Response of soil microflora, microbial biomass and some soil enzymes to baythroid (an insecticide)

Laboratory incubation experiments were conducted to study the response of bacterial and fungal population, soil microbial biomass, urease, amylase, invertase, and cellulase to Baythroid applied at 0, 0.4, 0.8, 1.6, 3.2 and 6.4 microgram g-1 soil (on an active ingredient basis). Generally, a positive effect on bacterial and fungal population was observed. Bacterial population increased from 13 to an average of 25 after 5 days of incubation of soil samples treated with different levels of Baythroid. Baythroid did not have a significant effect on fungal population, which was quite low after 5 days of incubation. After 15 days of incubation, however, Baythroid caused a substantial increase in fungal population although no consistent trends were observed with the rate of application. Carbon dioxide evolution from soil was almost unaffected by Baythroid except at the lowest and the highest levels of addition, where a negative and a positive effect, respectively, was obvious. Cumulative losses of CO2-C increased by 38 % at the highest level of Baythroid. The microbial biomass C varied between 138 and 147 microgram CO2-C g-1 soil in differently treated soils, a substantially positive effect of Baythroid was observed only at the highest rate of addition, while at lower levels a positive but non-significant effect was observed. Amylase activity increased by a maximum of 91.5 % at Baythroid level of 1.6 microgram g-1. At 6.4 microgram g-1 soil Baythroid, however, the activity was reduced by 47.9 %. Invertase activity also increased by 110.9 % at 1.6 microgram Baythroid g-1 soil followed by a decrease of 40.3 % at the highest level tested. Cellulase activity was not much affected, although an increase of 18.5 % was observed at 1.6 microgram g-1 soil Baythroid. At the highest level of Baythroid, however, cellulase activity was reduced by 25.9 %. Response of urease was almost similar to that of other enzymes. However, maximum increase of 40.9 % was achieved at 0.8 microgram g-1 soil Baythroid, while the decrease (9.1 %) at higher levels of Baythroid was less pronounced as compared to that for other enzymes. All the four enzymes showed a positive relationship in their response to different rates of Baythroid.