Reducing ammonia loss from urea by mixing with humic and fulvic acids isolated from coal

isolated from coal. Abstract: Problem statement: Ammonia volatilization is a major pathway for nitrogen loss from surface applied urea. While all top-dressed ammonia and ammonium based N fertilizers can volatilize, the potential loss is greatest with urea and fluids containing urea. As much as 20-50% of N applied to soils is lost through volatilization alone. Thus, the objective of this laboratory study was to reduce ammonia loss from urea via mixing with humic and fulvic acids isolated from coal. Approach: This study compared four different types of treatments which were urea without additives (T1), urea with humic acid-powdered form (T2), urea with fulvic acid-liquid form (T3) and urea with humic and fulvic acids-liquid form (T4). Comparisons were made based on ammonia loss, soil [NH.sub.4] and [NO.sub.3.sup.-] contents as well as exchangeable cations in the treated soils. Soil samples from typic paleudults (Bekenu series) were used. Humic substances were isolated using standard procedures. Daily ammonia loss from soil was measured using a modified closed-dynamic air flow system method. Results: All of the treatments with humic substances significantly reduced ammonia loss ranging between 13 and 25% compared to urea alone. The treatment with both humic and fulvic acids (T4) showed pronounced ammonia loss reduction. All treatments with humic substances significantly increased [NH.sub.4.sup.+] and [NO.sub.3.sup.-] content in soil samples compared to urea alone except for treatment having humic acid alone (T2). Treatments with fulvic acid (T3 and T4) also showed significant increase in exchangeable [K.sup.+] and [Na.sup.+] compared to urea alone. The increase in the formation of [NH.sub.4.sup.+] over [NH.sub.3], soil exchangeable cations and temporary reduction of soil pH may had retarded urea hydrolysis in the immediate vicinity of the fertilizer. Conclusion: Surface applied urea fertilizer efficiency could be increased if applied together with humic and fulvic acids.


INTRODUCTION
The use of urea accounts for approximately 51% of the world's agricultural N consumption (1) Widespread acceptance of urea was delayed in part due to its greater potential for N loss via ammonia volatilization. While all topdressed ammonia and ammonium-based N fertilizers can volatilize, the potential is greatest with urea and fluids containing urea with as much as 20-50% of N applied to soil lost through volatilization alone. According to Bundy (2), ammonia loss from urea-fertilized soils usually occurs when urea-containing fertilizer is surface-applied and not incorporated. The amount of nitrogen lost through ammonia volatilization from surface applied urea is greatly affected by soil and climatic conditions. Thus, understanding the factors that influence volatilization will enable urea users to select management practices that minimize volatilization. This will increase the quantity of applied N recovered by the crop as well as improving production efficiency and reduce potential impacts of N use on the environment (3).
To manage the risk of [NH.sub.3] loss, several studies have been done to alleviate the problem. Research has shown that one of the ways to enhance plant N use efficiency of urea and TSP mixtures is to mix them with humic acids (4) as these acids are known to have chemical properties such as high total acidity (CEC) useful in retaining [NH.sub.4.sup.+] as well as aiding in ammonia volatilization reduction (5). Humic substances are the most chemically active compounds in soils with cation and anion exchange capacities far exceeding those of clays (6) and having various capabilities beneficial to agricultural soils.
The major sources of humic substances generally can be found in coal as it contains the largest amount of humified substances particularly, HA (humic acid), FA (fulvic acid) and humin. Lobartini et al. (7) reported that commercial grade lignite from North Dakota compose substantial amount of HA (99.0%) in contrast to that of FA (1%). Lignite or leonardite samples were also noted, on average, to compose 99.5 HA and 0.5% of FA (8), (9).
Although much studies have been done regarding HA and its role in alleviating agricultural problems, much is yet to be known on FA and their role in ammonia loss reduction. Hence, this study aimed to investigate the effect of HA and FA isolated from coal in reducing ammonia volatilization from urea fertilizer as the concentration of HA is very high in coal.

MATERIALS AND METHODS
The soil used in this study was Bekenu series (Typic Paleudults). Soil samples were taken at a depth of 0-15 cm and the coal was taken from Mukah, Sarawak, Malaysia. Both of these samples were air dried and ground to pass through a 2 mm sieve.
The HA and FA extraction was carried out by the methods of Stevenson (10) and Susilawati et al. (11) with some modifications. Selected chemical and physical properties of soil, HA, FA and urea were determined using standard procedures. The pH of soil, urea, HA and FA were determined in a 1:2 soil: Distilled water suspension and KCl using a glass electrode (12).
Soil and HA organic carbon were determined using the Loss-on-ignition method (13). Soil CEC was determined by leaching 1M ammonium acetate buffer adjusted to pH 7.0 followed by steam distillation (14) Extraction of exchangeable K, Ca and Mg was done by the double acid method described in Tan (15). After extraction, the cations were measured using atomic absorption spectrophotometry (AAnalyst 800, Perkin Elmer Instruments, Norwalk, CT).
The soil texture was determined using the hydrometer method. Carboxylic-COOH, phenolic-OH and total acidity of HA and FA were determined using the method described by Inbar  These HA and FA rates were adopted because they gave better mixtures (based on several trials). In addition, studies have shown that less than 1 g HA [kg.sup.-1] soil is sufficient to condition soils (17). The treatments were prepared by first weighing (for each treatment) separately into plastic vials, then tightly closed and shaken on a reciprocal shaker at 150 rpm for 30 min to ensure thorough mixing.
Daily ammonia loss from soil was measured by a modified closed-dynamic air flow system method (4), (18), (19) The system comprised of an exchange chamber and a trap (250 mL Erlenmeyer flask), both stoppered and fitted with an inlet/outlet. The inlet of the chamber was connected to an air pump and the outlet was connected by polyethylene tubing to the trap containing boric acid solution. Soil (250 g) was placed in the exchange chamber and moistened to and maintained at 60% field capacity during the experiment.
The treatments were applied to the soil surface. Air was passed through the chambers at a rate of 3. 5 (15) and the extracts were analyzed as described previously.
The experimental design was completely randomized with three replicates for each treatment. Analysis of variance (ANOVA) was used to test treatment effects while means of treatments were compared using Tukey's test (21).

