Impact of glyphosate and chlorpyriphos on chemical and biological properties of lateritic soil

Abstract

The thesis entitled "Impact of glyphosate and chlorpyriphos on chemic I a d biological properties of lateritic soil" was done in the year 2012-2013 in the Dep rt e of Soil Science and Agricultural Chemistry, College of Horticulture, Vellanikara. The programme consisted of laboratory experiment viz., avoidance behaviour of earth orm, pot culture study on survival of earthworm and soil micro flora under the different pesticide treatments and two field experiments viz., application of (1) glyphosate and (2) chlorpyriphos in cropped area and lawn respectively. The laboratory and pot culture studies consisted of seven treatments. The treatme ts included two levels of application (recommended and double the recommended dose) of t 0 formulations of glyphosate (1.2 and 2.4 kg a.i. ha") and single chlorpyriphos formulati n (400 and 800 g a.i. ha") and an absolute control. Glyphosate formulations namely Roundup® and Glycel® were applied at 6 and 12 mL L-I respectively and chlorpyriphos formulation Dursban® was applied at 4 and 8 mL L-I. The main observations included in this research programme were biological properties of soil viz., earthworm count, total microbial count, dehydrogenase activity and chemical properties of soil viz., pH, organic carbon, available P, K Ca, Mg, Fe, Zn, Cu and Mn. Both laboratory and pot culture experiments were done with native earthworms. The laboratory experiment was done to understand the avoidance behavior of earthworm to t e two glyphosate formulations and chlorpyriphos. This study showed that all the chemicals were avoided by earthworms and the maximum avoidance was obtained with the application of Roundup® (12 mL L-I) followed by Glycel® (12 mL L-I) and Dursban® (8 mL L-I). Earthworm avoidance test done by two methods of application (mixing as well as spraying) showed similar trends. However, higher net response was obtained by mixing the pesticides in soil. Spraying on the surface imitated the field situation, in which the avoidance response ranged from 20.0 to 46.7 %, while in the case of mixing the pesticides with the soil, the avoidance response ranged from 33.3 to 73.3 %. When the soil was mixe with the chemicals, the earthworms had direct contact with the pesticides throughout the treated soil section and thus they had shown greater tendency to avoid the area. Pot culture studies on the survival of earthworms revealed that neither glyphosate nor chlorpyriphos had deleterious effects on their multiplication. The study also showed that both soil fungi and bacteria were affected by application of glyphosate and chlorpyriphos. The adverse effect of chemicals on the soil fungi followed the order Roundup 12 mL i.' > Dursban 8 mL r.:' > Glycel 12 mL r.:' > Dursban 4 mL i.' > Roundup 6 mL i.' > Glycel 6 mL r.'. Percent decline in the fungal population was 10.1 to 70.5 at one week and 5.8 to 62.9 at one month after application. It was noticed that Roundup and Glycel at the recommended rates of application did not cause adverse effect on soil fungi at one month after spraying, indicating that the effect was temporary. However, chlorpyriphos formulation caused a significant decline in the fungal population at both the sampling intervals. In the case of bacteria, percentage decline over control followed the order Roundup 12 mL i.' > Glycel 12 mL i.' > Roundup 6 mL r.' > Dursban 8 mL i.' > Glycel6 mL Cl> Dursban 4 mL i.', at one week after spraying and the extent of decline ranged from 14.3 to 71.6%. At one month, the effect of chlorpyriphos (Dursban) on bacteria was comparatively lower than that of glyphosate formulations at both the concentrations. Two field studies were conducted to find out the effect of application of glyphosate in cropped area (banana field) and chlorpyriphos in the lawn. Experiment in the banana field was conducted with five treatments namely Roundup® and Glycel® each at 6 and 12 mL i.' and an absolute control. The treat~ents were replicated four times. Studies with chlorpyriphos were carried out in the established lawns of the College of Horticulture, Vellanikkara with three treatments viz., Dursban® at 4 and 8 mL L-land seven replications. Both biological and chemical properties were studied. It was found that application of . glyphosate caused short term inhibitory effect on soil fungus up to 30 days after spraying, where as application of chlorpyriphos caused short term inhibitory effect on soil bacteria (up to 30 DAS). Population of soil micro flora did not vary between treatments at 60 days after spraying glyphosate and chlorpyriphos formulations. Dehydrogenase activity and available nutrients in the soil were unaffected by the application of glyphosate and chlorpyriphos formulations used in the study.