In vitro studies on side-effect of some insecticides on the growth of N2-fixing root-nodule bacteria

Abstract

Globally, the consumption of nitrogen (N) fertilizer- is significantly increased day after day. The biological N2-fixation is the main source of N input in agricultural soil. However, international emphasis on environmentally sustainable development with the use of renewable resources is likely to focus attention on the potential role of biological N2-fixation in supplying N for agriculture. Several environmental conditions are limiting factors to the growth and activity of the N2-fixing root nodule rhizobia. One of the most important and potentially limiting factors to biological N2 fixation is the use of pesticides including the insecticides. The objective of the study is to investigate the adverse effect of insecticides on N2-fixing rhizobia. Experiments were conducted to study the impact of seven insecticides [Anthio 33 EC (Formotion (33%), Sevin 85 WP (Carbaryl (85%), Rogor L-40 (Dimethoate (40%), Thiodan 50 WP (Endosulfan (50%), Phosphotion (Malathion (50%), Lannate 20 L (Methomyl (200 g/l), Pirimor 50 DP (Pirimicarb (50%)] on the growth of seven strains of N2-fixing root-nodule rhizobia (four strains belonging to Rhizobium leguminosarum, one strain belonging to R. phaseoli, one strain belonging to R. trifolii and one strain belonging to R. loti) in vitro by measuring optical density. The rhizobial strains were grown in yeast extract mannitol broth medium treated each with different concentrations (0, 0.1, 1, 10, and 100 mg/litre of active ingredient) of each insecticide and incubated for 48 hours in microfermentor at 28°C. The results illustrated that Phosphotion and Sevin had an adverse effect on the growth of the investigated rhizobial strains, whereas Thiodan was safe to these rhizobial strains. Whereas the strain Lóbab Z of R. leguminosarum was the most tolerant and R. trifolii Ló133/64 was the most sensitive one among the tested strains. The adverse effects of insecticides on rhizobia were observed at concentrations not normally expected to occur under field conditions. For further task, it is required to investigate the adverse effects of these insecticides on nodulation and N2-fixation capacity of specific host plants with their rhizobia in pot and field experiments as well as other vital microbial processes in a wide range of soils.