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Nutrient Management & Soil & Plant Analysis

Loblolly Pine Needles Retain Urea Fertilizer that Can Be Lost as Ammonia

^a Dep. of Crop and Soil Sciences, 3111 Miller Plant Sciences. Bldg., University of Georgia, Athens, GA 30602
^b Warnell School of Forest Resources, University of Georgia, Athens, GA 30602

^* Corresponding author (mcabrera{at}uga.edu)

Previous work showed that simulated rainfall applied immediately after urea application to a pine forest floor decreased NH₃ losses to negligible levels. The same work also showed that unless rain occurs before urea is dissolved, it might not be effective at leaching urea into the forest floor and decreasing NH₃ losses. This study evaluated the effects of diurnal humidity cycles and rainfall amounts on the proportion of urea leached from loblolly pine (Pinus taeda L.) forest floor fractions, and on subsequent ammonia volatilization. New and old pine needles, as well as the partially decomposed (O_e) horizon, were collected from a loblolly pine forest floor in Georgia, USA. Control samples and samples treated with urea at 200 kg N ha^–1 were exposed to zero, two, four, or eight simulated humidity cycles, after which they were leached with nine 20-mm increments of simulated rain. Rainfall (180 mm) applied immediately after urea application (0 humidity cycles) leached >98% of the urea from the three forest fractions. Increasing the number of simulated humidity cycles from zero to eight decreased the percentage of urea leached from new pine needles from 98 to 49%, but did not have a major effect on the amount of urea leached from old needles or partially decomposed fraction. All of the urea not leached from new pine needles by 180 mm of simulated rainfall was extractable by water when the new needles were ground. When new needles treated with urea were leached with 180 mm of simulated rainfall and then incubated at 25°C and 95% relative humidity (RH) for 15 d, NH₃ losses amounted to 8% of the urea applied. These results suggest that urea retained by recently dropped pine needles is found inside the needles, where it can be hydrolyzed and subsequently lost as NH₃.

Abbreviations: CRH, critical relative humidity • RH, relative humidity