Rejected Sago Starch as a Coating Material to Mitigate Urea-Nitrogen Emission

Abstract

Urea–nitrogen is commonly lost through ammonia (NH₃) volatilization, denitrification, and nitrate (NO₃⁻) leaching. Rejected sago starch (RSS), which is a by-product of sago flour extraction, could be used to minimize NH₃ volatilization from urea. Urea granules were coated with different concentrations of RSS (2%, 3%, 4%, 5%, and 6%), and their effects on NH₃ emission, soil pH, exchangeable ammonium (NH₄⁺), and available NO₃⁻ were determined. The urea was coated with RSS and homogenized using a mini rotary machine. The RSS-coated urea granules were dyed to differentiate their concentrations. The effectiveness of the RSS as a coating material was determined using a closed-dynamic air flow system. The soil used in the NH₃ volatilization was the Bekenu series (Sandy loam, Typic Paleudults). This study compared seven different mixture treatments: soil alone (S), 5 g of uncoated urea (U), 5 g of 2% RSS-coated urea (CU1), 5 g of 3% RSS-coated urea (CU2), 5 g of 4% RSS-coated urea (CU3), 5 g of 5% RSS-coated urea (CU4), and 5 g of 6% RSS-coated urea (CU5). Urea coated with RSS, particularly CU1, effectively minimized NH₃ loss and improved the retention of soil exchangeable NH₄⁺ and available NO₃⁻ compared with uncoated urea because the RSS serves as a barrier to minimizing the concentration of NH₃ from urea hydrolysis. Urea could be coated with RSS at the 2% concentration to enhance urea–N efficiency through a reduction in NH₃ emission from urea. RSS-coated urea could be an alternative for farmers because of its controlled release of N and economical benefits. Field planting using rice as a test crop to solidify the effectiveness of RSS-coated urea in improving N retention from urea is still ongoing.

1. Introduction

Nitrogen (N) is one of the major nutrients for plant growth and development. Owing to the lack of readily available N in soils, nitrogenous fertilizers such as urea are used to meet crop requirements. Urea is the most commonly used N fertilizer because of its ease of handling, cheapness, availability, and high N content (46%) [1]. However, in most intensive agricultural production systems, over 50% and up to 75% of the N applied are not used by crops because of loss due to ammonia (NH₃) volatilization, denitrification, and nitrate (NO₃⁻) leaching. For example, in paddy fields, 50% of the urea application is lost due to high soil pH. The N loss can cause environmental problems such as air quality degradation and global climate change (NH₃ volatilization and denitrification), eutrophication (NO₃⁻ leaching), and soil acidification (nitrification) [2]. In addition, N use efficiency is less than 30–40% of the applied N. Thus, it is important to develop effective methods for increasing soil N availability and N use efficiency. One way to improve N retention in soil is through the application of coated urea to prevent rapid transformation of N and fixed N in less stable N forms [3]. Coated urea is considered a slow-release fertilizer that reduces the rapid release of N through NH₃ volatilization by slowing down the conversion of N in urea, by delaying synchrony with the needs for ammonium (NH₄⁺) and NO₃⁻ in crops, by maximizing plant N uptake, and by minimizing N losses [3].

Several coating materials have been used to encapsulate urea granules. For example, urea has been impregnated with chemical additives such as N-(n-butyl) thiophosphoric triamide (NBPT) [4,5,6,7] and several polymers with different principles of action [8,9]. Moreover, chemical inhibitors include urease inhibitors [10] that hinder the hydrolysis of urea by urease enzyme, and nitrification inhibitors [11] that prevent the oxidation of NH₄⁺ have also been used as coating materials [12]. Organic or inorganic compounds [13,14,15,16,17,18], and micronutrients [19,20] are used for coating urea. However, most of the aforementioned coating materials are costly and not readily available. The aim of this research is to use bio-based coating material such as starch. There are many approaches to using starch from different sources [21,22,23,24]. However, these starches were mixed or modified with other binders or polymers before coating the urea granules. In this present study, an attempt was made to use rejected sago starch (RSS) as urea coating material. Using agro-industrial wastes such as RSS from sago starch flour extraction for urea coating is considered a lucrative means of producing cheaper coating material for urea granules.

