The production of NH+4-N following in vitro incubation of cattle urine was monitored for 24 h in the presence of ampicillin sodium salt (0, 32, 64, 128 mg/l), hydroquinone (0, 16·7, 33·4, 66·8 mg/l), wheat straw (0, 3·3, 6·6, 13·2 g/l) or spruce sawdust (0, 3·3, 6·6, 13·2 g/l) with (20 IU) or without urease. Each concentration of ampicillin, hydroquinone, wheat straw or spruce sawdust was tested in triplicate.

The equipment consisted of Woulff flasks containing 300 ml of a buffered solution (0·02 M; pH 7·50) with 1 ml of cattle urine. The cattle urine was characterized by measuring the main nitrogen contents, which were 6·52 mg total-N/ml, 5·96 mg urea-N/ml and 0·026 mg ammonium-N/ml. The initial pH of urine was 7·84. Ammonium and nitrate concentrations, and pH were monitored at zero- time and after 3, 6 and 24 h of incubation with the cattle urine.

The addition of urease to the flasks containing urine induced a significant increase in the production of ammonium-N, from 1·83 to 6·32 mg NH+4-N/flask after 24 h of incubation.

In the presence of urease, an inhibitory effect was recorded in NH+4-N production with ampicillin and spruce sawdust. In contrast, hydroquinone inhibited urease activity and wheat straw adsorbed the NH+4-N produced, both causing a dose-dependent relationship.

In the absence of urease, ampicillin, hydroquinone, wheat straw or spruce sawdust caused a dose- related decrease in NH+4-N production. However, the highest amounts of wheat straw (6·6 and 13·2 g/flask) exhibited a temporary increase in NH+4-N production during the first 6 h. This is probably due to a generation of extra NH+4-N as compared to the control flasks without straw. However, at 24 h, the situation paralleled the other materials.

Although pH increased approximately from 7·50 to 7·65 during the hydrolysis of urea in the presence of urease, no gaseous ammonia was volatilized into a 0·1 M HCl flask. Moreover, no increase in nitrate concentration was found during the incubation.

The present results suggest that ampicillin, hydroquinone and spruce sawdust could be used in order to reduce NH+4-N production from cattle urine. The NH+4-N already produced could probably be absorbed on wheat straw.