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In the United States, approximately 600,000 ha of pine trees are fertilized with urea each year, with N[H.sub.3] volatilization losses ranging from < 1% to >50% depending on environmental conditions. Previous work showed that the timing of rainfall after urea application plays a significant role in controlling N[H.sub.3] loss, but the effect of other environmental variables is not well understood. We conducted 10 29-d studies under different environmental conditions during 2 yr to identify important variables controlling N[H.sub.3] loss from urea applied to loblolly pine (Pinus taeda L.) at 200 kg N [ha.sup.-1]. Ammonia loss was measured with dynamic chambers that adjusted the rate of air flow through the system based on wind speed at 1 cm above the soil surface. Regression analysis indicated that a variable related to the initial water content of the forest floor and a variable related to the relative humidity (RH) during the study explained 85 to 94% of the observed variability in N[H.sub.3] loss. Relatively high initial water content followed by consistently high RH led to large N[H.sub.3] losses. In contrast, low initial water contents resulted in slow rates of N[H.sub.3] loss, which increased when elevated RH led to an increase in the water content of the forest floor. These results indicate that RH can play a significant role in N[H.sub.3] loss by accelerating urea dissolution and by increasing or decreasing the water content of the forest floor, which in turn can affect the rate of urea hydrolysis. Abbreviations: CRH, critical relative humidity of urea; RH, relative humidity; RH-days, relative humidity-days. doi:10.2136/ssaj2009.0220

Surface application of urea to pine forests may lead to ammonia (N[H.sub.3]) volatilization, which can reduce forest production. The objective of this study was to characterize urea reactions with the forest soil and to determine how various forest floor components interact with weather to affect N[H.sub.3] loss. Three field studies were performed in a midrotation loblolly pine (Pinus taeda L.) plantation with up to three treatments in each study, where N[H.sub.3] volatilization chambers were fertilized with 200 kg N [ha.sup.-1] that contained either (i) intact forest floor, (ii) forest floor removed by hand raking, or (iii) forest floor burned. In a complementary laboratory study, N[H.sub.3] loss was evaluated from cores comprised of: (i) mineral soil only, (ii) mineral soil + humus layer, (iii) mineral soil + humus + old needles, and (iv) mineral soil + humus + old needles + new needles. Loss of N[H.sub.3] in the field was generally greatest when urea was applied to a mineral soil with no forest floor, resulting in a loss of 53 and 59% of the applied N in two 29-d studies. Loss of N[H.sub.3] was intermediate from a complete forest floor (11 and 35% loss), and rates of loss appeared to be most affected by relative humidity. Losses were least from a recently burned forest floor (6 and 7% loss), due partly to slower urea hydrolysis. Loss of N[H.sub.3] in the laboratory study was also greater from the bare soil than from the complete forest floor. Retention of N[H.sub.4.sup.+] in the soil layers was related to the soil pH buffering capacity below pH 7. Because the humus layer was shown to have a large pH buffering capacity to retain N[H.sub.4.sup.+], these results suggest forest management that enhances humus formation should increase N[H.sub.4.sup.+]] sorption and decrease N[H.sub.3] volatilization. Abbreviations: CRH, critical relative humidity; DI, deionized; RH, relative humidity.

Previous work showed that simulated rainfall applied immediately after urea application to a pine forest floor decreased N[H.sub.3] 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 N[H.sub.3] 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.sub.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.sup.-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[degrees]C and 95% relative humidity (RH) for 15 d, N[H.sub.3] 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 N[H.sub.3].

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

The relationship between soil test P (STP) and dissolved reactive P (DRP) in runoff has been shown to vary with soil type due to differences in soil properties. This study was conducted to determine the effect of mineralogy on P sorption behavior and DRP losses in runoff using simulated rainfall. Nine different soft types were sampled from four different fields to provide a range in STP. Unamended soils were packed into runoff boxes for use in a rainfall simulation study (7.5 cm h-1 for 30 min). A mineralogical analysis and adsorption-desorption isotherm was conducted on one representative sample from each soil type. Results indicated that P retention for adsorption and desorption in separated day fractions and whole soils was well correlated to Al bearing minerals such as hydroxy-interlayered-vermiculite (HIV), gibbsite, and amorphous Al. However, P retention was negatively related to kaolinite content, which was also confirmed by isotherms conducted on pure day minerals. Based on the isotherm results, all soils were split into two groups based on the ratio of HIV/ksolinite. Soils with a HIV/kaolinite ratio >0.5 had a significantly lower concentration of DRP in runoff for a given soil water soluble P level compared with soils with a ratio <0.5. [ABSTRACT FROM AUTHOR]