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Published in Soil Sci. Soc. Am. J. 68:1744-1750 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-7—FOREST & RANGE SOILS

Rainfall Timing and Ammonia Loss from Urea in a Loblolly Pine Plantation

D. E. Kissel*, M. L. Cabrera, N. Vaio, J. R. Craig, J. A. Rema and L. A. Morris

Dep. of Crop and Soil Sci. and Agricultural and Environmental Services Lab. and School of Forest Resources, Univ. of Georgia, 2400 College Station Rd., Athens, GA 30602

* Corresponding author (dkissel{at}uga.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
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 midrotation 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.

Abbreviations: RH, relative humidity


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
STUDIES OF NH3 volatilization from urea applied to loblolly pine and slash pine (P. elliotti Engelm.) in the southeastern USA are limited, with relatively small amounts of NH3 losses being reported. Craig and Wollum (1982) found losses that ranged from a low of 4% from urea applied in the Winter to 13% when applied in the Spring. Volk (1970), Boomsma and Pritchett (1979), and Lucier (1983) found NH3 losses that ranged from 1 to 10%. Greater losses have been reported from geographical locations outside the southeastern USA. In Canada, Carrier and Bernier (1971) reported a loss of 30% of the urea when applied to jack pine (P. banksiana Lamb.) in the Summer. In Sweden, Nommik (1976) reported losses of 20% of the urea applied to Scots pine (P. sylvestris L.). Results from these studies were obtained under a limited range of possible environments, and in all cases the studies were relatively short in duration. Therefore, it is difficult to generalize from those results regarding the severity of NH3 losses under specific soil and climatic environments. Of particular interest is the effect of rain on NH3 losses from urea in the forest environment. Because the southeastern USA is a relatively humid environment with high rainfall, and because urea is highly water soluble, it might be expected to dissolve quickly when applied to soil, and to move easily into the soil with infiltrating rain (Paramasivan and Alva, 1997). In an agricultural soil, Black et al. (1987) found rainfall to be highly effective in reducing NH3 loss if applied within 3 h of the urea application, but the effect was greatly reduced if rain was applied at 48 h after urea application. It should be kept in mind, however, that the organic layer in a pine forest floor differs substantially from mineral surface soil in an agricultural field and may adsorb both urea and reaction products from urea hydrolysis. In a Canadian pine plantation, Carrier and Bernier (1971) found that rain was effective in slowing NH3 volatilization for up to 3 d after urea application, but its effect decreased with additional time.

Besides its effect of moving solutes such as fertilizer into the soil, rainfall will also increase the water content of the surface soil, and if all other factors are the same, an increase in soil water content of a dry soil surface will increase the rate of urea hydrolysis (Kissel and Cabrera, 1988; Freney et al., 1992). Faster urea hydrolysis at the soil surface will increase the rate of ammonia loss (Moe, 1967). Because rainfall can either decrease loss (by moving urea into the soil where it is adsorbed) or increase loss by increasing the rate of urea hydrolysis if urea or its hydrolysis products remain near the soil surface, rainfall timing and amount can affect the subsequent loss of ammonia in an uncertain way. With these effects in mind, the objective of this research was to determine how the amount of simulated rain and its timing affect the loss of NH3 from surface-applied urea in a loblolly pine plantation.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Field Studies
The field studies were performed in a loblolly pine plantation located at the Whitehall Forest of the University of Georgia near Athens, GA. The age of the loblolly pine stand was 24 yr, with a stem density of 830 ha–1. The height of dominant-codominant trees was 19.7 m and the stand basal area was 31 m2 ha–1. The quadratic mean diameter at breast height was 0.22 m. The soil at the study site is mapped as Cecil sandy loam (fine, kaolinitic, thermic Typic Kanhapludults), with an Oi horizon (pine needle layer) of 14300 kg ha–1 dry mass and below it, an Oe horizon (fragmented litter) and an Oa horizon (humified organic matter) that together amounted to 46900 kg ha–1 dry mass. The volatilization chambers developed and described by Cabrera et al. (2001) were used for measuring the loss of NH3 from surface-applied urea under field conditions. The chambers are designed to be open to normal environmental conditions most of the time. The lids to the chambers close for 115 s every 15 min, when air is drawn through the chamber headspace by a blower pump and the sweep air is bubbled through an NH3 trap. The flow of air through the chamber headspace is adjusted to be equal to windspeed at 1-cm height above the soil surface as measured by a needle anemometer (Bland et al., 1995; modified 229 Probe, Campbell Scientific Inc., Logan, UT). The amount of NH3 volatilized is estimated from the amount of N collected in the traps multiplied by 7.826. This multiplication factor was obtained by dividing the interval between samplings (15 min = 900 s) by the chamber sampling time (115 s). The amounts of NH3–N volatilized are then divided by the amounts of N applied to estimate the proportion of N volatilized.

