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Published online 4 August 2005
Published in Soil Sci Soc Am J 69:1525-1531 (2005)
DOI: 10.2136/sssaj2004.0246
© 2005 Soil Science Society of America
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Nutrient Management & Soil & Plant Analysis

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

Miguel L. Cabreraa,*, David E. Kissela, N. Vaioa, J. R. Craiga, J. A. Remaa and L. A. Morrisb

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)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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 (Oe) 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


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE PROCESS OF NH3 volatilization may lead to significant losses of N when urea is surface applied to crops (Nastri et al., 2000), pastures (Prasertsak et al., 2001), and forests (Kissel et al., 2004). These losses occur because the hydrolysis of urea generates NH3 (Koelliker and Kissel, 1988), which is subject to volatilization if not retained by soil or crop residues. Urea hydrolysis is performed by urease, an enzyme produced by microorganisms and present in soil in intracellular and extracellular forms (Klose and Tabatabai, 2000). Among the factors that affect urease activity are urea concentration, soil pH, soil temperature, and soil water content (Kissel and Cabrera, 1988). Ammonia volatilization is mainly affected by H+–buffering capacity (Ferguson et al., 1984), NH3 concentration, pH, temperature, water content, and wind speed (Sherlock and Goh, 1985; Ni, 1999). Because urea is very soluble in water (1.19 kg L–1 at 25°C; Gamsjäger, 1995), rain received soon after urea application would be expected to decrease NH3 losses by solubilizing urea and its hydrolysis products and transporting them into the soil. Black et al. (1987) found that 8 mm of rain received within 3 h after surface application of urea to a pasture reduced NH3 losses by 94%. Mugasha and Pluth (1995), working in a forested peatland, found that 40 mm of rain received 9 d after urea application reduced NH3 losses to background levels. In recent work, Kissel et al. (2004) reported that 20 mm of simulated rain applied 1 h before urea application to a pine forest floor resulted in a loss of 14% of the applied N, whereas 20 mm of rain applied 1 h after urea application decreased NH3 losses to 0.1% of the applied N. In the same work, 20 mm of simulated rain 1 to 3 d after urea application failed to eliminate NH3 losses (7% of applied N), apparently because part of the urea was retained by the forest floor. To further study this retention of urea by the forest floor, we evaluated the percentage of urea leached from the forest floor by simulated rain following different numbers of diurnal humidity cycles. A secondary objective was to evaluate the amount of NH3 loss from urea that was not leached from the forest floor by simulated rain.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sample Collection
Forest floor samples were collected from a loblolly pine forest located within the Whitehall Forest of the University of Georgia, near Athens, GA. The pine stand was 24 yr old, with a density of 830 stem ha–1. The height of dominant-codominant trees was 19.7 m and the stand basal area was 31 m2 ha–1. The soil at the sampled site is mapped as Cecil sandy loam (fine, kaolinitic, thermic Typic Kanhapludults), with an Oi horizon (pine needle layer) of 14300 kg dry mass ha–1, and below it, an Oe horizon (fragmented layer) and an Oa horizon (humified organic matter) that combined amounted to 46900 kg dry mass ha–1. In March 2003, we collected samples from three forest floor fractions: (i) recently dropped pine needles (called new pine needles), (ii) pine needles dropped in previous years (called old pine needles), and (iii) Oe horizon. All samples were stored in sealed plastic bags in a refrigerator at 4°C until used (<30 d).

Sample Analysis.
A subsample of each of the forest fractions was dried (65°C for 48 h), ground through a 100-µm screen, and analyzed for total N and C by dry combustion (Nelson and Sommers, 1982). Total C concentration was 561.5 g kg–1in new needles, 554.1 g kg–1 in old needles, and 477.0 g kg–1in the Oe horizon. Total N concentration was 5.0 g kg–1 in new needles, 8.9 g kg–1 in old needles, and 13.2 g kg–1 in the Oe horizon. The water content of the forest fractions (65°C for 48 h) at collection time was 0.081 g g–1 dry matter in new needles, 0.149 g g–1 dry matter in old needles, and 1.659 g g–1 dry matter in Oe horizon.

