Persistence and degradation of chlorothalonil in selected soils

Abstract

The study was taken up to determine the persistence and dissipation pattern of a commonly used fungicide chlorothalonil in two major soils of India differing widely in their physic-chemical properties under the influence of various moisture regimes at non sterile conditions. The two soil types were black clay soil from National Bureau of Soil Survey & Land Use Planning (NBSS & LUP), regional station, Bellary, Karnataka and loamy sand soil. Persistence and degradation of chlorothalonil in soils were determined by adding 100 ppm of toxicant to both the soil at sterile and sterile conditions and incubating them under different soil moisture regimes. The degradation reaction rate constants “K” and half life values (t1/2) for both the soils were computed from the amount of fungicide in soils at different periods following periodic residue analysis. The salient features of the investigation are summarized below: The two different soils collected from different locations represented fairly wide range of variation in soil characteristics such as textural make up, pH, organic matter content, CEC etc., (loamy sand Hessaraghatta soil:Haplustalf and black clay Bellary soil: Typic chromustents) were used for the study. The organic matter content was fairly high in black clay Bellary soil, while it was moderately low in loamy sand Hessaraghatta soil. There were similar differences in the other characteristic such as clay content, pH and CEC also. The recovery analysis of the fungicide residue in soils indicated that the recovery percentage were fairly high exceeding 90% indicating the analytical technique with modification was proved to be better for further studies. There was a marked difference in the persistence chlorothalonil in both the soil type. The degradation pattern of chlorothalonil residues indicated a close correspondence to first order exponential degradation kinetics in soils and mainly influenced by soil moisture. Increased degradation was observed with increased moisture content from air dry condition to submerged condition. Higher persistence chlorothalonil was noticed in black clay Bellary soil than that of loamy sand Hessaraghatta soil. Similarly marginally higher persistence was recorded in autoclaved soil condition than that autoclaved condition indicating that the involvement of micro-organisms in degradation was low at high concentration. The half life period (t1/2) of chlorothalonil in black clay soil for autoclaved condition ranged from 8.4 to 12.3 days, while for non autoclaved it ranged from 8.1 to 11.2 days at different soil moisture regimes. Similarly for loamy sand soil, it ranged from 8.0 to 11.1 days for autoclaved condition and 7.8 to 10.8 days for non autoclaved condition at different soil moisture levels. The half life period decreased with increased moisture content of soil, and the half life period was higher for autoclaved soil condition than non autoclaved condition. So also the half life values were fairly high for black clay Bellary soil than loamy sand Hessaraghatta soil. The degradation of soil retained fungicide residues followed first order reaction (R2>0.96). The degradation rate constant (K) increased with increasing moisture levels and K was always less in autoclaved condition than of non autoclaved condition. “Kdeg” was fairly high for loamy sand Hessaraghatta soil than that of black clay Bellary soil at particular point of time and treatments.