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 0-5-1-5kg NH3-N ha~' and increased with the N status of the canopy. In agreement with the small losses of ammonia, the content of '^N-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.

A farm-gate nutrient balance showed that the surpluses per hectare fell from 175 to 123 kg N, from 29 to 13 kg P and from 61 to 32 kg K from 1979/80 to 2003/04. Consequently, the nutrient use efficiency (total output in % of total input) increased for N from 20% in 1979/80 to 37% in 2003/04, for P from 23% to 52% and for K from 18% to 43%. Parallel to the peak in surplus of N around 1980, elevated nitrogen concentrations were observed in ground and marine waters. A series of agro-environmental action plans were initiated, and the discharge of N to Danish coastal waters was reduced, though less than the reduction in surplus and losses from agriculture. In contrast to N, the reduction in P surplus was not reflected in any reduction in discharge of P from agriculture during the last two decades. A farm-gate account of N surplus is a reasonable indicator of potential N loss, whereas loss of P is mainly related to the soil P saturation level and to specific loss events. By preparing accounts for subsectors such as the arable and animal production sectors or for products within these sectors, farm-gate nutrient balances show the development in agricultural production, in addition to the trend in potential environmental impacts. Nutrient balances may thus link the environmental aspect to the economic aspect of agricultural production and assist in the monitoring of the sustainability of agriculture.

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The activities of glutamine synthetase (GS) and glutamate synthase (GOGAT) in different leaves of field-grown spring barley were measured during the reproductive growth phase in 2 consecutive years. Concurrently, the contents of soluble ammonium ions and free amides in the leaves were determined. The studies were carried out to investigate the relationship between variations in these parameters and emission of NH3 from the plant foliage. GS and GOGAT activities declined very rapidly with leaf age. The decline in enzyme activities was followed by an increase in soluble ammonium ions and amides in the leaf tissues. During the same period, about 75% of leaf and stem nitrogen was reallocated to the developing ear. The amount of NH3 volatilized from the foliage during the reproductive growth phase amounted to about 1% of the reallocated nitrogen. 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. Ammonia volatilization occurring under conditions with declining GS and GOGAT activities and increasing tissue concentrations of NH4^ may be useful in protecting the plant from accumulation of toxic NH3 and NH4^ concentrations in the tissues.