The water potential of the forest fractions at different water contents was measured in triplicate samples by thermocouple psychrometry (Rawlins and Campbell, 1986). For that purpose, 5 g of each fraction was placed in an aluminum cup, water was added to saturate the samples, and then water evaporation was allowed under laboratory conditions until free water could no longer be observed on the samples. A subsample was then placed in a glass scintillation vial, which was capped to allow for further moisture equilibration. The aluminum cup with the remaining forest fraction was placed in an oven at 65°C and a subsample was removed periodically and placed in a separate scintillation vial for further moisture equilibration. This process generated samples with different water contents, which were subsequently analyzed for water potential by transferring a subsample to the chamber of a thermocouple psychrometer (model 85-2vc, J.R.D., Merril, Logan, UT) and allowing it to equilibrate in a water bath at 25°C for 4 h. Readings were taken with a psychrometer meter (Model 85, J.R.D. Merril) and water potentials were obtained from a calibration curve developed with NaCl standards (0,–2.5,–5.0,–7.5 MPa). Curves of water content versus water potential were used to estimate the water potential of the forest fractions at their original water contents.

Urea Leaching Studies—Laboratory
New and old pine needles were cut in segments 1 to 1.5 cm long. Subsamples (1 g dry-weight equivalent for pine needles, 1.5 g dry-weight equivalent for Oe horizon) of the different forest floor fractions at their original water contents were weighed into the bottom tray of small Petri dishes (3.85 cm ID, 1 cm deep), which were placed into acrylic plastic cylinders (4.45 cm ID, 120 cm long, 120 cm3 of headspace). The cylinders were closed with No. 10 rubber stoppers on both ends. The rubber stopper at the upper end had inlet and outlet tubing through which humidified air (95% RH) was circulated at 0.1 L min–1. The system was set up inside a Precision Model 815 incubator (Precision Scientific, Winchester, VA) at 25°C, and humidified air was circulated for 24 h to allow for moisture equilibration. Subsequently, 50 mg of reagent-grade, granular urea (2 mm OD) was added to each sample (200 kg N ha–1), and all samples were returned to the incubator, where a diurnal cycle of temperature and RH was implemented in the flow-through system (Fig. 1). This diurnal cycle was accomplished by bubbling the sweep air through a carboy containing distilled water at a laboratory temperature of approximately 23°C. The sweep air then entered the incubator, which was set at 15°C during nighttime (1900 until 0700 h) to obtain approximately 95% RH. At 0700 h, the sweep air was changed to laboratory air (no longer bubbled through water) and the temperature of the incubator was raised to 25°C within a 3-h period to obtain a minimum RH of approximately 50% during daytime (0700 until 1900). At 1900 h, the sweep air was bubbled again through water at 23°C and the temperature of the incubator was decreased to 15°C within a 3-h period to return to 95% RH for the nighttime. The main purpose of establishing a diurnal humidity cycle was to have a daily period with RH above the critical relative humidity (CRH) for urea followed by a daily period with RH below the CRH for urea, as is commonly observed under field conditions. The CRH for urea, which is the humidity of the atmosphere above which urea will spontaneously absorb moisture, decreases from 85 to 75% as the air temperature increases from 0 to 30°C (National Fertilizer Development Center, 1979). To visually confirm the dissolution of urea, we placed 50 mg of granular urea in a beaker inside an extra plastic cylinder in the flow-through system. Temperature and RH of the sweep air was measured inside the incubator with an in-line Vaisala HMP45C sensor (Vaisala Inc., Woburn, MA) connected to a Campbell Scientific CR10X datalogger (Campbell Scientific Inc., Logan, UT). The sweep air circulated over each sample was subsequently bubbled through 50 mL of 0.05 M H2SO4 to trap any NH3 volatilized. Treatments consisted of leaching the samples with simulated rain (180 mm in 20-mm increments) at 0, 1, 2, 4, and 8 d after urea application. Control samples without urea addition were also included. All treatments were arranged in a completely randomized design with four replications.



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  		Fig. 1. Temperature and relative humidity of the sweep air circulating over samples in an incubator (bars are standard errors; CRH = critical relative humidity for urea).