The volatilization chambers were set up under a canvas roof at a height of approximately 3 m with open sides that prevented rain from falling onto the volatilization chambers, but allowed normal airflow across the fertilized volatilization chambers. Nine volatilization chambers were placed in a straight line under the canvas roof, with approximately 1 m between chambers.

The soil placed inside the volatilization chambers was obtained in the following manner. Undisturbed blocks of the forest floor with dimensions 18 by 18 by 10 cm deep (to fit the volatilization chamber dimensions) were removed with a cutting tool and then placed inside the volatilization chambers, being careful that the surface of the soil block was flush with the top of the chamber sidewalls. Although this method of obtaining undisturbed soil blocks removes active uptake by severing roots, this was not thought to affect the behavior of urea since its hydrolysis and loss processes are a surface phenomenon that appears to occur over relatively short time periods of 30 to 60 d. Measurements of NH3 volatilization were begun in each study immediately after agricultural grade granular urea was applied at a rate of 200 kg ha–1 N. Ammonia was trapped by 650 mL of 0.05 M H2SO4 as sweep air was bubbled through it (Cabrera et al., 2001). The trapping solution was changed on Days 1, 2, 3, 5, 7, 11, 16, 21, and 29 after application of urea, made to 1 L volume, mixed well, and a sample frozen for later analysis. A similar trap change schedule was followed during the second 29 d in the study that lasted 58 d. Ammonium in the trapping solution was analyzed on an OI Analytical Flow Solution 3000 (College Station, TX) using the automated phenate colorimetric procedure EPA-600/4-79-020 (USEPA, 1983). The water contents of the litter and underlying organic layer and mineral soil were measured at the beginning of each study on samples taken just adjacent to the soil block. Water contents were determined by drying the Oi, Oe, and Oa horizon samples at 65°C for 48 h and the mineral soil at 105°C for 24 h. The water contents are given in Table 1. Air temperature and relative humidity (RH) were also monitored throughout the studies with a HMP35C Temperature/RH probe connected to a CR10X datalogger (Campbell Scientific, Inc.).


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  		Table 1. Dates, duration, rainfall amount, mean air temperature, and water content of pine needles, organic layer, and soil for four field studies conducted during 2002.

 
To understand the effect of rain on loss of NH3, we built and tested rainfall simulators for applying simulated rain to the volatilization chambers. The simulators were constructed from 256 syringe needles (23 gauge by 25.4 mm) glued to an acrylic plastic manifold (30 by 30 cm). The syringes were spaced 2 cm in both horizontal dimensions to provide uniform coverage of the area. The manifold was mounted on springs at a height of 2.5 m so that the raindrops (8.4 mg each) would achieve terminal velocity and fall at spatially random locations on the plots. A Masterflex peristaltic pump and speed controller (model 7553-70; Cole-Parmer Instrument Co., Vernon Hills, IL) were used to deliver water to the manifold so that different rates of simulated rainfall could be achieved. An aluminum catch basin was placed around the volatilization chamber during rainfall simulation so that simulated rain fell only onto the fertilized 18- by 18-cm area inside the volatilization chamber and not onto the electrical components of the chamber system. The basin was constructed with 20-cm-high sidewalls to prevent any urea from splashing out of the volatilization chamber. Simulated rain was applied for 2 min, followed by 3 min without rain, with this cycle repeated until the desired amount of rain was applied. Application of 24 mm of rain required approximately 40 min using this protocol. This rate of simulated rain application was comparable with that often received in rainstorm events in the southeastern USA.