Sarawak, Malaysia, is among the world’s leading producers of sago starch flour and exports approximately 40,000 t annually to different Asian countries [25]. It is expected that the demand will increase annually to meet the global demand. Moreover, with an increase in sago starch demand, the sago starch industry is facing waste management problems, which have resulted in environmental pollution and public health hazards. During starch processing, significant amounts of residual solid wastes are generated. The wastes are sago bark, pith residues, and RSS. These wastes are burned or washed off to nearby streams as a means of disposal [25]. Starch flour produced from sago plant (Metroxylon sagu) generates approximately 20–25 t ha⁻¹ year⁻¹ of RSS, and this waste pollutes water bodies and aquatic life [26]. In this present study the approach used to consider RSS as a coating material for urea granules is based on the availability of RSS from a natural polymer derived from plants.

Previous studies using starch from various sources such as cassava, potatoes, maize, and sago to encapsulate urea granules targeted the low production cost, the biodegradability, the environmentally friendliness, the abundant availability, and the hydrophilic properties [21]. In spite of these advantages, there is a lack of information on the effectiveness of urea coated with RSS and its effects on minimizing N loss through NH₃ volatilization. Considering the acidic nature of RSS (pH 3.58 to 5.95) [27], the starch could be put to good use by using it as a coating material for urea granules to decrease NH₃ loss from urea. This is because NH₃ volatilization in flooded or waterlogged soils is largely controlled by pH and NH₄⁺ ion concentrations. We expected that if urea granules are coated with RSS with a pH of approximately 3, it could regulate the equilibrium between NH₄⁺ and NH₃ in the soil system and could decrease the soil microsite pH temporarily.

In this study, the approach was to produce urea coated with RSS at different concentrations, after which the coated urea was tested on its dissolution rate; coating thickness; N release in a NH₃ volatilization study; and N retention in the forms of soil exchangeable NH₄⁺, and available NO₃⁻. To this end, this study was carried out (i) to produce urea coated with RSS at different concentrations and (ii) to determine the effects of RSS-coated urea on NH₃ volatilization and retention of soil-exchangeable NH₄⁺ and available NO₃⁻. The incorporation of urea coated with RSS provides opportunities for N fertilizer management to mitigate N loss through NH₃ volatilization and to improve soil N availability. Using RSS as a coating material for urea encapsulation is an attempt to delay urea hydrolysis, in which the dissolution rate of urea could be delayed by providing a thin layer of RSS film to avoid a reaction with water. The slow degradation of RSS would retard the rapid loss of N from urea hydrolysis, and N could be retained in the soil in its available forms (NH₄⁺ and NO₃⁻) for crop uptake. This present study provides information on the mechanism of N releases and N retention from RSS-coated urea over uncoated urea.

2. Materials and Methods

2.1. Soil Sampling Site, Preparation, and Initial Physico-Chemical Characterization

The soil used for the NH₃ volatilization experiment was sampled at a depth of 0–20 cm from an uncultivated area in Universiti Putra Malaysia Bintulu Sarawak Campus (UPMKB), Sarawak, Malaysia (Figure 1).

The soil was air-dried for seven days, crushed using a soil mortar and a pestle, after which a 2 mm sieve was used for a laboratory analysis and the NH₃ volatilization study. The laboratory analyses and the NH₃ volatilization study were carried out at Soil Science Laboratory in the Department of Crop Science, UPMKB, Sarawak, Malaysia.

The soil pH in water and 1 M KCl and the electrical conductivity (EC) were determined at a ratio of 1:2.5 (soil: distilled water/KCl) using a digital pH meter and EC meter [28]. The soil texture was determined using the hydrometer method, whereas the bulk density of the soil was determined using the core ring method [28]. The total organic matter (TOM) and total organic carbon (TOC) in the soil were determined using the loss-on-ignition method [28], whereas total N was determined using the Kjeldahl method [29]. Exchangeable NH₄⁺ and available NO₃⁻ were extracted using the Keeney and Nelson method [30], and the analyses were carried out using the steam distillation method [29]. The soil cation exchange capacity (CEC) was determined using the ammonium acetate (NH₄OA_c) method [28], followed by steam distillation [26]. The soil available P and exchangeable cations were extracted using the double acid method [28]. The available P was determined using the molybdenum blue method [31], whereas the exchangeable cations were determined using Atomic Absorption Spectrophotometry (AAnalyst 800, PerkinElmer, Norwalk, CT, USA). There were three replications analysed for the selected soil chemical properties.

