Studies on the Effects of Herbicides on Soil Nitrification*

Abstract

Summary. A perfusion technique was used to examine effects of selected herbicides (concentration range 50–1000 ppm) on the rate of oxidation of ammonia to nitrate in garden loam soil. Effects on the nitrification rate of soil, previously enriched with nitrifying organisms, were used to estimate direct action on the processes of ammonia oxidation. The kinetics of nitrification in fresh soil were used to estimate effects on bacterial growth and to detect possible adaptations to herbicide toxicity. The herbicides used could be ranged in the following order of increasing effectiveness as inhibitors of ammonia oxidation: dichlobenil < paraquat < picloram < 2,3,6‐TBA = chlorthiamid < bromoxynil < chlorflurazole < ioxynil < propanil. Endothal only stimulated ammonia oxidation, even up to 1000 ppm. Endothal, paraquat and 2,3,6‐TBA had no apparent effects on bacterial proliferation; the inhibitory effects of the remaining herbicides were in the order: picloram < dichlobenil = bromoxynil < ioxynil < chlorthiamid = chlorflurazole = propanil. With the exception of paraquat and endothal all herbicides seemed to become less toxic during perfusion of fresh soil, i.e. over the enrichment period of 26–28 days. In the case of propanil and chlorthiamid this could have been due to the destruction of the herbicide by soil micro‐organisms. With picloram and 2,3,6‐TBA it is probable that there was an adaptive loss of sensitivity of the nitrifying organisms to the toxic action of the herbicides. The evidence available suggests that Nitrosomonas is more sensitive than Nitrobacter to herbicide action. Recherches sur les effets des herbicides sur la nitrification Résumé. Une technique de perfusion a été utilisée pour examiner les effets d'une série d'herbicides (á des concentrations de 50 á 1000 ppm) sur le taux d'oxydation de l'ammoniac en nitrate dans un sol de jardin limoneux. Les effets sur le taux de nitrification du sol, préaiablement enrichi avec des organismea nitrifiants, ont été utilisés pour évaluer l'action directe sur les processus d'oxydation de l'ammoniac. Les données cinétiques de la nitrification dans un sol frais ont été utilisées pour évaluer les effets sur la croissance des bactéries et pour déceler les adaptations possible h la toxicité de l'herbicide. Les herbicides utilisés purent être classés comme suit dans i'ordre croissant d'activité inhibitrice de I'oxydation de I'ammoniac: dichlobénil<paraquat<piclorame<2,3,6‐ TBA = chlorthiamide<bromoxynil<chlorflurazole<ioxynii<propanil. Scul l'endothal provoqua une stimulation de I'oxydation de I'ammoniac jusquà 1000 ppm. L'endothal, le paraquat et le 2,3,6‐TBA ne présentèrent pas d'activité apparente sur la proiifération bactérienne; les effets inhibiteurs des autres herbicides furent, dans I'ordrc: piclorame < dichlobenil = bromoxynil<ioxynil<chlorthiamide = chlorflurazole = propanil. A l'exception du paraquat et de l'endothal, tous les herbicides parurent devenir moins toxiques pendant la perfusion sur un sol frais, c'est‐à‐dire au deli delà période d'enrichissemenl de ...

Effect of Picloram on Some Soil Microbial Activities

“…Goring et al (1967) reported that conversion of ammonia to nitrite in the soil was partially inhibited by picloram at 1000 ppm but not at 100 ppm, while conversion of nitrite to nitrate was not significantly inhibited at either 100 or 1000 ppm. Similar results were recently obtained by Debona & Audus (1970), using equally high rates, and by Tu & Bollen (1969), working with low rates of picloram. Dubey (1969), on the other hand, showed that nitrification processes were significantly reduced in two of the three soils studied at rates as low as 2 ppm, while inhibition was complete at 100 ppm.…”

Effects of repeated applications of ten soil‐active herbicides on weed population, residue accumulation and nitrification*

“…Burns & Audus (1970) showed that paraquat is rapidly adsorbed on inorganic soil components where it becomes unavailable even to microbiological degradation. Paraquat had no effect on ammonification and nitrification in soil (Debona & Audus, 1970;Tu & BoMen. 1968).…”

Effect of Three Organophosphorous Nematicides on Non-target Nematodes and Soil Microbial Community

“…The number of ammonia oxidizers was not reduced by the nematicides either (Table 4), although fumigants (e.g. Toyota et al 40) ) and certain herbicides and fungicides, such as propanil and 2,4-D 8,9) , bensulfuron-methyl 46) , prosulfuron, mancozeb and chlorothalonil 24) , are known to decrease numbers of nitrifying bacteria.…”