 
To leach the samples with simulated rain, the material in each Petri dish was transferred to a 5-cm ID Buchner funnel that had a Whatman No. 41 filter paper at the bottom. The funnel was placed over a sidearm flask to collect the simulated rain that percolated through the forest fraction. Rain was simulated 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 long) uniformly spaced on a 4.45-cm ID, plastic disk. The needles generated 10.4-mg droplets and the manifold delivered a total flow rate of about 1.1 mL min–1. The manifold was placed 10 cm above the Buchner funnel, which was rotated during the rain simulation to obtain uniform distribution of water over the forest floor fraction. Simulated rain was applied in nine 39.3 mL (20-mm) increments and leachate was collected separately for each increment to be analyzed for urea content by colorimetry (DeManche et al., 1973) After the simulated rainfall, the leached forest fraction was freeze dried, ground through a 100-µm sieve, extracted with water (0.1 g material: 200 mL water) for 30 min, and the extract was analyzed for urea content by colorimetry. The amount of urea leached by rainfall was added to the amount of urea extracted by water from the ground samples to calculate the total amount of recovered urea. Because of space limitations, the rainfall treatments were applied at different times to each of the forest fractions (new needles, old needles, Oe horizon), resulting in three separate studies.

Because one of the main differences in the composition of new and old pine needles is the concentration of terpenes (Kainulainen and Holopainen, 2002), we conducted a study to determine whether terpenes had an effect on the amount of urea retained. For that purpose, samples (1 g dry-weight equivalent) of new needles were extracted with 25 mL of hexanol for 60 min to remove terpenes, after which the hexanol was allowed to volatilize and the samples were treated with urea as described before. Simulated rainfall (180 mm) was applied after zero, one, and two humidity cycles, as described before. The results were compared with those obtained with samples that had been treated similarly but had not been previously extracted with hexanol. Control samples (no urea added) with and without hexanol extraction were also included. The treatments were arranged in a completely randomized design with four replications.

Urea Leaching Study—Field
To determine if the effect of diurnal humidity cycles generated in the laboratory was similar to that of humidity cycles in the field, samples (1 g oven-dry equivalent) of new needles were weighed into the bottom tray of Petri dishes (3.8 cm ID; 1 cm deep) and then reagent-grade, granular urea was applied at a rate equivalent to 200 kg N ha–1 on an area basis. Control samples without urea were also prepared. The samples were taken to the Whitehall Forest on 26 Mar. 2003, where they were left on the forest floor for 0 or 5 d, after which, they were retrieved, returned to the laboratory, and leached with three-consecutive 20-mm simulated rains (60 mm total), as described above. Treatments were arranged in a completely randomized design with four replications.

Temperature and RH sensors located at the study site malfunctioned during the study. Therefore, temperature and RH from a weather station located at approximately 2 km from the study site were used to calculate temperature and RH at the study site. For that purpose, 5-min data (n = 1437) collected at the study site during a different 5-d period with similar ranges of temperature and RH were regressed against data collected by the weather station. The regression equation for temperature was T = –2.21 + 1.14 TWS (r2 = 0.97, p < 0.0001), where T is temperature at the study site (°C) and TWS is temperature at the weather station. The regression equation for RH was RH = 5.05 + 0.96 RHWS (r2 = 0.96, p < 0.0001), where RH is relative humidity at the study site (%) and RHWS is relative humidity at the weather station.

Ammonia Volatilization Study
A laboratory study was conducted to determine whether NH3 losses could occur after forest floor fractions that had received urea were leached with simulated rainfall. Samples of field-moist new needles (1.4 g dry-weight equivalent; 0.075 g H2O g–1 dry matter) or Oe horizon (1.3 g dry-weight equivalent; 1.659 g H2O g–1 dry matter) were weighed into the bottom tray of small (3.85 cm ID, 1 cm deep) Petri dishes and placed in the flow-through system described above. Air was circulated over each sample at 0.1 L min–1, and a diurnal cycle of temperature and RH was implemented as described before (Fig. 1). After 24 h, reagent-grade, granular urea was applied to each Petri dish at a rate of 200 kg N ha–1, and the dishes were returned to the incubator for five diurnal humidity cycles (5 d). Air circulating over each sample was bubbled through 50-mL of 0.05 M H2SO4 to trap any NH3 volatilized. Subsequently, the material in each Petri dish was transferred to a Buchner funnel to be leached with nine 20-mm increments of simulated rain, as described before. After leaching, the materials were returned to Petri dishes, which were placed in the flow-through system at 25°C and 95% RH for 15 d. Air circulated over each sample was bubbled through 50 mL of 0.05 M H2SO4 to trap any NH3 volatilized. The traps were changed on Days 1, 2, 4, 6, 9, and 15 after the simulated rainfall. Control samples without added urea were also included, and all treatments were arranged in a completely randomized design with four replications.