All the field studies were performed during 2002 and used three replications for each treatment, arranged in a randomized complete block design. Study RSU1 was conducted in March and April and evaluated the effects of 0, 4, and 11 mm of simulated rain. The basis for these application rates of water was that 11 mm was approximately two times the amount of water contained in the organic layer and believed to be sufficient to stop loss. The 4-mm treatment was estimated to be insufficient to eliminate all ammonia loss. Study RSU2 took place in May and evaluated the effect of 0 or 40 mm of rainfall. In these first two studies, rainfall treatments were applied after urea dissolved on the forest floor (either 4 or 5 d after urea application). Study RSU4 was performed in August and September and measured the effect of 24 mm of rain applied to (i) bare soil (forest floor removed) immediately after urea application; (ii) forest floor immediately after urea application; and (iii) forest floor after urea completely dissolved (Day 7). The final study, RSU5, took place from September through November and evaluated NH3 loss from (i) no rain, (ii) 24 mm of rain on Day 16, and (iii) 24 mm of rain on Day 30 after urea application. Ammonia volatilization measurements were performed for 29 d in all field studies except for the last one (RSU5), in which the measurements were continued for 58 d to determine if loss of NH3 continued at significant rates during the second month after urea was applied.

Laboratory Study
This study was performed to gain a better understanding of how the timing of rainfall relative to time of urea application affects leaching of urea by infiltrating rainfall and NH3 loss. Treatments were the timing of 20 mm of simulated rain: (i) within 1 h before urea application; (ii) within 1 h after urea application; (iii) 1 d after urea application, (iv) 2 d after urea application, and (v) 3 d after urea application. Controls (without urea application) were performed for each rainfall treatment, and both treatments and controls were arranged in a completely randomized design with three replications. The experimental units used were acrylic plastic cylinders, 4.5 cm i.d. and 20 cm long, in which the layers of the forest floor had been reconstructed by adding first the A horizon, (304 g oven-dry weight equivalent packed to a depth of 14.6 cm), then the Oa and Oe horizons (3 g total, dry basis), then the Oi horizon (0.5 g of old pine needles, and 0.5 g of new pine needles, dry basis) in the approximate proportions and densities found in the forest. At the time of packing, the water content was 16.4 g kg–1 for the A horizon, 1470 g kg–1 for Oa and Oe horizons considered together, 150 g kg–1 for old needles, and 111 g kg–1 for new needles in the Oi horizon. Right after packing, all cylinders received a simulated rainfall of 5 mm (7.7 mL) and were allowed to equilibrate for 24 h at 25°C before initiation of the study.

Reagent-grade, granular (average diam. of 2 mm) urea at a rate of approximately 200 kg ha–1 N was applied to all cylinders by carefully weighing the urea granules on an analytical balance, and then transferring them from the weighing paper to the surface of each cylinder. The actual weight of urea was recorded for each cylinder, and the calculations of loss based on the actual amount applied. Simulated rain was added with a device consisting of a peristaltic pump connected to plastic tubing that delivered deionized water to a manifold with 21 hypodermic needles (22 gauge by 37.5 mm), uniformly spaced on a 4.45-cm-diam. plastic disk placed on top of the treated cylinders. The needles generated 10.4-mg droplets and the manifold delivered a total flow rate of about 1.1 mL min–1. During the rain simulation, the cylinders were rotated to obtain uniform distribution of rain over the surface of each cylinder.

The cylinders were placed in a flow-through system in which air was circulated over the headspace of each cylinder at 0.1 L min–1 and then bubbled through 50 mL of 0.05 M H2SO4 to trap any volatilized NH3. The acid solution in the NH3 traps was changed on Days 1, 2, 3, 5, 7, 11, 16, 18, 20, 22, 26, and 31, and then frozen until analyzed. The incubating conditions were designed to create an environment similar to the daily 24-h wetting and drying cycle that results from normal dew in early morning and drying later in the day. This was accomplished by bubbling the sweep air through a carboy containing distilled water at a laboratory temperature of approximately 23°C which was then cooled to 15°C in the incubator during nighttime (from 2000 h until 0800 h), resulting in a RH of approximately 95%. At 0800 h, the sweep air was changed to laboratory air (not humidified) and the temperature raised to 25°C during a 3-h period, resulting in 40% RH for the daytime period (0800 h until 2000 h). At 2000 h, the sweep air was changed again to humidified air and the temperature decreased to 15°C during a 3-h period, returning to a RH of 95% for the nighttime period.

