nitrogenase activity of different soils varied despite almost
identical levels of the populations of Ngfixlng bacteria, under
the influence of the pesticides.
Water regime influenced the effect of pesticide or
pesticide combinations on -Mg fixation in paddy soil with and
without rice straw amendment*
In a nonflooded alluvial soil
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single'or combined application of butaehlor and carbofuran
significantly stimulated Mg fixation *
Thiofoenearb had no
effect on Mg fixation whan applied alone but in combination
with carbofuran resulted in higher E^ fixation 0
Both
oxadiazon© and thiotoencarb had no effect on Bg fixation under
flooded condition but in combination with carbofuran the Kg
fixation was high,
Addition of rice straw had some stimulatory
effect ©n the Kg fixation in this soil under flooded conditions.
In acid, sulphate saline Pokkali soil# rice straw
application stimulated Kg fixation# the stimulatory effect of
carbofuran either alone or in combination became more pronounced
under flooded conditions,.
Kg fixation was low in a late rite
Sukinda soil and the effect of pesticides was not altered by
rice straw amendment*
A uniform stimulation of Kg fixation
occurred in soils when carbofuran was applied either singly
or in combination with other herbicides tested*
Results
indicated that tha effect of pesticides on Hg fixation varied
with the rice straw application# water regime and soil type#
Further evidence was provided for a uniform stimulatory effect
of carbofuran irrespective of soil type, water regime# organic
amendment and pesticide combination at least in the. three soils
tested.
No additive effect of pesticides in the presence of
carbofuran in stimulating Ng fixation was noticed.
Soil salinity affected the nitrogenase and Mg-fixing
bacterial populations under both flooded and nonflooded
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conditions* ; $2 fixation was low in saline soils and also in a
normal soil amended with salt mixture (salinity level 4 and
30 ds/m)
under flooded and nonflooded conditions.
Submerged
soils amended with rice straw had higher l?2 fixation irrespective
of-the salinity levels used in the study*
leaching the saline
soil improved M2 fixation in the soil despite varying levels
of soil salinity*
An increase in the .soil salinity effected a decrease
in the populations of at least three groups of N2~fix±ng
microorganisms*
Leaching the saline soil inp roved the
population density of anaerobic
fixers and Asosplrlllam,
Rice straw amendment to a saline soil further improved the
populations of above two groups of N2 fixers,
1’he salinity
levels tested did not have a pronounced effect on the Asototoacter
populations*
Results indicate that soil amelioration for
salinity with leaching and organic matter addition would
improve the inpHeated microbial populations and H2 fixation
in salt-affeeted rice soils*
Aaospirlllum sp« isolated from the salt-affected soils
possessed certain degree of salt tolerance*
Cultures from
highly saline soils were less sensitive than cultures from
medium saline and non-saline soils with regard to the nitrogenase activity upon fresh exposure to the salt mixture.
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Asospirlllum sp.# closely associated with rice plants
and soils# produced considerable quantities of indole acetic,
acid*
Both indole acetic acid (IAA) accumulation and fL>
fixation were increased in Azospirillum cultures isolated
from rice roots and soils by earbofuran* an insecticide widely
used in rice cultivation.
Mg fixation varied among the
,
isolates of Azosplrlllum from soils and roots of different
rice cultivars •
Addition of earbofuran at 250 and 500 jag/
flash significantly stimulated
fixation in these cultures.
The stimulation was more pronounced .v/ith earbofuran at lower
level.
XAA accumulation by Asosblrlllum cultures was more
pronounced at lower level (250 jug/flash) of earbofuran•
Evidence was also provided for earbofuran degradation by
Azospirilium cultures.
However# 7-phenol# a major degradation
product of earbofuran did not enhance the
iaa
accumulation. .
The higher accumulation of IAA in Asogpirllluro culture® in
the presence of earbofuran is probably related to the. increased
growth due to fixed M present in the insecticide.
Results
indicated the involvement of parent compound earbofuran
and/or its metabolites other than ?-phenol in the higheraccumulation of IAA by Asosplrillum sp.
Such increased
accumulation of IAA in addition to higher Kg fixation might
explain the stimulatory effect of earbofuran on plant growth
in situations of close plantHbaeteria associations«
Root and/or seed bactsrisatiom with Aaosoirl 1 lam,
improved paddy yields under upland and lowland conditions.
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The performance o£ teospi rill urn was better in mixed cultures
than in pure culture with respect fc© three rice varieties ■
studied.
Further, the grain yield signifleanfcly increased
with the increase in the level of fertiliser R and iteosplrlllum
inoculation.
In the presence of R (at 40 kg R/ha) the
combined application of Asoaplrilliari and P (at 60 % 'P/ha}
significantly i rip roved the rice yields under upland conditions#
However, under lowland conditions the response of P and
Asosoirlllum was more pronounced at low P level (20 kg P/ha)«
These results indicate the interaction between bacteria and
the fertiliser practices in improving the rice yields*
The following are the conclusions from the study s
*
.Application of cellulose or green manure (Sesbanis sp.)
enhanced nitrogeoase activity in three paddy soils
incubated under two water regimes*
*
Casbofuran, a carbamate insecticide widely used In rice
cultivation, at field recommended levels stimulated tha
populations of
soil system*
bacteria associated with rice
Growth, Rg fixation, and indole acetic acid
accumulation in teosoirillum sp« isolated from rice roots
and soils were significantly increased in the presence
of carbofuran in laboratory incubation studies.
*
Uniform stimulation of Rg-fixing bacteria and Ng fixation
occurred due to carbofuran applied either singly or in
combination with other pesticides in the presence and
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absence .of rice straw under two water regimes in three
paddy soils#
Application of lower levels of agrochemicals (fertiliser
13 and pesticides) stimulated fjg fixation in pure bacterial
cultures and in soils under two water regimes while
.higher levels exerted an inhibitory effect#
An increase in soil salinity decreased N2 fixation and the
populations of M2-fixing bacteria in flooded and nonflooded
soils*
teaching the saline soil Improved the H2 fixation
and M2-fixing bacterial populations.
h certain degree of salt tolerance was observed in
Aaospirtlluro isolated from salt«af£ected paddy soils*
Root and seed -bacterisation with Asosplrl Hum isolated
from rice soil system improved the paddy yield with
different fertiliser management under upland and lowland
conditions*