FIELD INVESTIGATIONS OF AMMONIA EXCHANGE BETWEEN BARLEY PLANTS AND THE ATMOSPHERE .1. CONCENTRATION PROFILES AND FLUX DENSITIES OF AMMONIA

The exchange of ammonia between the atmosphere and the canopy of spring barley crops growing at three levels of nitrogen application (medium N, high N and excessive N) was studied over two consecutive growing seasons by use of micrometeorological techniques. In most cases, ammonia was emitted from the canopy to the atmosphere. The emission started around 2 weeks before anthesis, and peaked about or shortly after anthesis. The volatilization of ammonia only took place in the daytime. During the night-time, atmospheric ammonia was frequently aborbed by the canopy. Occasionally, plants in the medium and high N treatments also absorbed ammonia from the atmosphere during the daytime. Daytime absorption of ammonia never occurred in the excessive N canopy. The loss of ammonia from the canopy amounted in both years to 0.5-1.5 kg NH3-N ha-1 and increased with the N status of the canopy. In agreement with the small losses of ammonia, the content of N-15-labelled nitrogen in the plants did not decline during the grain-filling period. The experimental years were characterized by very favourable conditions for grain dry matter formation, and for re-utilization of nitrogen mobilized from leaves and stems. Consequently, a very high part of the nitrogen in the mature plants was located in grain dry matter (80-84% in 1989; 74-80% in 1990). The efficient re-utilization of nitrogen may have reduced the volatilization of ammonia.