Urethanes and Soil Nitrification

Urethanes have marked physiological activity in plants and animals. Lefevre (1939) found that phenyl urethane affects wheat germination, and subsequently Templeman and Sexton (1946) showed that a variety of urethanes suppress the germination of wheat and oats without affecting charlock. The most active urethanes (for example, isopropylphenylcarbamate;ethylphenylcarbamate) are effective at a concentration of a few parts per million. These arylcarbamic esters are known to affect mitosis in cells and this fact led to an investigation of urethanes as possible inhibitors of tumor growth. Haddow and Sexton (1946) obtained positive results in animals with isopropylphenylcarbamate, ethylphenylcarbamate and ethyl urethane, though the last is apparently inactive in plants. Ethyl urethane is used in the treatment of leukemia (Patterson et al., 1946), but in mice its administration gives rise to lung tumors (Larsen and Heston, 1945; Larsen, 1947). These effects of urethane take place at much smaller concentrations than those required to bring about its well known narcotic affects. Esterases are inhibited by urethanes (Stedman and Stedman, 1931, 1932), the inhibition of choline esterase by such urethanes as eserine and prostigmine being well known. Schweitzer, Stedman and Wright (1939) conclude that the urethane grouping is essential for esterase inhibition. Meyerhof (1916) showed that urethanes inhibit the respiration of nitrite oxidising organisms, the inhibitory power increasing with increasing size of the aliphatic radicle; for example, methyl urethane, inhibits the oxygen uptake of these organisms by 50 per cent at a concentration of 0.3M, but isoamyl urethane is equally effective at a concentration of 0.022M. Ethyl urethane also inhibits respiration of isolated pure cultures of Nitrosomonas by 42 per cent at a cocentration of 0.016M and gives 4 per cent inhibition of Nitrobacter respiration at a concentration of 0.11M. The inhibitory effect of urethane was confirmed by Lees and Quastel (1946), who found, however, a higher sensitivity of the process of nitrification to ethyl urethane in soil culture. The inhibitory effect of ethyl urethane on soil nitrification was also shown to be reversible. Quastel and Scholefield (1949) pointed out that while 0.01M ethyl urethane completely suppresses the nitrification of 0.01m ammonium chloride in garden soil for about twenty days, the subsequent rate of nitrification proceeds normally. The final nitrate