Ammonia volatilization from urea-based fertilizers applied to tall fescue pastures in Georgia, USA

The use of urea fertilizers in grasslands is likely to increase in areas with concentrated animal feeding operations as restrictions on manure applications are implemented. Concerns have been raised about the economic and environmental impacts of N[H.sub.3] loss from these urea fertilizers. This study evaluated N[H.sub.3] losses from Nitamin (a urea polymer), urea-N[H.sub.4]N[O.sub.3] (UAN), and granular urea applied to tall fescue (Festuca arundinacea Schreb.) plots at 50 kg N [ha.sup.-1] in fall and spring for 2 yr. Fertilizers were applied to circular plots (30-m diameter) and N[H.sub.3] loss was measured by the modified passive flux method for 69 to 120 d after application. In a separate laboratory study, Nitamin, UAN, and urea were surfaced applied to rescue thatch at 100 kg N [ha.sup.-1] and treatments were incubated at 24[degrees]C and 90% relative humidity for 31 d. In fall applications, urea lost more N[H.sub.3] (19% in 2004, 46% in 2005) than UAN or Nitamin, which were not different from each other (6% in 2004, 34% in 2005). In contrast, there were no differences among fertilizers in spring applications, with average losses of 13% in 2005 and 17% in 2006. In the laboratory study, urea lost significantly more N[H.sub.3] (24%) than UAN or Nitamin, which were not different from each other (average 9% loss). These results indicate that Nitamin and UAN undergo similar N[H.sub.3] losses, and that both fertilizers may lose less N[H.sub.3] than urea under conditions favorable to volatilization. Abbreviations: CRH, critical relative humidity; RH, relative humidity; UAN, urea--ammonium nitrate.