Study of dissipation and degradation of malathion and dimethoate from the soil and the pea plant (Pisum sativum) using a radioisotope tracer technique

Abstract

The dissipation and degradation of 14C-malathion {S-l, 2- bis (ethoxy carbonyl) ethyl 0, 0- dimethyl phosphorodithioate} and T'Csdimethoate {O, O-dimethyl S-methyl carbamoyl methyl phosphorodithioate} from the soil and the garden pea (Pisum sativum) have been studied by use of radioisotope tracer techniques. Field experiments were conducted on the dissipation and degradation of 14C-malathion in Nairobi and Kisii areas. Greenhouse experiments were conducted at the University of Bayreuth in Germany, where the dissipation and degradation of "Cvdimethoate and 14C-malathion were conducted with the potted garden pea plants and the soil. Weather conditions close to those prevailing in the Kenyan situation were simulated. "Cvlabelling on the malathion molecule was done at the two methoxy groups in one set of experiments and at the 2 nd , 3 rd carbons in the diethyl malonate of the malathion molecule in another set of experiments. Only the two carbons on the methoxy groups of dimethoate were labeled. The extractable residues of the pesticides were quantified by liquid scintillation counting while the bound residues were first combusted by a biological materials oxidiser to 14C02 , which was trapped by a cocktail before counting. Metabolites from the degradation of malathion and dimethoate were identified by TLC, autoradiography and confirmed by GC-MS. In greenhouse studies, the dissipation and degradation studies on the garden pea, show that "Cvmalathion dissipated fast from the foliar surface of the garden pea. There was only 5.7% of the initial pesticide dose on the foliar surface of the garden pea as surface (dislodgeable) residues aft\ 16 days. The extractable residues initially increased in the plant cells due to penetration t~ugh the cuticle of the plant from the surface, reaching a maximum level after 3 days and then started decreasing. Sixteen days after inoculation, 9% was present as extractable residues. The non-extractable (bound) residues were only 4.2 % of the initial t!"SB~OU !!VFks:n I. ~P. S. LI8IJelA,l(:1I(V IV dose and 16.7% was translocated from the treated leaf surface to the rest of the plant. A high proportion of the pesticide (17%) was lost through evapotranspiration due to the high rate of translocation of the malathion residues. The dissipation of the total residues of malathion from the pea plant was exponential and a half-life value as determined by first order kinetics model, was 11 days. The metabolites, 0, O-dimethyl phosphorodithioate and diethyl mercaptosuccinate which result from chemical hydrolysis of malathion, were detected both in the soil and plant samples. Malaoxon, an oxygen analogue, was only detected in the soil. "Cvdimethoate was comparatively more persistent than "Cvmalatbion in the pea plant. There were 21%, 19.4% and 5% of dislodgeable, extractable and bound residues of dimethoate, respectively recovered from the pea plant. Only 4.6% of applied 14C-dimethoate was translocated. The half-life value of the total residues of dimethoate in the pea plant, as determined from first order kinetics model, was 29 days. Dimethoxon, the oxygen analogue of dimethoate, was detected both on the surface of the leaves and inside the pea plant. The dissipation rate of 14C-malathion was lower from the soil than from the garden pea plant in the greenhouse. The extractable residues in the soil dissipated quickly, with only 1.5% of the initial pesticide dose remaining 32 days after inoculation. The bound residues constituted a higher proportion of the total residues in the soil. The bound residues rapidly built up in the soil up to 42% after 8 days. Thereafter, the bound residue levels decrease until there was only 20.6% remaining in the soil after 32 days. The decrease in the bound residues has been attributed to th~iOdegradation by microorganisms to 14C02.The total residues of malathion dissipated from the soil with a half-life value of 17 days. The uptake of 14C-malathion from the soil by the garden pea was low with 2.9% of the pesticide being taken up. 14C-dimethoate was more persistent than "Cunalathion in the soil. Only the extractable residues of dimethoate were degraded to CO2 while the bound residues remained at a v maximum level of 41.3% after 32 days. Due to the high solubility of dimethoate in water, its uptake from the soil was higher (4.1 %) than malathion (2.9%) uptake by the garden pea. The soil bound more pesticide residues than the pea plant as expected. Under field conditions, malathion dissipated slightly faster during the long rain season (tll2 = 36.7 days) than in the short rain season (t1l2= 41 days) in Kisii. In both seasons, the bound residue levels decreased due to biodegradation to CO2 after 21 and 7 days for the short and long rains seasons respectively. In Nairobi soil, malathion dissipated more slowly with a half-life value of 770 days. The difference between these rates may be attributed to the prevailing weather conditions at the two sites. However, the dissipation patterns of malathion in Kisii and Nairobi areas, separately, correlated well with the prevailing weather conditions and soil characteristics. The adsorption and desorption patterns of malathion and dimethoate conformed to the Freundlich isotherm equation, which assumes a multi-layer type of heterogeneous adsorption. Malathion, being more lipophilic, was adsorbed to the Kenyan soil with higher organic matter.