On Day 16, all treatments received 5 mm of rain to increase their water content and the rates of ammonia loss so that treatment differences would be accentuated. On Day 31, each cylinder's contents were divided into new needle, old needle, organic layer (Oe and Oa horizons), and mineral soil (A horizon). The mineral soil was sectioned into the upper (5 cm), middle (5 cm), and lower (4.6 cm) parts of the column. New needles and old needles were separately extracted with 25 mL of 1 M KCl in a 50-mL centrifuge tube by shaking for 30 min followed by filtering the extract through Whatman (Clifton, NJ) No. 1 filter paper. The organic layer was extracted in a similar manner, but with 50 mL of 1 M KCl in a 100-mL, wide-mouth bottle. Mineral soil was extracted with 500 mL of 1 M KCl in a 1-L, wide-mouth bottle.

Urea in the extracts was determined by measuring the pink color formed when urea is heated with diacetyl monoxime (2,3-butanedione-2-monoxime) and semicarbazide HCl under acidic conditions (DeManche et al., 1973). The concentration of NO2– + NO3– in extracts was determined with the Gries-Ilosvay procedure after reduction of NO3– to NO2–, and the concentration of NH4+ was determined with the salicylate-hypochlorite method (Mulvaney, 1996). Cumulative amounts of NH3 loss from each cylinder were estimated by multiplying the NH4+ concentration in the traps times the trap volume, and adding the amounts lost during the different sampling times. Cumulative amounts of NH3 loss from each treatment were estimated by subtracting cumulative amounts from controls. The amounts of inorganic N and urea present in each of the soil layers were calculated by multiplying concentrations in solution by the extracting volumes. The amounts of inorganic N and urea recovered from the different treatments were calculated by subtracting corresponding amounts from controls.

Statistical Analyses
An ANOVA was performed with cumulative NH3 losses, and Fisher's LSD was used to separate treatment means (SAS Institute, 1999).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Field Studies
Results of the first rainfall study (RSU1) are shown in Fig. 1a and Table 2. There were no differences in NH3 loss between 0, 4, and 11 mm of simulated rainfall, with an average loss of 17% of the applied urea. These results contrast with those of Black et al. (1987), who reported that a simulated 8-mm rainfall 24 h after applying urea at 100 kg ha–1 N to a pasture reduced NH3 losses by 35% (from 31% of the applied urea with no rain to 21% with rain). We interpreted the lack of a rain effect in our study to mean that the water added was not sufficient to remove much of the urea from the surface layer of the forest floor. For these reasons, we performed a second study (RSU2) in which 0 or 40 mm of simulated rain was applied. Results showed that cumulative losses by 29 d after urea application were not significantly different between treatments (Table 2), although there was a trend for a larger loss with 40 mm (Fig. 1b). In this study we did not measure downward movement of urea, but we concluded from the high losses that the rainfall was not effective in moving the urea downward. If rainfall had moved urea into the soil, NH3 from urea hydrolysis would have been adsorbed by the soil, preventing its loss. Apparently, the urea was either adsorbed by the O horizon or physically protected in some way against downward movement. A possible mechanism of retention by the O horizon may be abiotic retention of NH4+, as reported by Axelsson and Berg (1988) for Scots pine litter. Abiotic immobilization of NH4+ has been reported for a variety of forest soils (Johnson et al., 2000; Fitzhugh et al., 2003) and grassland soils (Barrett et al., 2002). In our study, losses tended to be greater on the treatment receiving 40 mm of simulated rainfall, probably because of the greater water content and faster urea hydrolysis from the water application. Ammonia losses were greater than in the first study, probably because the average air temperature was higher (18.9 vs. 14.9°C, Table 1) and because the initial water content of the pine needles was greater than in the first study (0.832 vs. 0.325 g g–1, Table 1). In other unpublished data (Kissel, Cabrera, and Craig, 1999–2002) we found water content of the pine forest organic layer to be equally important to air temperature in affecting the loss of NH3, with greater losses at greater water contents.



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  		Fig. 1. (a) Cumulative loss of NH3 during the period of 14 Mar. to 12 Apr. 2002 following application of 200 kg ha–1 urea-N to midrotation pines as affected by application of 0, 4, and 11 mm of simulated rain on Day 4 following urea application. (b) Cumulative loss of NH3 during the period of 2 to 31 May 2002 following application of 200 kg ha–1 urea-N to midrotation pines as affected by application of 0 and 40 mm of simulated rain on Day 5 following urea application. (c) Cumulative loss of NH3 during the period of 9 Aug. to 7 Sept. 2002 following application of 200 kg ha–1 urea-N to midrotation pines as affected by application of 24 mm of simulated rain to either a bare soil or a normal forest floor the same day as the urea is applied or on Day 7 to a normal forest floor following urea application (bars are standard errors).

