AMMONIA SURFACE-EXCHANGE ABOVE AN AGRICULTURAL FIELD IN SOUTHEAST ENGLAND

A micrometeorological method and a chamber method were used to measure ammonia exchange over a mineral-fertilised wheat crop and to monitor the seasonal variation of NH3 under a variety of environmental and soi l conditions. Ammonia emission was favoured regardless of fertilisatio n in dry and warm conditions in summer with an average NH3 flux of abo ut 0.03 mu g N m(-2) s(-1). In contrast, a consistent NH3 deposition w as observed during wet conditions in winter with an average flux of - 0.068 mu g N m(-2) s(-1) and an average apparent deposition velocity o f about 1.6 cm s(-1). The effect of temperature was clearly observed t hrough a diurnal variation in the rate of NH3 emission with the maximu m occurring at midday. The results also showed that, whilst negligible NH3 was emitted from the soil, much higher fluxes (emission or deposi tion) were observed above the crop. Ammonia exchange patterns througho ut the whole experimental study showed a dependence on the ambient NH3 concentration with a compensation point of 3-4 mu g m(-3). Loss of NH 3 to the atmosphere accounted for approximately 1% of the NH4NO3 ferti liser applied to the soil.