Ammonia Volatilization: Soil, Plant, and Microclimate Effects on Diurnal and Seasonal Fluctuations

Nitrogen balance studies in pastures have shown unexplained losses of applied N with much of the loss assumed to be denitrification and NH₃ volatilization. This project studied soil, plant, and microclimate effects on diurnal and seasonal fluctuations of gaseous NH₃ flux in a subtropical pasture fertilized with urea. Soil and microclimate measurements were taken concurrently with NH₃ flux density determinations to relate these factors to magnitude and direction of NH₃ transport. Average daily NH₃ volatilization varied with seasonal soil and microclimate conditions and time after area application. A short period of large NH, efflux was observed after urea application during warmer seasons whereas smaller efflux for longer duration was observed during cooler seasons. Soil water content and rainfall after urea application modified seasonal NH₃ losses. Diurnal NH₃ cycling was apparent, with large efflux occurring during daytime and small efflux or influx at night. Prior to each urea application, average diurnal NH₃ transport was into the soil-plant system possibly due to low soil mineral and plant N. Brief periods of NH, influx were common throughout all seasons, particularly around sunset and sunrise. Soil surface temperature was the most highly correlated factor influencing NH3 flux density during the summer season. During the remainder of the year evapotranspiration had the highest correlation, although the increased midday fluxes were probably due to paralleled increase in soil temperature and windspeed. All of the major influencing factors are. interrelated through their dependence on solar radiation. Rainfall distribution and amount after urea application appeared to control the total NH₃ losses from applied urea. The rainfall influence was probably the result of rainfall dispersing urea which prevented high concentrations of NH₃ and NH₄⁺ from building up around urea-prills.