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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* 156 * 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 157 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 158 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 159 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 160 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. 101 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. 162 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 163 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*