MSc

The study entitled „Persistence and transformation of carbosulfan in laterite and coastal alluvium soils of Kerala and its effect on soil organisms‟ was conducted at Department of Soil Science and Agricultural Chemistry and the laboratory attached to the All India Network Project (AINP) on Pesticide Residues, College of Agriculture, Vellayani, Thiruvananthapuram, Kerala during 2015-16. The main objectives of the experiment were to study the persistence, mobility and transformation of carbosulfan in laterite and coastal alluvium soils of Kerala (in cropped and non cropped situation) and to assess its effect on soil organisms. Laterite and coastal alluvial soils were collected from Vellayani and Kazhakkoottam respectively. The physico-chemical analysis of the soils were done which revealed that coastal alluvial soils had an organic matter content of 0.84 per cent while laterite had only 0.41 per cent. The primary and secondary nutrients were comparatively higher for coastal alluvial than laterite soil. The method for the estimation of carbosulfan residues from the soils were validated at 0.05, 0.25 and 0.50 μg g-1 level of carbosulfan. Modified QuEChERS method with acetonitrile as extracting solvent and Primary Secondary Amine (PSA) sorbent for clean up was found to be suitable for the estimation of carbosulfan from the soil. Mobility of carbosulfan was assessed by loading 3 levels viz., 100, 150 and 200 μg of carbosulfan 25 EC separately on top of soil columns in PVC pipes and eluting with 20, 40, 80 and 160 mL of water. In the laterite soil, carbosulfan moved down the soil column and resulted in residue levels ranging from 1.50-0.04, 2.29-0.27 and 3.55-0.05 mg kg-1 at 100, 150 and 200 μg levels, respectively when eluted with water. In the coastal alluvial soil, the corresponding residues ranged from 1.78-0.32, 2.20-0.51 and 3.07-0.72 mg kg-1 at 100, 150 and 200 μg level after elution with water. The residues found on the leachate ranged from 0.001-0.01mg kg-1 for laterite and 0.001-0.03 mg kg-1 for coastal alluvial soil.
The persistence of carbosulfan in the laterite and coastal alluvial soils under laboratory and cropped (grow bag with chilli Ujwala variety) conditions was studied
using two formulations viz., Emulsifiable Concentrate (EC) and granular formulations each at 1, 2.5 and 5 mg kg-1 levels. The half lives (t1/2) of carbosulfan 25 EC in the laterite soil when applied at 1, 2.5 and 5mg kg-1 levels were 5.08, 7.69 and 10.53 days respectively in the laboratory condition, while in the cropped condition they were 2.17, 4.60 and 5.24 days, respectively. In coastal alluvial soil, application of carbosulfan 25 EC at 1, 2.5 and 5 mg kg-1 level resulted in half lives of 2.35, 2.91 and 4.96 days respectively in the laboratory condition and 2.95, 4.59 and 5.13 days respectively under cropped condition. The persistence of carbosulfan granule in the laterite soil at the same level resulted in half lives of 9.88, 10.50 and 11.50 days in the laboratory condition and 3.26, 5.16 and 7.30 days respectively in cropped condition. In coastal alluvial soil, the half lives of carbosulfan granules were 8.99, 9.45 and 10.45 days in the laboratory condition and 5.70, 6.50 and 9.80 days respectively in the cropped condition at 1, 2.5 and 5 mg kg-1 level of application. The three toxicologically important metabolites of carbosulfan viz., carbofuran, 3-hydroxy carbofuran and 3-keto carbofuran were monitored and the metabolite concentration was in the order of carbofuran > 3-keto carbofuran > 3-hydroxy carbofuran in the two soils. The effect of EC and granular formulation of carbosulfan on the microbial load was monitored after normal (250 g ai ha-1) and double (500 g ai ha-1) dose and found that in laterite soil the bacterial population increased to 9.45 x 106 cfu g-1 soil from the control population (6.86 x 106 cfu g-1) at normal dose of EC formulation. Granule application in the normal dose resulted in a higher population of actinomycetes (6.59 x 104 cfu g-1) than control (4.95 x 104 cfu g-1). In the coastal alluvial soil, application of EC and granules in the normal dose increased the bacterial population to 19.53 x 106 cfu g-1 and 20.35 x 106 cfu g-1 soil respectively from the control population (11.74 x106 cfu g-1). The population of arthropods declined in the two soils by carbosulfan treatment at both levels.
The study concluded that the mobility of carbosulfan was found to be higher in the coastal alluvial soil compared to laterite soil. The persistence of carbosulfan
was higher in granular formulation than in EC in both soils. Transformation of carbosulfan gives carbofuran as the major metabolite which on further degradation gives 3-keto and 3- hydroxy carbofuran in soil. The normal dose application of carbosulfan had certain positive effect on the soil organisms but the double dose application resulted in a considerable reduction in the population of microbes in the two soils.