Inhibition of soil ammonia oxidation by a fungicide, chlorothalonil

We investigated the degrees of inhibition of soil ammonia oxidation by a fungicide, chlorothalonil (tetrachloroisophthalonitrile) and the mechanisms. Chlorothalonil, thiram, triflumizole, tolclofos-methyl, iprodione, flutolanil, hexaconazole, isoprothiolane, benomyl, and metalaxyl (50 mg kgsup(-1)) were applied to the soil (1ight-colored andsol) in combination with ammonium sulfate (200 mg N kgsup(-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 (100 mg kgsup(-1)) and chloramphenicol (500 mg kgsup(-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 10E4 gsup (-1) to 10E2 gsup(-1) or less for 14 days, while those in the chloramphenicol-treated soil maintained 10E4 gsup(-1) for 21 days. These results suggest that chlorothalonil inhibits soil ammonia oxidation by acting on ammonium-oxidizing bacteria lethally, 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.