 

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  		Table 2. Ammonia losses in four field studies conducted during 2002.

 
An unanswered question from the first two field trials was whether a rain received immediately after urea application would have a different effect from a rain received 4 to 5 d after urea application, which was the protocol followed in the previous studies. The results of the third trial (RSU4), designed to answer that question, are shown in Fig. 1c and Table 2. Ammonia losses during this study were relatively low because of the dry soil and environmental conditions (Table 1). Both the pine needles and the organic layer were drier at the start of this study than at the beginning of the other two studies. What is clearly shown in Fig. 1c is that the application of simulated rain on Day 7 did not stop NH3 loss, but instead increased it (Table 2), apparently because the rain increased the soil water content without moving all urea to deeper soil layers. In contrast, when rain was applied immediately after the urea (to bare soil or forest floor), NH3 losses were reduced to <1% of the applied urea by 29 d after application. These results are in agreement with those of Malhi et al. (1996), who found that simulated rainfall immediately after a surface application of urea to zero-tillage barley increased the recovery of urea-N by plants. Apparently, when rain dissolves the fertilizer granules, the urea is not retained by the forest floor or by the no-till residue and is moved downward with the infiltrating water, thereby greatly reducing NH3 loss and increasing urea N recovery. On the other hand, when rain does not occur right after urea application to a pine forest floor, the dissolution of urea by morning dew and subsequent reprecipitation or adsorption may be protecting the urea against further downward movement, although it is not clear how this protection occurs.

The results from the final field study (RSU5) are shown in Fig. 2 and Table 2. Rates of NH3 loss were large throughout the study, mainly because it was relatively warm and rainy during this time, with 181 mm of rain received (Table 1). Although the plots were sheltered from direct rainfall, a moist and humid environment caused by the rain apparently enhanced loss. Losses were relatively uniform from all plots until Day 16, when 24 mm rain was applied to one of the treatments, at which time its rate of loss tended to increase slightly. At Day 30, 24 mm of rain was applied to another treatment, which also tended to increase the rate of NH3 loss during the final 20 d of the study. By the end of the study, however, there were no significant differences between treatments, with losses ranging from 49 to 58% (Table 2). Results from this study indicated that rain did not reduce NH3 losses and that NH3 losses can be sustained for up to 58 d after urea application when environmental conditions are favorable. These results contrast with those of Mugasha and Pluth (1995), who found that a 40-mm rain occurring 9 d after urea application (200 kg ha–1 N) reduced NH3 losses to background levels in forested Canadian peatlands [tamarack, Larix laricina (Du Roi) K. Koch; and black spruce, Picea Mariana (Mill.) Britton et al.]. Thus, it would appear that some of the components of the pine forest floor may be involved in adsorbing urea and reducing its susceptibility to leaching by rainfall.



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  		Fig. 2. Cumulative loss of NH3 during the period of 16 Sept. to 14 Nov. 2002 following application of 200 kg ha–1 urea-N to midrotation pines as affected by application of 24 mm of simulated rain on Days 16 or 30 of the study (bars are standard errors).

 
The losses of NH3 from the four field studies varied widely (Table 2), and appeared to be influenced by environmental conditions (water content and temperature), although the effect of these two factors need further study. Losses were at their lowest in Study RSU4, which had the highest average air temperature at 24.6°C but the lowest water content in the forest floor (Table 1). Losses were at their highest when temperatures were moderate in Studies RSU2 and RSU5, 18.9 and 17.1°C, respectively, but conditions were relatively moist, from relatively wet initial conditions of the soil and/or rainy weather conditions.