RESULTS
The soil used in this study was slightly acidic, in both water and KCl ( The selected chemical properties of HA are presented in Table 2. The range of the organic carbon content was consistent with that of Tan (5). The phenolic-OH, carboxylic-COOH and total acidity range were also comparable with those reported by Schnitzer (23) and Tan (5)  The average yield of HA was 3.92% with the highest being 5.00% while the lowest was 3.00%. In comparison with leornardite which contains an average of 80% of HA (6), the content of HA in this indigenous coal was very low. A similar study was reported by Fong et al. (24) using coal from Mukah, Sarawak yielding as much as 9.83% of HA.
Conversely, the yield of FA was generally much higher than HA. This is because the FA was not purified during the extraction and fractionation processes as it still contained substantial amounts of NaOH and HCl.
The [NH.sub.3] loss started on the second day of incubation for all treatments except for T1 where the occurrence was on the first day (Fig. 1) 24.7% compared to the total loss in T1 (Table 3).
There was no significant difference between T1 and T2 in the accumulation of [NH.sub.4.sup.+] for both depths.
[ For exchangeable cations, there were no significant differences between T0, T1 and T2 regardless of depth, However, T3 and T4 were statistically different from T0, T1 and T2 at both depths except for exchangeable K at 1.5-3 cm depth, where T4 showed no significant difference from T0, T1 and T2. At 1.5-3.0 cm depth, T3 showed the highest accumulation of all exchangeable cations followed by T2 ( Table 4). The increment was attributed to the fact that FA was not purified, containing high amounts of NaOH from the extraction process. The lowest accumulation of exchangeable cations occurred for T4. This finding contradicts the study of The pH (water and KCl) at 0-1.5 cm of the treatments with urea mixtures were significantly different from the control (T0) except for T4 which was not significantly different from T0 for pH in water. However, no such differences were observed for all the treatments at 1.5-3 cm depth for pH in water (Table 5).
Conversely, this was not true for pH in KCl at the same depth where all of the treatments with urea additives were significantly different from T0. It was also found that the pH for T1 and T2 were not statistically different regardless of depth and medium used for the pH determination. Treatment with the highest amount of mixture particularly T4 gave the lowest pH throughout the study as it contained both HA and FA that had very low initial pH.

DISCUSSION
The high pH, total organic carbon, exchangeable Ca, Mg and Na in this study could be attributed to the adulteration of the soil in previous years. It was found that exchangeable Ca was substantially higher than the reported values.
This result suggests that liming activities had probably been carried out in the area, thus causing the differences in the selected chemical properties of the soil. According to Havlin et al. (27), liming raises soil pH which in turn affects the CEC of the soil. As pH increases, it will result in an increase in effective CEC, consequently increasing the availability of basic cations such as [K.sup.+], [Mg.sup.2+] and [Na.sup.+]. As pH increases, more microorganisms are able to thrive in the soil and are able to actively breakdown organic materials in the soil, therefore increasing the total organic carbon in the soil.
The effectiveness of washing HA with distilled water is supported by the fact that the ranges of the phenolic-OH, carboxylic-COOH and total acidity of the HA were found to be within the ranges reported by other authors as previously mentioned in Table 2 with a relatively large molecular size or high molecular weight. This molecule has high carbon content, but is relatively low in oxygen (O), carboxylic (COOH) group and total acidity. On the other hand, several studies have shown that FA has relatively low molecular weights as compared to HA but has higher oxygen and lower carbon contents than the high molecular weight HA. FA was also found to contain more functional groups of an acidic nature, particularly COOH (5), (29).
The type of coal used in this study was sub-bituminous, which ranked higher than lignite coals due to its higher combustible energy, thus higher carbon content. It was also found that the carbon content of the coal used in this study ranged between 38.6-43.5%, typical of those reported for sub-bituminous coal, ranging between 35-45% of carbon. The low yields of HA could be attributed to increasing coalification (from lignite, sub bituminous, bituminous to anthracite coal), where the material is more likely to become less extractable with dilute alkali solutions (30).  The high soil pH for T1 and T2 (Table 5)  The result in Table 5 show decreasing pH values from T1-T4 at 0-1.5 cm depth for both pH determinants (water and KCl solution). This is typical for HA and FA as the pH of FA was much lower than HA due to the presence of HCl in the unpurified FA solution. Therefore the pH of T3 (urea + FA) was lower than T2 (urea + HA). However, T4 had the lowest pH value which was attributed to the presence of both HA and FA. volatilization from urea may also contribute to the reduction of environmental pollution from excessive utilization of N fertilizers. It must be stressed that the results obtained in the volatilization experiment using a slightly acidic (pH 6.13-6.9) soil may only be applicable to similar type of soils.
The effectiveness of this study may be improved by using lignite coals that can yield higher amounts of HA instead of using sub-bituminous coals.

ACKNOWLEDGEMENT
The researchers acknowledge the financial support of this research by the Ministry of Higher Education, Malaysia.