MEASUREMENTS OF AMMONIA EMISSION FROM GRAZED GRASSLAND

Some of the factors influencing NH3 emission from grazed grassland were examined. The large day-to-day variation in rates of loss were only partially explained by variation in the measured environmental factors (wind speed, soil and air temperature, soil moisture status, relative humidity, rainfall and potential evapotranspiration). Of the measured variables, wind speed had the largest effect but the best multiple linear regression model of daily NH3 loss had an adjusted R2 value of only 0·406. The mechanisms controlling NH3 flux were therefore unclear. There were marked diurnal rhythms in NH3 loss and the concentration gradients above the sward were strongest during the period which included the 3 h each side of midday. This pattern of NH3 release has important implications in relation to atmospheric mixing processes and chemical reactions. A comparison of two means of determining NH3 concentrations, i.e. a bubbler collection system or denuder tubes, indicated that different forms were collected which could be related to the environmental conditions. Such differences and effects should be considered when models are developed to describe the behaviour of nitrogenous and other species in the atmosphere. © 1990.