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 soil conditions. Ammonia emission was favoured regardless of fertilisation in dry and warm conditions in summer with an average NH3 flux of about 0.03 μg N m−2s−1. In contrast, a consistent NH3 deposition was observed during wet conditions in winter with an average flux of −0.068 μg N m−2s−1 and an average apparent deposition velocity of about 1.6 cm s−1. The effect of temperature was clearly observed through a diurnal variation in the rate of NH3 emission with the maximum occurring at midday. The results also showed that, whilst negligible NH3 was emitted from the soil, much higher fluxes (emission or deposition) were observed above the crop. Ammonia exchange patterns throughout the whole experimental study showed a dependence on the ambient NH3 concentration with a compensation point of 3–4 μg m−3. Loss of NH3 to the atmosphere accounted for approximately 1% of the NH4NO3 fertiliser applied to the soil.