Quantification of ammonia exchange between agricultural cropland and the atmosphere: Measurements over two complete growth cycles of oilseed rape, wheat, barley and pea

The exchange of ammonia between the atmosphere and the canopy of barley, wh eat, oilseed rape and pea crops was studied over two growing seasons by use of a modified aerodynamic gradient technique in which passive horizontal f lux samplers were applied with a wind profile in gradient configuration. Th e crop foliage was a net source of NH3 to the atmosphere, with NH3 emission s on a seasonal basis between 1 and 5 kg NH3-N ha(-1). The amount of NH3 lo st constituted between 1 and 4% of the applied nitrogen and between 1 and 4 % of the actual amount of nitrogen present in the mature shoots. The volati le NH3 losses depended on seasonal variations in climatic conditions affect ing the growth and nitrogen economy of the crops and increased under condit ions with excessive N absorption by roots and a high N concentration in the foliage. The accumulated NH3 loss was positively correlated with the above -ground crop N content at anthesis, but not with that at final maturity. Th ere were no indications that NH3 emissions were larger under conditions unf avourable for nitrogen remobilization from vegetative plant parts (low N ha rvest index). Nevertheless, a distinct peak in NH3 emission occurred during senescence. It is concluded that crops in many areas will represent a sign ificant input of ammonia to the atmosphere and that NH3 losses may become l arge enough to significantly affect crop N budgets.