Saturday, 15 July 2006

Comparison of Two Passive Sampler Methods for Measuring Ammonia Losses.

Miguel Cabrera, Nicolas Vaio, John Rema, and David Kissel. Univ of Georgia, 3111 Plant Science Building, Athens, GA 30602

Understanding the magnitude of ammonia losses from surface-applied nitrogen fertilizers is important for agronomic and environmental reasons. Some of the simple methods developed to measure ammonia losses under field conditions use passive flux samplers, which do not require power or sophisticated instrumentation. The passive sampler method described by Schjoerring et al in 1992 consists of measuring ammonia losses from a circular plot by mounting passive flux samplers at different heights in four masts located at ninety-degree angles on the periphery of the plot. Each passive sampler is composed of two glass tubes connected by silicone tubing, with a nozzle connected to one of the tubes. The method requires the use of two passive samplers at each height, one with the nozzle pointing towards the inside of the circular plot and one with the nozzle pointing towards the outside of the plot. The tubes are coated with oxalic acid on the inside to trap any ammonia present in the air that circulates through them. After a period of exposure, the samplers are brought to the laboratory and the two tubes in each sampler are extracted separately with water to dissolve the ammonium oxalate present and determine the ammonia retained. Tubes facing the inside of the plot are considered to contain ammonia derived from the plot whereas tubes facing the outside of the plot are assumed to contain background ammonia. The simplicity of this method has allowed researchers to carry out many measurements of ammonia losses during the past 15 years. In spite of its simplicity, however, the method requires a large number of passive flux samplers. For example, if the flux is to be measured at five heights, it is necessary to use 10 samplers per mast, which would require extracting 20 separate tubes per mast, or 80 tubes per circular plot. A modified method proposed by Wood et al. in 2000 consists of a single, rotating mast located at the center of the circular plot, with a single passive flux sampler at each height. A wind vane rotates the mast so that the nozzle of each passive flux sampler is always pointing towards the wind source. A separate mast, away and upwind from the treated plots is used to measure background fluxes of ammonia. This method requires fewer tubes to be extracted and has been shown to perform similarly to another passive sampler method that uses a single-height, rotating sampler located at the center of the plot. To our knowledge, however, no study has been conducted to compare the passive flux sampler method of Schjoerring et al. to that of Wood et al. This study was carried out to compare these two methods. Two field studies were conducted for this purpose on a Cecil soil planted to tall fescue (Festuca arundinacea Schreb). In the first study, three circular plots, 40-m in diameter were fertilized with 67 kg N ha-1 as urea-ammonium nitrate solution. Four masts were mounted on the periphery of the plots and one mast was mounted at the center of each plot. Each mast had passive flux samplers at 0.4, 0.75, 1.5, 2.25, and 3 m. Samplers were replaced at 4, 8, 12, and 26 d after application. On average ammonia losses measured with the rotating mast were 36% greater than those measured with periphery masts. In the second study, 210 kg N ha-1 as urea was applied to three, 30-m circular plots and tubes were changed every 5 d for 36 days. Similarly to the previous study, ammonia losses measured with the rotating mast were 37% greater than those measured with periphery masts. The smaller estimates obtained with the periphery masts appear to have been caused by a bypass of ammonia from the tubes facing the inside of the plots to the tubes facing the outside of the plots. Tubes facing the outside of the plots are used to estimate background fluxes of ammonia, so any bypass of ammonia into them would decrease the estimation of fluxes derived from the plot.

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