2.2. Rejected Sago Starch Characterization

The RSS used in this study was obtained from Sago Link Sdn. Bhd. at Mukah, Sarawak, Malaysia. The RSS was air-dried and ground to pass through a 2 mm sieve for initial characterization using the standard procedures described by Tan [28], as previously outlined in Section 2.1. The swelling ability and solubility of RSS were determined using the method of Adebowale et al. [32]. The water-holding capacity (WHC) of RSS was determined using a standard method [28]. Analyses for each standard procedure were conducted in triplicates.

2.3. Preparation of Rejected Sago Starch-Coated Urea

The coating material from RSS involved two steps, whereby the first step was the dilution of solid RSS in 5 mL and the second step involved adding the diluted RSS in 15 mL of distilled water. The coating materials of RSS were prepared by diluting different amounts of RSS (0.4 g, 0.6 g, 0.8 g, 1 g, and 1.2 g) with 5 mL of the distilled water in 50 mL beakers to obtain different concentrations (2%, 3%, 4%, 5%, and 6%) of the RSS coating materials (

Table 1. Composition of rejected sago starch formulations for urea coating.

Coated Urea	Composition
	Urea (g)	Starch Slurry (mL)	RSS (g)
CU1(2%)	200	0.4 g in 20 mL distilled water	50
CU2(3%)	200	0.6 g in 20 mL distilled water	50
CU3(4%)	200	0.8 g in 20 mL distilled water	50
CU4(5%)	200	1.0 g in 20 mL distilled water	50
CU5(6%)	200	1.2 g in 20 mL distilled water	50

Note: CU1 to CU5 are urea coated with rejected sago starch at different concentrations; RSS is rejected sago starch.

).

The-urea coating process was carried out at the laboratory scale using a self-built mini rotary machine (Figure 2). A mini rotary machine was assembled from two stainless-steel basins with 30 cm diameters and 10 cm depths attached to the driller to rotate the basins. A fan regulator was used to control the speed of the rotary machine (Figure 2).

Thereafter, the different concentrations of RSS were poured into 15 mL of distilled water, which was boiled in a water bath at 90 °C (Figure 3). The mixtures of RSS slurries were stirred for five seconds in water baths to obtain a homogenous mixture and were removed from the water bath instantly to avoid gelatinization of the starch. The dissolved RSS concentrations were dyed with different dyes to differentiate their concentrations, as indicated in Figure 3. The dyes used for RSS colouring were made for food colouring and did not contain any toxic pigments. Thus, the dyes had no effects on the soil pollution or any reactions with urea granules. The method of coating materials preparation in this study was slightly modified from that of Himmah et al. [21]. The modification involved changing the coating solution that was made from 2% w/v of starch, carboxymethyl cellulose (CMC), and 1 g of polyethylene glycol (PEG) [21] to entirely RSS only at different concentrations to determine the optimize concentration value(s) that could be used as urea coating materials to reduce NH₃ loss. It is worth noting that we chose to use RSS without any addition in this study because RSS is inexpensive, is abundantly available from sago starch flour extraction, and exhibits a good coating property. Our attempt at using RSS only as the urea coating material was to benefit from the use of the rejected part of sago starch in an economical way because the addition of synthetic polyether such as polyethylene glycol (PEG) is laborious, time consuming, and costly.

To obtain similar sizes of the coated urea, urea was sieved to pass through a 2 mm sieve, after which they were weighed to obtain 200 g for each formulation and 25 g of dried RSS powder was added. The mixtures were rotated in the rotary machine at 42 rpm (revolution per minute) until the urea granules were fully covered with the dried RSS powder (Figure 3). Afterwards, the urea granules were thoroughly sprayed with RSS slurry and added with another 25 g of dried RSS flour to disperse the coated urea granules and to avoid stickiness. The urea coating process was further continued by mixing sprayed urea granules and dried RSS flour in a mini rotary machine and rotated for another 10 min at 42 rpm until the RSS coating slurry and dried RSS flour thoroughly coated the urea granules. It must be noted that the urea coating process was carried out separately for different concentrations of RSS coating material slurries. The RSS-coated urea granules were oven-dried at 50 °C for one hour to remove the moisture and then stored in a sealed container for further analysis.

2.4. Characterization of Uncoated Urea and Rejected Sago Starch-Coated Urea

After the coating, both RSS-coated and uncoated urea were tested for N content, RSS coating thickness, and dissolution rates.

