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A review is given of the state of the art of testing side‐effects of pesticides on nitrogen conversions in soil.
Ammonification and nitrification are the most important processes of the nitrogen cycle. Usually tests for effects of pesticides are carried out with nitrogen‐containing amendments like lucerne meal. In soils with a low carbon content such amendments may result in disturbance of soil populations. The ammonium ions released by nitrification are converted by nitrifying bacteria via nitrite in nitrate. Nitrification can also be studied by addition of ammonium directly to the soil. In this way, ethylenethiourea, a conversion product in soil of ethylenebisdithiocarbamate fungicides (e.g. maneb, zineb) was found to be a strong inhibitor of nitrification in soil.
A rapid test on ammonium oxidation was designed by inhibiting the conversion of nitrite into nitrate by potassium chlorate. Determination of the nitrite formation gives a rapid prediction of the inhibition of ammonium oxidation in soil by chemicals.
Denitrification and nitrogen fixation were not considered to be suitable processes to study side‐effects of pesticides in soil. An exception should be made for pesticides applied to legume crops (Papilionaceae) where symbiotic nitrogen fixation is an essential process.