Ammonia surface-exchange above an agricultural field in Southeast England

Abstract

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 NH 3 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 NH 3 flux of about 0.03 μg N m −2s −1. In contrast, a consistent NH 3 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 NH 3 emission with the maximum occurring at midday. The results also showed that, whilst negligible NH 3 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 NH 3 concentration with a compensation point of 3–4 μg m −3. Loss of NH 3 to the atmosphere accounted for approximately 1% of the NH 4NO 3 fertiliser applied to the soil.