Laboratory chamber system for measuring ammonia emissions from intact soil cores: studies of cattle feedlots and forest soils

Wet deposition of ammonium in Rocky Mountain National Park (RMNP) has reached a “critical load” where negative impacts on alpine ecosystems are evident. Many different sources are likely contributing to this deposition; however, very few measurements of ammonia emissions from potential sources have been made, limiting our ability to apportion deposited nitrogen to the appropriate sources. A laboratory chamber system was developed to measure ammonia fluxes from intact soil cores (13 cm diam.) taken from sites with widely varying characteristics (e.g., cattle feedlot pens, forest soils). The system routed ammonia-free air from a compressor, through a chilled water bath, and to modified vacuum desiccators containing the intact soil cores. Twelve chambers were operated simultaneously. Acid trap bubblers collected ammonia volatilized from the cores, and analysis was done using a Flow Injection Analyzer. Studies were conducted using soil cores collected from cattle feedlot pens in Lamar and Greeley, Colorado. During 5 to 7 day tests at 22 C, ammonia fluxes from feedlot cores typically ranged between 4 and 18 g m^-2 d^-1 with the highest fluxes during first-day from urine patches and duff material (i.e., loose mixture of dry manure, fresh manure, urine, and soil on the pen surface). The lowest fluxes (1 to 4 g m^-2 d^-1) occurred from pens that had been recently cleaned/scraped. The system was also capable of detecting differences in ammonia emissions among pens where cattle had been fed diets with different protein contents. Data collected in areas bordering RMNP showed fluxes from grassland and forest soils were very low - less than 0.01 g m^-2 day^-1. Nevertheless, sample calculations suggested that these forest flux rates may be large enough to account for observed diurnal patterns of background NH3 in the park. The laboratory chamber system is a useful tool for initial evaluation of ammonia fluxes and testing mechanistic models of the volatilization process..