After 15 d, the samples were freeze-dried, ground through a 100-µm sieve, and separate samples were extracted with water (0.1 g: 200 mL; 30 min) for urea determination, or with 1 M KCl (0.1 g: 200 mL; 30 min) for inorganic N determination. The concentration of NO–2 + NO–3 in extracts was determined with the Gries-Ilosvay procedure after reduction of NO–3 to NO–2. The concentration of NH+4 was determined with the salycilate-hypochlorite method (Mulvaney, 1996). Ground samples were also analyzed for total N by dry combustion (Nelson and Sommers, 1982).

The amount of total N measured in control samples was subtracted from the amount of total N measured in treated samples to determine the amount of urea-derived N present in ground samples. Similarly, the amount of inorganic N measured in control samples was subtracted from the amount of inorganic N measured in treated samples to determine the amount of urea-derived inorganic N. Because control ground samples did not have measurable amounts of urea, all urea measured in ground treated samples was assumed to come from the applied urea. The amount of urea-derived inorganic N and the amount of urea extracted by water from ground treated samples were subtracted from the total amount of urea-derived N found in ground samples to calculate the amount of organic N derived from urea that was remaining in the samples after incubation.

Statistical Analysis
The amounts of urea leached by simulated rainfall, urea extracted by water from ground samples, and total urea recovered were subjected to an analysis of variance (SAS Institute, 1999) and the means were tested with Fisher's LSD at a 0.05 probability level. A similar analysis was performed for the amounts of NH3 lost, and for the amounts of inorganic N and organic N derived from urea. A nonlinear regression procedure in SAS was used to determine the relationship between the percentage of urea leached from new needles by simulated rainfall and the number of humidity cycles and cumulative rain applied.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
We measured negligible amounts of NH3 loss (<0.1% of applied N) during exposure of the samples to the different numbers of diurnal humidity cycles in the flow-through system. These results suggest that negligible amounts of urea were hydrolyzed during that period, in agreement with the observed average urea recovery of 100.5% for treatments shown in Table 1. The lack of urea hydrolysis was probably caused by the relatively low water content of the forest fractions and by the overall drying conditions that existed during exposure to the diurnal humidity cycles. From the curves of water content versus water potential, we estimated water potentials to be less than –6 MPa for new needles, less than –2.5 MPa for old needles, and less than –0.6 MPa for the Oe horizon. Although specific information for forest floor fractions is not available, urea hydrolysis is known to decrease as water availability decreases (Kissel and Cabrera, 1988). In addition, when urea granules dissolved on the forest fractions, concentrations near the point of dissolution may have been near saturation (15 M at 15°C and 20 M at 25°C), which could have inhibited urea hydrolysis. Lal et al. (1993) found that urea concentrations > 4 M inhibited urease activity in wheat residue, probably because of denaturation of the urease enzyme. This denaturation action of urea on proteins has been previously documented (Nozaki and Tanford, 1963).


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  		Table 1. Urea leached by simulated rainfall (180 mm in 20-mm increments) and subsequent urea extracted by water from ground samples of new needles, old needles, and Oe horizon exposed to different numbers of simulated diurnal humidity cycles in an incubator.

 
Urea Leaching Studies—New Needles
In this and all other leaching studies, control samples (no urea added) did not contain detectable amounts of urea. In new needles treated with urea, the percentage of urea leached by a given amount of cumulative rain depended on the number of humidity cycles to which the sample had been exposed (Fig. 2a). When simulated rain was applied to new needles immediately after urea application (0 humidity cycles), the first 20 mm leached 95% of the urea and the total amount of simulated rain (180 mm) leached 98% of the urea (Fig. 2a and Table 1). In contrast, when rain was applied after one humidity cycle, the first 20 mm of rain leached only 25% of the urea and the total amount of applied rain (180 mm) leached only 67% of the urea. Slightly smaller amounts of urea were leached after 2, 4, and 8 d of humidity cycles. The percentage of urea leached (% Lch) as a function of the number of humidity cycles (d) and the amount of cumulative rain (mm) could be adequately described (r2 = 0.99; p < 0.0001) by the following equation, which may be useful as a predictive tool under similar conditions:



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  		Fig. 2. Effect of amount of simulated rainfall and number of diurnal humidity cycles on the percentage of urea leached from (a) new needles, (b) old needles, and (c) Oe horizon (bars are standard deviations).

