Ammonia transport in a temperate grassland .2. Diurnal fluctuations in response to weather and management conditions

Ammonia emission into the atmosphere is of concern because of its potential impact on atmospheric aerosol chemistry and fertilizer N use efficiency. The purpose of this study was to evaluate the effects of N surplus and deficit on soil-plant-atmosphere NH3 exchange in a temperate humid grassland. Plant and soil N measurements were made along with NH3 fluxes using flux-gradient techniques. Ammonia flux was related to the plant ammonia compensation point (NH3 CP), which was controlled by plant N concentration and management practices (such as harvest and fertilization). The measured fluxes were generally much larger during daytime than during nighttime. The daily variation in flux was apparently caused by higher diffusive resistance at night. Generally, there were no significant differences between daytime and nighttime NH3 CP. Ammonia was absorbed in dew and released during dew evaporation; however, nighttime absorption exceeded daytime release, with some of the N apparently being captured by the crop.