2.4.1. Nitrogen Content

The total amount of N in the uncoated urea and RSS-coated urea were determined using the standard Kjeldahl method [29].

2.4.2. Thickness of Coating

The average thickness of different concentrations of RSS-coated layer was carried out by comparing the diameters of uncoated urea and different concentrations of RSS-coated urea. The ten granules, approximately similar in size and shape, of the uncoated urea and RSS-coated urea were randomly selected from each sample, and their diameters were measured with a digital Vernier caliper [33].

2.4.3. Urea Dissolution Rate

The urea dissolution rates were determined by agitating five grams of uncoated urea and RSS-coated urea in 100 mL of distilled water for each of the three replicates, after which the times for complete dissolution were recorded.

2.5. Ammonia Volatilization Study

An ammonia volatilization study was conducted using a closed-dynamic airflow system [34,35]. This system consisted of exchange chambers with a 500 mL conical flask containing a 250 g soil sample and a trap chamber with a 150 mL conical flask containing 75 mL of the 2% boric acid-indicator solution, which were both stoppered and fitted with an inlet and outlet facility. The inlet of the exchange chamber was connected to an air pump, and the outlet was connected by polyethylene tubing to the trap chamber containing the boric acid solution to trap NH₃ gas (Figure 4).

The treatments evaluated in the NH₃ volatilization study per 250 g of soil were as follows:

• S: soil alone
• U: soil + 5 g of uncoated urea
• CU1: soil + 5 g of urea coated with RSS (2%)
• CU2: soil + 5 g of urea coated with RSS (3%)
• CU3: soil + 5 g of urea coated with RSS (4%)
• CU4: soil + 5 g of urea coated with RSS (5%)
• CU5: soil + 5 g of urea coated with RSS (6%)

The rate of urea tested in the NH₃ volatilization study was based on the standard recommendation for urea application in wet rice cultivation by Muda Agricultural Development Authority, Malaysia [36], whereby the amount of urea used was 151 kg ha⁻¹ N. The conical flasks containing soil (labelled as the exchange chamber or chamber A) (Figure 4) were moistened at 60% of the field capacity (105 mL of distilled water for 250 g soil) and left overnight to equilibrate. Then, distilled water was added to exchange chamber A up to 3 cm from the soil surface. The water levels in each exchange chamber were maintained throughout the incubation period (30 days) by adding distilled water to replace the water lost due to evaporation. All of the treatments were surface applied in each exchange chamber and replicated for three replications. Air was pumped using an air pump, and the flow rate was adjusted to an appropriate rate of 2.5 L min⁻¹–3.5 L min⁻¹. The volatilized NH₃ from uncoated urea and RSS-coated urea were captured in 75 mL of the 2% boric acid solution with bromocresol green and methyl red indicators (trap chamber) (Figure 4). The boric acid solution was replaced every 24 h and back titrated with 0.01 M HCl to determine the NH₃ loss from uncoated urea and RSS-coated urea. The endpoint was reached when the colour changed from green to red purplish. The estimation of NH₃ loss in a closed dynamic airflow system was continued until the NH₃ loss declined to 1% of the N added in the system [35]. The daily amount of NH₃ loss through volatilization was calculated as follows:

$$\mathrm{Ammonia}\mathrm{loss}(\%)=\frac{\mathrm{Volume}\mathrm{of}\mathrm{HCl}\left(\mathrm{mL}\right)\times \mathrm{Molarity}\mathrm{of}\mathrm{HCl}\left(\mathrm{M}\right)\times 14.01}{\left(\mathrm{Soil}\left(\mathrm{g}\right)+\mathrm{Urea}\left(\mathrm{g}\right)\right)\times \left(\mathrm{Urea}\left(\mathrm{g}\right)\times 0.46\right)}\times 100\%$$

(1)

where 14.01 = the atomic mass of nitrogen and 0.46 = the nitrogen percentage of urea.

After 30 days of incubation, the soil samples were air-dried and analysed for pH, soil exchangeable NH₄⁺, and available NO₃⁻ based on the standard procedures previously stated in Section 2.1.

2.6. Experimental Design and Statistical Analysis

The NH₃ volatilization study was arranged in a completely randomized design (CRD) with three replications for each treatment. Analysis of variance (ANOVA) was used to test treatment effects, whereas means of treatments were compared using Duncan’s New Multiple Range test (DNMRT) at p ≤ 0.05. The Statistical Analysis System (SAS) version 9.4 was used for the statistical test.

