Decrease of 4,5,6,7-Tetrachlorophthalide in Paddy Field Soil After Aerial Application

Abstract

Variation in the fungicide, 4,5,6,7-tetrachlorophthalide (phthalide), in paddy field soil was investigated in order to evaluate its decrease after aerial application by a radio-controlled helicopter. The maximum concentrations of phthalide were 309-320 microg/kg dry, which were 83%-96% of the applied phthalide. The organic carbon normalized soil sorption coefficient (K(oc)) was calculated to be 94-96 mL/g at 1 h after the application. The calculated K(oc) values increased to 620-1,300 mL/g from 1 through 9 days after the application and then 4,700-7,200 mL/g 14 days after the application. The half-life of the phthalide was calculated to be 20-31 days.

Decrease of Herbicide Bromobutide and its Debromo Metabolite in Paddy Field Soil during 24 weeks After Application

“…Qin et al (2004) reported that the halflife of a nematicide, fosthiazate, was correlated with the organic matter content and clay content of soil. In contrast, the half-life of phthalide in the paddy soil with the higher organic content was shorter than that in the paddy soil with lower organic content (Iwashita et al 2008). The t 1/2 S values of bromobutide (21 days in paddy soil 1, 15 days in paddy soil 2) with organic contents of 7.21 % ± 1.34 % and 7.90 % ± 2.03 %, respectively, were larger than those (12 days) with contents of 4.31 % ± 0.86 % in paddy soil 3; the t 1/2 S values in paddy soils 1 and 2 with silt and clay contents of 36.5 % and 44.8 %, respectively, were larger than that with the content of 12.9 % in paddy soil 3.…”

“…The amounts reached maximum levels (about 20 % of the applied amounts) at 3 and 4 days after application, respectively (Amano et al 2001). We previously investigated variations in the concentrations of a fungicide, phthalide, in two paddy field soils at 137 days after application (Iwashita et al 2008). Phthalide was detected at maximum levels within 24 h after application, and was observed for 112 days after application in both paddy fields.…”

“…3. The decrease can be interpreted using first-order reaction kinetics (Qin et al 2004;Iwashita et al 2008) as Eq. (1):…”

“…The soil densities were 1.29 ± 0.12 g/cm 3 (paddy soil 1), 1.28 ± 0.093 g/cm 3 (paddy soil 2) and 1.22 ± 0.10 g/cm 3 (paddy soil 3); the silt and clay contents were 36.5 % (paddy soil 1), 44.8 % (paddy soil 2) and 12.9 % (paddy soil 3). The organic contents were estimated as the loss upon ignition (550°C, 15 h) and calculated as the percent dry weight (Iwashita et al 2008). The organic contents of paddy soils 1, 2 and 3 were 7.21 % ± 1.34 %, 7.90 % ± 2.03 % and 4.31 % ± 0.86 %, respectively.…”

“…The authors evaluated the runoff ratios of bromobutide from the paddy fields to drainage channels as 28 % ± 16 % (Morohashi et al 2012). Therefore, adsorption into the paddy soil was the main route of the residual bromobutide, just as it is for herbicides thiobencarb, mefenacet, pyrazosulfuron-methyl, and dymron (Amano et al 2001;Ishii et al 2004), as well as the fungicide phthalide (Iwashita et al 2008). Ishii et al (2004) reported the half-lives of three herbicides (mefenacet, pyrazosulfuron-methyl, and dymron) based on paddy field experiments conducted during 12 weeks after application.…”