Four-year monitoring of atmospheric ammonia using passive samplers at a single-crop rice paddy field in central Japan

摘要

The aim of this study was to clarify the atmospbere-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 and1.5 m) on a weekly basis over a four-year period. The mean NH3 air concentrations 士 standard deviations at 4.0 m were 2.5 士 0.8 and 3.0 ±1.0 (20°C, 1013 hPa) in the cropping and fallow seasons, respectively. Sharp increases in the NH_3 concentrations coincided with the application of poultry manure and post-harvest fieia-burning activities; those values exceeded the 95% range of the weekly mean concentrations. The NH_3 exchange fluxes were calculated using a gradient method in wnich theflux was expressed as the product of the difference in air concentrations multiplied by the diffusion velocity between the two heights. The mean NH_3 exchange fluxes 士 standard deviations were 0.016 士 0.022 and 0.019 士 0.113 (ig N m~(-2) s~(-1) of 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 NH_3, with an annual net deposition of 7-9 kg N ha~(-1) yr~(-1). However, the application of poultry manure in February 2008 induced a strong NH_3 emission of approximately10 kg N ha~(-1) during three weeks, which nearly counterbalancedthe annual deposition for an entire year. Passive samplers are a convenient option for long-term monitoring, although the weekly mean exchange fluxes of NH_3 had a systematic error; a case study showed a 66% overestimation for the weekly mean exchange flux.