Abstract
We investigated the degrees of inhibition and the mechanisms by which soil ammonia oxidation is inhibited by a fungicide, chlorothalonil (tetrachloroisophthalonitrile). Chlorothalonil, thiram, triflumizole, tolclofos-methyl, iprodione, flutolanil, hexaconazole, isoprothiolane, benomyl, and metalaxyl (50mg kg^<-1>) were applied to the soil (light-colored andosol) in combination with ammonium sulfate (200mg N kg&<-1>). Chlorothalonil and thiram strongly inhibited soil ammonia oxidation among the fungicides tested. The duration of the inhibition induced by chlorothalonil was longer than that by a nitrification inhibitor, dicyandiamide. The activity of the inhibition of soil ammonia oxidation by chlorothalonil depended on its application rate. The inhibitory activity of 4-hydroxy-2, 5, 6-trichloroisophthalonitrile (TPN-OH), a major metabolite of chlorothalonil in upland soils, was lower than that of chlorothalonil and dicyandiamide. The activity of tetrachloroterephthalonitrile (TTPN), an isomer of chlorothalonil, was lower than that of chlorothalonil. Isophthalonitrile (IPN), terephthalonitrile (TePN), phthalonitrile (PN), benzonitrile (BN), and 1, 2, 3, 5-tetrachlorobenzene (TCB) did not inhibit soil ammonia oxidation. Chlorothalonil (100mg kg^<-1>) and chloramphenicol (500mg kg^<-1>) were added to soil in which ammonium-oxidizing bacteria had been accumulated by repeated application of ammonium sulfate. Both chemicals inhibited soil ammonia oxidation. The number of the ammonium-oxidizing bacteria in the chlorothalonil-treated soil decreased from 10^4g^<-1> to 10^2g^<-1> or less for 14 days, while those in the chloramphenicol-treated soil maintained 10^4g^<-1> for 21 days. These results suggest that chlorothalonil inhibits soil ammonia oxidation by acting on ammonium-oxidizing bacteria lethally, and that both nitrile and chlorine in the structure of chlorothalonil are indispensable to its inhibitory action and that their arrangement in the molecules significantly affects chlorothalonil's activity.
