Influence of Pesticides on Azospirillum sp. Population and Its Nitrogen Fixation in Groundnut (Arachis hypogaea L.) Soils

Aim: To study the impact of selected pesticides on Azospirillum sp. population and its nitrification in groundnut (Arachis hypogaea L.) soils.

Study Design: Black clay and red sandy loam soils with known pesticide history were collected from groundnut (Arachis hypogaea L.) cultivated fields and were investigated to elucidate the impact of pesticides on Azospirillum sp. population and its nitrification in both the soils.

Place and Duration of Study: The soil samples were collected from groundnut cultivated fields of Anantapur District, Andhra Pradesh (A.P) and the study was carried out for 3 months.

Methodology: Ten gram portions of each soil sample were placed in (25 × 150 mm) test tubes and treated with different concentrations of pesticides, (10, 25, 50, 75 and 100 µg g-1 soil) which were equivalent - 1.0, 2.5, 5.0, 7.5 and 10 kg ha-1. Soil samples without pesticides served as controls. The soils with and without pesticides were incubated at room temperature (28 ± 4°C) in the laboratory and moisture content was maintained at 60% water holding capacity (WHC) throughout the experimental period. After 7 and 14 days of incubation, triplicate soil samples were used to estimate the population size of Azospirillum sp. using the MPN method. Five ml aliquots of semi – solid malate medium were added to five MPN tubes and inoculated with 0.5 ml of a soil suspension from 10-1 to 10-5 soil dilutions, and incubated at 37°C.

Results: The population of Azospirillum sp. in both soils increased when pesticides were applied @ 2.5 - 5.0 kg ha-1 and incongruity, when the pesticides concentration increased from 7.5 - 10.0 kg ha-1, the Azospirillum sp. population gradually decreased in both soils.

Conclusion: The present study aimed at determining the influence of selected pesticides such as oxydemeton methyl, emamectin benzoate, dithane Z-78 and benomyl on the population of Azospirillum sp. and nitrogen fixation in black clay soil and red sandy loam soils in groundnut cultivated fields of Anantapur District, Andhra Pradesh, India. Insecticides and fungicides applied up to 5.0 kg ha-1, enhanced the population of Azospirillum sp. and its nitrogen fixation also increased significantly after 7 and 14 days of incubation in both soils. However, the population of Azospirillum sp., decreased with increasing period of soil incubation in both treated and untreated soils.