3. Results

3.1. Selected Physical and Chemical Properties of Soil

The soil used in this study was Bekenu Series (Typic Paleudults), whereby most of the physical and chemical characteristics were similar to the characteristics described by Paramananthan [37] (

Table 2. Selected physical and chemical properties of Bekenu Series.

Property	Current Study	Range * (0–36 cm)
Colour	Dark yellowish brown	Yellowish brown
pH in water	4.67 ± 0.012	4.6–4.9
pH in 1M KCl	3.83 ± 0.010	3.8–4.0
EC (µS cm−1)	52.00 ± 1.155	NA
Total organic carbon (%)	1.44 ± 0.330	0.57–2.51
Organic matter (%)	2.48 ± 0.563	NA
Total N (%)	0.09 ± 0.009	0.04–0.17
Exchangeable NH4+ (mg kg−1)	6.30 ± 0.701	NA
Available NO3− (mg kg−1)	4.67 ± 0.234	NA
Available P (mg kg−1)	1.51 ± 0.180	NA
---------------------------------------------- (cmol (+) kg−1) ----------------------------------------
Cation exchange capacity	11.67 ± 0.21	3.86–8.46
Exchangeable K+	0.16 ± 0.007	0.05–0.19
Exchangeable Ca2+	0.03 ± 0.004	NA
Exchangeable Mg2+	0.03 ± 0.005	NA
Exchangeable Na+	0.22 ± 0.013	NA
Exchangeable Fe2+	0.19 ± 0.001	NA
Exchangeable Cu2+	0.02 ± 0.016	NA
Exchangeable Zn2+	0.17 ± 0.011	NA
Exchangeable Mn2+	0.02 ± 0.001	NA
Sand (%)	66	72–76
Silt (%)	22	8–9
Clay (%)	16	16–19
Texture (USDA)	Sandy loam	Sandy clay loam

Note: The values given are on a dry-weight basis; NA: not available; * subjected to the soil development range stated in Paramananthan [37].

). The soil colour was dark yellowish brown, whereas the texture was sandy loam with a composition of 66% sand, 22% silt and 16% clay. The soil pHs were low in distilled water (4.67 µS cm⁻¹) and potassium chloride (3.83 µS cm⁻¹), low in CEC and TOM, and low in EC (52 µS cm⁻¹). The Bekenu Series (Typic Paleudults) soil was considered infertile soil, which was reflected by the low content of major plant nutrients such as total N, available P, and exchangeable cations and the low CEC (Table 2).

3.2. Selected Physical and Chemical Properties of Rejected Sago Starch

The nature of RSS is acidic as indicated by the low pH in distilled water (3.43) and KCl (5.10), whereas the EC, OC, and OM of RSS were high. The RSS has low exchangeable cations, except for K, and low solubility but high swelling power (

Table 3. Selected physical and chemical properties of rejected sago starch.

Property	Rejected Sago Starch
pHwater	3.43 ± 0.050
pH1M KCl	5.10 ± 0.009
EC (µS cm−1)	1459.00 ± 58.000
Total organic carbon (%)	43.05 ± 2.666
Organic matter (%)	74.23 ± 4.596
Exchangeable NH4+ (ppm)	3.74 ± 0.234
Available NO3− (ppm)	2.34 ± 0.467
Available P (mg kg−1)	0.87 ± 0.065
Exchangeable K+ (cmol (+) kg−1)	170.27 ± 68.311
Exchangeable Ca2+ (cmol (+) kg−1)	0.78 ± 0.020
Exchangeable Mg2+ (cmol (+) kg−1)	20.13 ± 0.811
Exchangeable Fe2+ (cmol (+) kg−1)	0.41 ± 0.064
Exchangeable Na+ (cmol (+) kg−1)	0.12 ± 0.062
Swelling power (g/g)	10.0 ± 0.153
Solubility (%)	0.56 ± 0.006

).

3.3. Characterization of Uncoated and Coated Urea

3.3.1. Nitrogen Content of Uncoated Urea and Urea Coated with Rejected Sago Starch

The N content in the uncoated urea is 46%, and the urea coated with 2% sago starch decreased from 46% to 45%. The other RSS concentrations (3%, 4%, 5%, and 6%) showed the decreased in N contents from 44 to 42%, with the increased RSS concentrations as the coating material (

Table 4. Nitrogen content of uncoated urea and rejected sago starch-coated urea.

