Abstract
The aim of this study was to clarify the atmosphere-paddy exchange of ammonia (NH3) at a single-crop rice paddy field in central Japan. Measurements of NH3 air concentrations were carried out using passive samplers at two heights above the ground surface (4.0 and 1.5 m) on a weekly basis over a four-year period. The mean NH3 air concentrations ± standard deviations at 4.0 m were 2.5 ± 0.8 and 3.0 ± 1.0 (20°C, 1013 hPa) in the cropping and fallow seasons, respectively. Sharp increases in the NH3 concentrations coincided with the application of poultry manure and post-harvest fieldburning activities; those values exceeded the 95% range of the weekly mean concentrations. The NH3 exchange fluxes were calculated using a gradient method in which the flux was expressed as the product of the difference in air concentrations multiplied by the diffusion velocity between the two heights. The mean NH3 exchange fluxes ± standard deviations were 0.016 ± 0.022 and 0.019 ± 0.115 μg N msof deposition in the cropping and fallow seasons, respectively; where the weekly mean flux was assumed to be zero if the difference in the air concentrations between the two heights was not significant (p < 0.05). In general, the paddy field acted as a net sink of atmospheric NH3, with an annual net deposition of 7–9 kg N hayr-. However, the application of poultry manure in February 2008 induced a strong NH3 emission of approximately 10 kg N haduring three weeks, which nearly counterbalanced the annual deposition for an entire year. Passive samplers are a convenient option for long-term monitoring, although the weekly mean exchange fluxes of NH3 had a systematic error; a case study showed a 66% overestimation for the weekly mean exchange flux.
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