Rainfall timing and ammonia loss from urea in a loblolly pine plantation

Surface application of urea to pine forests may lead to ammonia (NH3) loss. It is generally believed that rainfall received soon after urea application will wash the urea and its hydrolysis products into the soil and stop NH3 loss, but quantitative data are lacking, especially for the forest environment. The objective of this study was to quantify the effect of rainfall on loss of NH3 when received at different times following urea application. Four field studies were performed in a mid-rotation loblolly pine (Pinus taeda L.) plantation, where NH3 volatilization chambers were fertilized with 200 kg ha(-1) N and NH3 losses were measured for either 29 or 58 d. In a complementary lab study, both NH3 loss and movement of fertilizer N into the soil were measured following simulated rain. Loss of NH3 from urea was either increased or not affected by simulated rainfall applied after the urea granules were dissolved by dew. Increased NH3 loss due to simulated rainfall was attributed to inefficient downward leaching of urea and increased water content, which is known to increase the rate of urea hydrolysis. In contrast, simulated rainfall applied immediately after urea application reduced NH3 losses to <1% of the applied urea. Our results show that unless rain occurs before urea is dissolved by morning dew, it may not be effective at leaching urea into the soil and reducing NH3 losses. Further research should be conducted to elucidate the mechanism of urea retention by the O horizon in pine forests.