Samples	Nitrogen (%)
U	46 ± 0.12
CU1	45 ± 0.99
CU2	44 ± 0.40
CU3	44 ± 0.57
CU4	43 ± 1.46
CU5	42 ± 0.53

Notes: Values are the means followed by standard deviation.

).

3.3.2. Dissolution Rate of Uncoated Urea and Rejected Sago Starch-Coated Urea

There was a significant difference between the uncoated and coated urea in the dissolution rate. The time required for complete dissolution of the uncoated urea was shorter compared with the RSS-coated urea. The uncoated urea was dissolved at 16.33 s, whereas the dissolution rates of RSS-coated urea ranged from 145.13 to 220.4 s (Figure 5). Among the RSS-coated urea, the dissolution rates increased with decreasing RSS concentrations from 6% to 2% (Figure 5).

3.3.3. Granule Diameters of Uncoated Urea and Rejected Sago Starch-Coated Urea

The uncoated urea differed from urea coated with RSS in their diameter, whereas the diameter of RSS-coated urea was not significantly different irrespective of the RSS concentration (Figure 6).

3.4. Ammonia Volatilization from Uncoated Urea and Rejected Sago Starch-Coated Urea

Soil alone (S) did not contribute to NH₃ loss throughout the incubation period. At day two of the NH₃ volatilization study, uncoated urea (U) did not show any NH₃ loss and remained stable until day two. However, the daily NH₃ loss in uncoated urea rapidly increased starting from day three, and the highest NH₃ loss was seen on day 13, with 33.4% NH₃ loss (Figure 7). The NH₃ loss in the uncoated urea declined on days 14–17, after which the amount of NH₃ loss was approximately 1% of the N in the uncoated urea on days 25 to 29 (Figure 7). The treatments with RSS-coated urea (CU1, CU2, CU3, CU4, and CU5) showed a gradual increase in NH₃ loss on day five and continued to emit NH₃ on days 10–12, at which point the NH₃ loss in all chambers with RSS-coated urea started to decrease on days 21–29 (Figure 7).

Total NH₃ loss was higher in soil with uncoated urea compared with all RSS-coated urea except for urea coated with RSS at the 6% concentration (Figure 8). Urea coated with RSS in CU1 (2%), CU2 (3%), CU3 (4%), and CU4 (5%) significantly reduced NH₃ loss compared with uncoated urea (U) (Figure 8).

3.5. Soil pH, Exchangeable Ammonium, and Available Nitrate of Uncoated and Coated Urea

pH was similar in all soil (U, CU1, CU2, CU3, CU4, and CU5) except for soil alone (S) (Figure 9). The higher soil pH in all treatments added with urea (U, CU1, CU2, CU3, CU4, and CU5) after volatilization showed increased pH compared with the acidic nature of the Bekenu series soil (Table 2).

Urea coated with 2% RSS (CU1) showed higher retention of soil exchangeable NH₄⁺ and available NO₃^- compared with uncoated urea (U) (

Table 5. The effects of uncoated urea and coated urea on exchangeable ammonium and available nitrate.

Treatments	Exchangeable NH4+(mg kg−1)	Available NO3− (mg kg−1)
S	8.640 d	7.005 c
U	81.025 b	17.279 b
CU1	88.730 a	31.289 a
CU2	82.893 b	20.081 b
CU3	85.298 ab	18.213 b
CU4	64.446 c	25.685 b
CU5	61.644 c	25.919 ab

Note: Mean values with different letter(s) indicate significant differences between treatments using Duncan’s test at p ≤ 0.05.

). No significant effects in the retention of soil exchangeable NH₄⁺ and available NO₃⁻ were detected in uncoated urea and RSS-coated urea at 3% (CU2) and 4% (CU3) (Table 5). Urea coated at 5% (CU4) and 6% (CU5) were not effective in retaining soil exchangeable NH₄⁺, which was indicated by the lower accumulation in soil exchangeable NH₄⁺ compared with the uncoated urea (U) (Table 5).

4. Discussion

4.1. Selected Physical and Chemical Properties of the Bekenu Series Soil

The dark yellowish brown colour of the soil is related to the acidic nature of the Bekenu Series, which is reflected by the low pH (Table 2). The soil used in this study is not categorized as saline because of the low EC value compared with the optimal value (<1000 µS cm⁻¹). In addition, Bekenu soil with a high percentage of sand (sandy soil) has a low EC with low organic matter levels. The low content of soil CEC corroborated the low content of all soil soluble minerals owing to the weathering and run off from excessive rain fall and dry climate [37]. The lower content of major and micronutrients suggested that the Bekenu Series soil is infertile and needs fertilization to improve the fertility status [38].

4.2. Selected Physical and Chemical Properties of Rejected Sago Starch

The low pH of RSS (Table 3) is consistent with the pH of aqueous extract of industrial (4 minimum) and edible (4 minimum) grades of sago starch (Malaysia Standard Specification 468 and 470, respectively) [39]. The OM and OC of RSS were high, and this is common for starch derived from the plant (Table 3). The higher OM and OC of the RSS are because starch is complex carbohydrate that has six carbon atoms. Both the swelling and water solubility properties of sago starch suggest the interaction between the amorphous and crystalline areas [40]. In addition, it is influenced by the amylose and amylopectin characteristics [41]. Sago starch has a high swelling power and a low solubility (Figure 5) because of the lower amylose content. Moreover, heating a starch at a high temperature (95 °C) results in a high swelling power. According to Egharevba [42], when unprocessed or native starch granules are heated in an adequate amount of water, it causes swelling and the amylose dissolves. The low mineral contents in RSS (Table 3) could be attributed to several factors such as poor processing conditions, the presence of metal ions during processing, the freshness of the sago raw pith, the presence of polyphenol compounds, and the consequent activity of polyphenol [43].

4.3. Characterization of Uncoated and Coated Urea

4.3.1. Nitrogen Content of Urea Coated with Rejected Sago Starch

The lower N content in RSS-coated urea could be due to loss during the coating progress, which is affected by temperature during coating (Table 4). When the coated urea was oven-dried, the loss of N occurred because a higher air temperature increases NH₃ volatilization due to the increased speed of hydrolysis of the urea. As stated by Moritsuka and Matsuoka [44], during heat-drying, more than 75% of N lost in inorganic N is due to NH₃ volatilization. Although the effect of temperature had been known to increase the risk of NH₃ volatilization from urea, we used oven-drying to encapsulate the urea with RSS because oven-drying is necessary for a dry coating method. In addition, the dry method is preferrable for encapsulating urea particularly for convenient storage and field application. However, there are other methods for urea coating that do not involve oven-drying, for example using a fluid bed coater. The drying process of a fluid bed coater involves exhaustion by air humidity, whereby the drying rates are measured by means of a humidity meter. The humidity is measured both for inlet air humidity and outlet air humidity. In this present study, we used a self-built mini rotary machine for the urea coating, which was built from recycled items (Figure 2). In addition, the lower N content of urea coated with RSS is related to the lower inherent N content of the RSS.

4.3.2. Dissolution Rate of Uncoated Urea and Rejected Sago Starch-Coated Urea

The faster dissolution rate of the uncoated urea over RSS-coated urea is due to the absence of a coating layer observed in this study to slow down the process of urea hydrolysis (Figure 5). Among the RSS-coated urea, the dissolution rates increased with decreasing RSS concentration (6% to 2%) (Figure 5). The lower dissolution rate of CU1 suggests that 2% of RSS concentration is suitable for encapsulating urea compared with other concentrations (3%, 4%, 5%, and 6%). The lower dissolution rate with a lower RSS concentration for the urea coating material contradicting the findings of Naz and Sulaiman [33] is related to the characteristics of the coating material. In the study by Naz and Sulaiman [33], the authors reported that the N release time increased with an increase in coating thickness. A similar finding on increases in the coating solution composition in line with the releases time was also observed in a study of urea coated with waterborne starch biopolymer [45].

4.3.3. Diameter of Uncoated Urea and Rejected Sago Starch-Coated Urea

The difference in the diameters of uncoated urea and RSS-coated urea suggest that the RSS served as a thin film covering the urea granules and protected them from being hydrolysed (Figure 6). Among the RSS-coated urea, the diameter increased with increasing composition, and the order was CU5 > CU4 > CU3 > CU2 > CU1 (Figure 6). This order was in line with the findings of Naz and Sulaiman [33].

