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The effect of pesticides on nitrogenase activity in rhizosphere soil from rice fields was investigated. The differential response of insecticides on nitrogenase depended on the method of field application. Results also showed that the differential response to the pesticides of. specific groups of nitrogen-fixing micro-organisms depended upon the method of application.Soil incorporation of carbofuran stimulated the rhizosphere nitrogenase, while endosulfan and hexachlorocyclohexane inhibited it. Carbofuran and hexachlorocyclohexane stimulated nitrogenase when applied to the standing water. Seedling root dips of isofenphos stimulated nitrogenase, while endosulfan, BPMC and carbaryl showed a variable effect. Quinalphos inhibited nitrogenase irrespective of method of application.

Summary The influence of phosphorus application on soil nitrogenase and N~-fixers in tropical paddy soils differing in their properties was investigated in a laboratory incubation study. Application of P stimulated the soil N 2-ase in an alluvial soil and in a P-deficient soil under both flooded and nonflooded conditions. The stimulation of N 2-ase by P was more pronounced under nonflooded conditions. A corresponding increase in N: -ase occurred with an increase in the P level at least up to 80 ppm level. A depressive effect of P on N2-ase occurred after 16 days under nonflooded conditions when the level of P was increased to 100 ppm. But under flooded conditions the stimulation was almost continuous. Results indicate that the effect of P on N2-ase depended on the water regime, level of P and soil type. Addition of P had a little effect on the population of N2-fixing micro-organisms in alluvial soil. On the contrary, addition of P stimulated the population of Azospirillum and Azotobacter in a P-deficient soil. Data suggested that the alteration in the N2-fixing microbial populations and the levels of available P might be responsible for changes in the N2-ase activity in these soils. Application of superphosphate and dicalcium phosphate stimulated N2-ase activity; while the rock phosphate exhibited an innocuous effect in alluvial and P deficient soils. In Sukinda soil, however, super phosphate slightly stimulated N2-ase at early stages, while other P sources had innocuous effect. Results indicate that the level and source of applied P exhibited differential influence on N2-ase and N2-fixers in tropical paddy soils.

SUMMARYThe influence of addition of sulphate on acetylene reduction in three paddy soils differing in their properties under two water regimes was investigated in a laboratory experiment. Nitrogenase activity was high in a P-deficient alkaline soil and addition of sulphate further enhanced the activity under two water regimes, with a pronounced stimulation under non-flooded conditions. Sulphate application to submerged alluvial soil enhanced nitrogenase activity with no apparent effect under non-flooded conditions. In acid sulphate saline Pokkali soil sulphate addition had little effect on the nitrogenase activity. Sulphate addition did not result in significant changes in the soil pH and redox potential. No relationship seemed to exist between the sulphate disappearance and stimulation of nitrogenase in these soils. A differential stimulation of N2-fixing microorganisms was noticed as a result of sulphate application. Results suggest that sulphate-induced stimulation of nitrogenase activity occurs in non-flooded soils.