 
Results of grinding and extracting the leached needles with water showed that the urea that was not leached by simulated rainfall was extractable by water (Table 1). The results also showed that the percentage of urea retained by the needles increased from 8 to 56% when the humidity cycles increased from zero to four, and remained constant between four and eight humidity cycles. Thus, up to four humidity cycles were required to reach maximum urea retention, which indicates that the retention process is not instantaneous, but instead requires time. The observation that the retention process required time and that urea not leached by simulated rainfall was extractable by water suggests that retention may have been caused by urea diffusing into the needles. This diffusion may have occurred across the cuticle or through the cut ends of the needles. Diffusion of urea across the cuticle of citrus leaves has been shown to increase as temperature increases from 19 to 28°C, and as RH increases at 28°C (Achor et al., 2001). The cut ends of the needles may also have provided points of entry for urea. However, because urea retention by the forest floor was also observed by Kissel et al. (2004) working with intact needles under field conditions, it is likely that diffusion may primarily occur across the cuticle.

Diffusion into the needles as the mechanism of retention is further supported by results from hexanol treatment. There were no differences in the percentages of urea leached and retained by the needles between original samples and those that had been previously extracted with hexanol (Table 2). These results suggest that terpenes present on the surface of new needles (Kainulainen and Holopainen, 2002) were not involved in the observed retention of urea. Thus, slow diffusion of urea into the needles, rather than surface sorption may explain urea retention.


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  		Table 2. Mean effect of extraction with hexane on urea leached from new needles by simulated rainfall (180 mm in 20-mm increments) and on subsequent urea extracted by water from ground samples after exposure to 0, 1, or 2 simulated diurnal humidity cycles in an incubator.

 
Urea Leaching Studies—Old Needles
When simulated rain was applied immediately after urea application to old needles, the first 20 mm leached 96% of the urea and the total amount of applied rain (180 mm) leached all of the urea (Fig. 2b). These results are similar to those obtained with new needles (Fig. 2a). Also, when rain was applied to old needles after one humidity cycle, the first 20 mm of rain leached 80% of the urea and the total amount of applied rain (180 mm) leached all of the applied urea. Although smaller amounts were leached by the initial rain increments after two, four, and eight humidity cycles, there were no differences in the total amounts of urea leached by 180 mm of rain (Fig. 2b). Consequently, very little extractable urea was left in the old needles after 180 mm of simulated rainfall (Table 1). These results indicate that the effect of diurnal humidity cycles on the amount of leachable urea was not as marked in old needles as it was in new needles. The reason for this may be that as pine needles decompose, their anatomical structure becomes more open, making it easier for water to penetrate and leach soluble compounds.

Urea Leaching Studies—Oe horizon
As with the other two residues, the first 20 mm of simulated rainfall leached most of the urea from the organic fraction when the rain occurred immediately after urea application. Smaller percentages of urea were leached when the rain was applied after one or more humidity cycles, but as in the case of old needles, 180 mm of rain leached most of the urea (Fig. 2c, Table 1) from the Oe horizon. The smallest percentages of urea leached were observed after two humidity cycles, but the reason for this was not clear

Urea Leaching—Field
Needles treated with urea and left on the forest floor for 5 d were exposed to four diurnal humidity cycles in which RH increased above the CRH for urea (Fig. 3). When these needles were subsequently leached with 60 mm of simulated rainfall (in 20-mm increments), the percentage of urea leached was 23% and the percentage of urea extracted from ground needles was 72% (Table 3). The percentage of urea leached was similar to that observed when new needles were leached with 60 mm of rain after four simulated humidity cycles in the laboratory (30%, Fig. 2a). These results indicate that the retention of urea observed in the laboratory also occurs under field conditions.



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  		Fig. 3. Air temperature and relative humidity during exposure of new pine needles to field conditions (CRH = critical relative humidity for urea).