Results from the laboratory study are shown in Fig. 3 and 4 and Table 3. Cumulative loss of NH3 by Day 31 was highest (14% of the applied N) from the treatment that received 20 mm of simulated rain within 1 h before application of urea. Losses were probably at the highest from this treatment for two reasons: (i) the simulated rain increased soil water content, thereby creating near-optimum water content for rapid urea hydrolysis (Kissel and Cabrera, 1988), and (ii) the applied urea would have remained at or near the soil surface, resulting in both high ammoniacal N concentrations and high surface pH, both of which are conducive to loss of NH3 (Ferguson et al., 1984). The smallest loss of 0.1% of the applied N was from the treatment that received 20 mm of simulated rain within 1 h after urea application. Apparently, the loss for this treatment was small because the simulated rain dissolved the urea and moved it more deeply into the soil where its reaction products were adsorbed by the soil (Fig. 4). These results agree with those of Cabrera and Vervoort (1998), who found that a simulated 40-mm rainfall immediately after a surface application of broiler litter reduced NH3 losses by 49% when compared with the same amount of simulated rainfall applied immediately before litter application. Broiler litter contains urea as well as uric acid, which decomposes to urea (Eiteman et al., 1994). Treatments that received 20 mm of rain 1, 2, or 3 d after urea application lost between 6.7 and 8.6% of the applied N, intermediate between the other two treatments. Under the conditions of this study, delaying the rain by 1 d was as effective as delaying it by 3 d in immobilizing part of the urea-N against further leaching by rain. Because losses were intermediate for these three treatments, the rainfall applied on Days 1, 2, and 3 apparently leached part of the urea to deeper layers, but some urea-N remained at or near the surface by a mechanism that may be abiotic retention (Johnson et al., 2000). Recovery of N derived from urea was complete when rain was applied within 1 h after urea application (Table 3). In contrast, recovery was only 73 to 79% when rain was applied either before urea application or 1 to 3 d after urea application. Thus, it would seem that in these latter cases, the urea was retained by a layer of the forest floor (needles or organic layer) in a way that was not fully extractable by KCl.



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  		Fig. 3. Cumulative loss of NH3 during a 31-d laboratory study from 200 kg ha–1 urea N, in which 20 mm of simulated rain was applied to a forest floor (packed into acrylic plastic cylinders) at different times with respect to the urea application. Diurnal cycles of temperature (15 to 25°C) and relative humidity (40 to 95%) mimicked the natural cycle (bars are standard errors).

 


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  		Fig. 4. Urea-derived N found in O and A horizons at the end of a 31-d laboratory study in which 20 mm of simulated rain was applied to a forest floor (packed into acrylic plastic cylinders) at different times with respect to a urea application of 200 kg ha–1 urea N (bars are standard errors).

 

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  		Table 3. Ammonia losses and N recovered in a laboratory study in which simulated rain was applied to forest floor (packed into acrylic cylinders) at different times with respect to urea application. Samples were kept for 31 d in an incubator with diurnal cycles of temperature (15 to 25°C) and relative humidity (40 to 95%).

 

   SUMMARY AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
The results from four field trials and one laboratory study consistently indicate that when urea is applied to a typical southern pine forest floor and is allowed to slowly dissolve from morning dew, simulated rain is apparently ineffective in completely leaching the urea into the soil, based on the high losses of NH3 that continued following the simulated rain. For example, up to 40 mm of simulated rain did not stop NH3 loss in the field studies, but tended to increase NH3 loss, probably because of increased water content of the soil from adding water. Increased soil water content leads to faster rates of urea hydrolysis, which in turn leads to higher surface soil pH and greater rates of NH3 volatilization. As noted in the results from the field studies, it appears that urea can be applied during dry weather with relatively low losses, even when temperatures are high. This was clearly the case for the study performed during August and September when conditions were hot and dry and losses were only 5% of the applied urea after 29 d. However, because rains that occur after urea is dissolved by dew are ineffective in leaching urea into the soil, rain received following a period of dry weather such as this one can increase loss of NH3 from the applied urea. The increased loss is because of increased soil water content, which increases the rates of urea hydrolysis and NH3 loss.

The mechanism by which the urea is absorbed and protected against downward leaching was not determined from this study. It will be helpful to determine the exact mechanism by which this occurs, so that fertilizer can be managed more effectively to reduce these losses. The importance of these processes and their great variation illustrates this point.

The total loss of NH3 by the end of each of the field studies ranged from a low of around 1% of the applied urea when rain was applied the same day as the urea, to a high of 58% of the applied urea from the 58-d study performed from mid-September to mid-November. Loss from the soil surface does not mean that the NH3 is totally lost from the forest system, because absorption by the foliage is probably an important mechanism of N uptake by pine trees. While the degree of NH3 retention by the forest floor is still poorly understood, any means to reduce NH3 volatilization from the soil surface is likely to improve N fertilizer use efficiency. If urea is adsorbed or retained by pine needles, their removal by raking or by burning before the application of urea may improve the efficiency by which urea fertilizer is used by southern pines. Further research should be conducted to elucidate the mechanism of urea retention by the O horizon.

Received for publication November 24, 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 




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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome