Biol Fertil Soils (1994) 18:37-41 9 Springer-Verlag 1994 A.C. Das 9 D. Mukherjee Effect of insecticides on the availability of nutrients, nitrogen fixation, and phosphate solubility in the rhizosphere soil of rice Received: 18 February 1993 Abstract A field experiment was conducted to investi- gate the effects of 1,2,3,4,5,6-hexachlorocyclohexane (BHC), phorate, carbofuran, and fenvalerate, at their rec- ommended doses, on some chemical and microbiological properties of the rhizosphere soil in relation to rice yields. In general, the insecticides had a beneficial effect on rhizosphere soil properties. Carbofuran strongly stimulat- ed the mineralization of organic C. BHC and phorate led to the retention of less total N in the soil. BHC released more NH2-N than the other insecticides. Phorate, how- ever, liberated the most NO 3-N. Phorate and fenvalerate released more available P than BHC and carbofuran did. All the insecticides stimulated the proliferation of aerobic non-symbiotic N2-fixing and phosphate-solubilizing mi- croorganisms, resulting in an overall increase in rice yield. BHC had the greatest effect on rice yields, followed by phorate. Key words Available nutrients 9 Insecticides 9 Microor- ganisms 9Nitrogen fixation 9Phosphate solubilization 9 Rhizosphere soil 9 Rice yield Introduction In modern agriculture, insecticides are frequently used in the field to increase crop production. Besides combating insect pests, insecticides also affect the populations and activity of beneficial microorganisms in soil (Nayak and Rao 1982; Ogunseitan and Odeymi 1985; Jena et al. 1987; Moorman 1989; Singh and Prasad 1991; Bhuyan et al. 1992). However, no definite conclusions can be drawn on the effects of insecticides on these soil microorganisms since different groups of insecticides exhibit manifold variations in toxicity (Matsumura and Boush 1971; Si- mon-Sylvestre and Fournier 1979). We therefore studied A.C. Das (~) 9D. Mukherjee Department of Agricultural Chemistry and Soil Science, Bidhan Chandra Krishi Viswavidyalaya, Mohanpur-741 252, India the effects of BHC, phorate (0,0-diethyl-S-etyhlthiometh- yl-dithiophosphate), carbofuran (2,2-dimethyl-2,3-dihy- drobenzafuranyl-7-N-methyl carbamate) and fenvalerate [(RS)-cyano(3-phenoxyphenyl) (RS)-methyl-4-chloro-a (1-methylethyl) benzene actate], at their recommended doses, on the availability of plant nutrients and on the growth and activity of N2-fixing and phosphate-solubi- lizing microorganisms in the rhizosphere soil of rice. Materials and methods Experimental site and design A field experiment was laid out in 4 mx4 m plots following a ran- dom block design. Some characteristics of the soil are given in Ta- ble 1. Fertilizer consisting of 50, 22, and 42 kg ha -I of N, P, and K, as urea, single superphosphate, and muriate of potash, respec- tively, was mixed thoroughly with the soil during land preparation. Thirty-day-old seedlings of rice (Oryza sativa L. cv. Manik kumar) were transplanted at two seedlings per hill, with a spacing of 15 cm by 20 cm between hill and row. Twenty-one days after transplanting, another 50kg ha -~ of N as urea was applied as topdressing, and 30 days after transplanting, BHC, phorate, carbofuran, and fen- valerate at the rate of 7.5, 1.5, 1.0, and 0.35 kg a.i. ha -1, respec- tively, were sprayed onto the crop field separately, each mixed with 6001 ha -t of distilled water. The crop was cultivated following usual agronomic practices. Sampling of rhizosphere soil There were three replicates for each treatment. Rhizosphere soil samples were collected from each plot 0 (1 h), 15, 30, 45, and 60 days after the insecticides had been applied, by uprooting 12 plants from six hills at random, keeping the root system intact as far as possible. After the pieces of plant root had been removed, rhizo- shpere soils from the replicated plots of each treatment were ana- lysed. Analysis of soil samples Chemical analysis. The soil samples were analysed for organic C, total N, NH~-N, and NO~--N according to Jackson (1973). Avail- able P was estimated in a sodium bicarbonate extract (Olsen et al. 1954) colorimetrically (Jackson t973).