Effects of Field Methods and Soil Cover on Estimating Ammonia Loss from Nitrogen‐15‐Urea

Ammonia loss from urea fertilizers is difficult to measure accurately in the field. In a field study we compared two methods for measuring NH 3 loss from 15 N‐urea surface applied to bare and bermudagrass ( Cynodon dactylon L.) covered Captina silt loam (fine‐silty, mixed, mesic, Typic Fragiudult). The two methods were semiopen chambers and open microplots using 15 N recovery. Labeled urea was applied at 100 kg N ha −1 , and chambers and microplots were shaded and protected from rainfall. Estimates of NH 3 losses from the bare soil after 357 h were 24.6 and 48.0% of the applied N from the semi‐open chambers and open microplots, respectively. Estimates of NH 3 losses from the bermudagrass after 357 h were 34.5 and 42.7% of the applied N for the semi‐open chambers and open microplots, respectively. In the bermudagrass microplots, 78.6% of the applied 15 N either remained in the plant‐thatch layer above the soil or was volatilized 18 h after urea application. Transport and transformations in the soil had little influence on NH 3 loss from bermudagrass plots. These data indicated that measured NH 3 loss from bare soil in semi‐open chambers was significantly less than NH 3 loss in open microplots using 15 N recovery techniques. Cumulative NH 3 losses from urea retained on bermudagrass and thatch above the soil were similar in the semi‐open chambers and open microplots.