 

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  		Table 3. Urea leached from new needles by simulated rainfall (60 mm in 20-mm increments) and subsequent urea extracted by water from ground samples after exposure to 0 and 5 d of field conditions.

 
To summarize, results of these leaching studies suggest that rainfall does not eliminate NH3 volatilization losses when the rain occurs after one or more humidity cycles because the forest floor, and in particular new needles, retain part of the applied urea in a form that is not leachable by water. The retained urea appears to be inside the needles because it is extractable by water when the needles are ground.

Ammonia Volatilization Study
In this study, new needles and Oe horizon were exposed to five humidity cycles, after which 180 mm of simulated rainfall was applied. The percentage of urea leached by the simulated rainfall was larger for the Oe horizon (93%) than for new needles (56%), in agreement with studies described above (Table 4, Fig. 4a). As a result, only 0.3% of the urea applied to the Oe horizon was lost through NH3 volatilization when the leached samples were subsequently incubated at 25°C and 95% RH for 15 d (Table 4, Fig. 4b). In contrast, 7.9% of the urea applied to new needles was lost through NH3 volatilization when the samples were incubated after the simulated rain (Fig. 4b). These results indicate that urea remaining inside the needles can be hydrolyzed and lost as NH3. The presence of urease activity in pine tissues has been previously documented (Johansson et al., 2002) and would explain the hydrolysis of urea inside pine needles. The reason why there was urea hydrolysis after the simulated rain but not during the diurnal humidity cycles is that after the simulated rain the new needles had a greater water content (0.983 g H2O g–1 dry matter) than during exposure to the humidity cycles (0.075 g H2O g–1 dry matter).


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  		Table 4. Urea leached by simulated rainfall (180 mm in 20-mm increments) after five diurnal humidity cycles, NH3 volatilized during a subsequent 15-d incubation (25°C and 95% relative humidity), urea and NH4 extracted by water from ground samples at the end of the study, and urea-derived organic N remaining in ground samples of Oe horizon and new needles.

 


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  		Fig. 4. (a) Effect of cumulative, simulated rainfall on the percentage of urea leached from new pine needles and Oe horizon. (b) Ammonia loss from new pine needles and Oe horizon after being leached by 180 mm of simulated rainfall (bars are standard deviations).

 
Analysis of the new needles after the volatilization study was concluded showed that of the total urea applied, 1.3% remained as urea, 11.3% as NH4–N, and 8.6% as organic N. There was no (nitrite + nitrate)-N derived from the applied urea. Thus, apparently the NH4–N released through urea hydrolysis was partly assimilated by microorganisms decomposing the needles. The total amount of urea-N recovered was only 85% for new needles, probably because of losses of NH3–N during freeze-drying of the samples. Wood and Hall (1991) have previously shown that freeze-drying can lead to losses of N as NH3 from manures.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
A 20-mm simulated rainfall occurring immediately after urea application (0 humidity cycles) leached more than 95% of the applied urea from the three forest fractions. In contrast, a 20-mm, simulated rainfall occurring after one diurnal humidity cycle leached 25% of the applied urea from new pine needles, 80% from old pine needles, and 72% from the Oe horizon. Regardless of the number of humidity cycles, a total cumulative rainfall of 180 mm leached 83 to 100% of the urea from old pine needles and Oe horizon, but only a maximum of 67% of the urea from new pine needles. These results show that retention of urea against leaching by simulated rainfall is stronger for new pine needles than for the other two fractions. The urea not leached by simulated rainfall was extractable by water from the ground samples, which suggests that retained urea was inside the needles. We hypothesized that as urea dissolved, it diffused into the new needles. When new needles treated with urea were first exposed to five diurnal humidity cycles, then leached with 180 mm of simulated rainfall, and finally incubated at 25°C and 95% RH for 15 d, NH3 losses amounted to 7.9% of the applied urea. These results indicate that urea retained against leaching by rainfall can be hydrolyzed and lost as NH3 under favorable conditions.

Our findings suggest that when recently dropped pine needles are removed from the forest floor, as done by those who sell pine needles as mulch, rainfall may better leach applied urea into the soil, thereby decreasing NH3 losses. Our results also suggest that an important management consideration may be the use of large granule sizes that would cause urea to fall through new needles to old needles, where retention against leaching by rainfall would not be as great.

Received for publication July 18, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 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