Evaluation of the acetylene inhibition method for measuring denitrification in soilless plant culture systems

Denitrification measurements by means of the acetylene inhibition method require a continuous presence of acetylene to block the microbial reduction of N₂0 to N₂. To examine the effect of such steady exposures on the growth of plants, roots of cucumber and tomato seedlings were treated with different acetylene concentrations. Acetylene concentrations of ≥1 vol% in the gas phase, which were necessary for complete inhibition of N₂ formation, led to a significant retardation of root growth. This was partly due to trace amounts of ethylene contained in the acetylene gas which could not be removed with the usual prescrubbing through a sulfuric acid train. As a result of the growth impairment, oxygen consumption in the root zone decreased after 4 days of exposure. In order to avoid these side effects, the denitrification measurements in soilless cultures were performed on individual plants over a limited period of 2–3 days. The flow-through chamber method proved to be suitable for determining the gaseous N losses in a closed-loop system. It avoided greater air variations from the environmental conditions (substrate temperature, airflow and plant composition) and excluded errors in measurement caused by injury to roots and spatial variability of denitrification activity in the root medium. For exact estimation of the gaseous N losses, preceding 24-h acetylene fumigation was necessary. Subsequently at least three gas samples had to be taken throughout the day, because the N₂O+N₂ emissions were subject to a pronounced diurnal variability.