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

Surface application of urea to pine forests may lead to ammonia (N[H.sub.3]) 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 N[H.sub.3] 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 N[H.sub.3] when received at different times following urea application. Four field studies were performed in a mid-rotation loblolly pine (Pinus taeda L.) plantation, where N[H.sub.3] volatilization chambers were fertilized with 200 kg [ha.sup.-1] N and N[H.sub.3] losses were measured for either 29 or 58 d. In a complementary lab study, both N[H.sub.3] loss and movement of fertilizer N into the soil were measured following simulated rain. Loss of N[H.sub.3] from urea was either increased or not affected by simulated rainfall applied after the urea granules were dissolved by dew. Increased N[H.sub.3] 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 N[H.sub.3] losses to