Controls of Volatile Ammonia Losses from Loblolly Pine Plantations Fertilized with Urea in the Southeast USA

Abstract

Ammonia volatilization losses from surface applied urea could reduce the N‐use efficiency in loblolly pine (Pinus taeda L.) plantations. Three field studies were conducted to assess the effectiveness of two urea formulations: coated‐urea fertilizer (CUF) and the urease inhibitor N‐(n‐butyl) thiophosphoric triamide‐treated urea (NBPT) at reducing ammonia (NH3) losses in loblolly pine plantations under different forest floor moisture (dry vs. wet), substrate (forest floor vs. mineral soil), and site/soil type conditions (Piedmont vs. Coastal Plain). An additional laboratory study under controlled environmental conditions helped validate the results from the field. Ammonia volatilization losses were influenced more by the initial forest floor moisture than by the fertilizer formulations and were 43% lower when the fertilizer treatments were applied directly on the forest floor than on the exposed mineral soil. The average NH3 losses, expressed as a percentage of applied N, in the Piedmont ranged from 1 to 9%, and from 7 to 16%, 7 and 30 d after fertilization, respectively. At Day 7, both CUF and NBPT reduced NH3 losses, as compared with untreated urea, by 39 and 80%, respectively. In the Coastal Plain, NH3 losses ranged from 7 to 17%, 7 d after fertilization. Both CUF and NBPT reduced NH3 losses, as compared with untreated urea, by 35 and 25%, respectively. This difference in treatment ranking between sites suggests a fertilizer by site/soil type interaction which the laboratory study confirmed. These results highlight the importance of knowing the environmental and site conditions before fertilization and can help decide where and when these formulations might be used more effectively.