4.4. Comparison of Ammonia Loss from Uncoated Urea and Rejected Sago Starch-Coated Urea

The lower NH₃ loss through volatilization from soil with RSS-coated urea than in soil with uncoated urea was consistent with the findings of other researchers [19,46,47,48,49,50]. The higher NH₃ loss in U (uncoated urea) than those of CU1, CU2, CU3, CU4, and CU5 (RSS-coated urea) (Figure 7 and Figure 8) was due to the hydrolysis and ammonification of urea, which consumes hydrogen ions to produce NH₄⁺ ions and results in an increase in soil pH, as shown in Figure 9. More NH₃ are formed over NH₄⁺ when soil pH increases; thus, the equilibrium of NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ shifted to the left (dominated by the higher concentration of NH₃), after which it led to hydrolysis and susceptibility to NH₃ volatilization. The ability of RSS-coated urea (CU1, CU2, CU3, CU4, and CU5) to reduce the NH₃ loss is related to the physical barrier of the RSS coating to water. When urea was encapsulated by coating with RSS, the reaction of urea in the soil could be delayed, which positively minimized the loss of N from the soil in the form of NH₃. The reduction in NH₃ loss is related to a slower urea hydrolysis and N release from the urea when urea was coated with RSS. The coating of urea significantly reduced the hydrolysis process by reducing the mineralization of N [51].

Additionally, RSS minimized the amount of NH₃ emission because of its acidity (3.58 to 5.95). During urea hydrolysis, a higher concentration of NH₃ in soil solution is formed due to the higher pH. When urea is coated with an acidic material such as RSS, the NH₃ loss can be significantly minimized because the RSS may have temporary acidified the soil surrounding the urea. When the soil pH is less than 5.5, urea hydrolyses slowly [52,53]. As a result, this process effectively increases the time required for complete urea hydrolysis. Moreover, the properties of RSS in water can be used to reduce NH₃ loss. Although sago starch is hydrophilic, they are highly water-sensitive, which can prevent water from entering the starch film and thus slows down the hydrolysis of urea by preventing water to react with urea.

Despite the RSS-coated urea (CU1, CU2, CU3, and CU4) significantly reducing NH₃ loss compared with the uncoated urea (U), there was no significant different between U and CU5 (Figure 8). According to Osorio et al., [54] and Reddy and Bhotmange [55], a higher starch concentration in diluted solutions increases the connectivity between starch granules, generating less filling spaces in the solutions. Thus, the starch absorbs more water and swells faster than with very low starch concentrations. This reaction may explain the insignificant effect of the highest RSS concentration used in CU5 (6%) to coat urea because, when starch swells faster, the coating becomes more porous, after which the urea granules dissolve and hydrolyse immediately. In accordance with the dissolution time, CU1, with a lower concentration of RSS (2%), shows more promising effects in minimizing NH₃ loss compared with other concentrations of RSS.

4.5. Soil pH, Exchangeable Ammonium, and Available Nitrate of Uncoated and Coated Urea

The increase in soil pH in all of the treatments except for soil alone (S) was due to the absence of urea undergoing hydrolysis in soil alone (Figure 9). The increases in soil pH with uncoated and RSS-coated urea are related to the ammonification of urea, which consumes hydrogen ions to produce NH₄⁺ ions when urea is hydrolysed [56]. Despite the insignificant difference in soil pH among all the treatments (U, CU1, CU2, CU3, CU4, and CU5), the NH₃ loss was higher in U (uncoated urea) and indicated that RSS-coated urea has an impact on mitigating NH₃ loss compared with uncoated urea. All of the treatments increased in pH during incubation compared with before incubation, which was due to the nature of waterlogged soil. As reported by [57], when soil is waterlogged, the soil pH decreases for 14 days, reaches a minimum pH, and increases to neutral pH. Moreover, the base cation pool seemingly increases in the soil as RSS contains basic cations such as potassium, magnesium, and calcium (Table 3). The base cations that are alkaline counteract the acidic pH of the soil and therefore raise the soil pH [58].

The higher soil exchangeable NH₄⁺ and available NO₃⁻ in CU1 compared with uncoated urea (U) (Table 5) suggests that the coated urea at 2% improved the retention of soil exchangeable NH₄⁺ and available NO₃⁻. The higher accumulation of soil exchangeable NH₄⁺ and available NO₃⁻ in CU1 compared with U (Table 5) corroborated the higher NH₃ volatilization in soil added with urea without coating of RSS. Moreover, the lower NO₃⁻ content in all treatment seemingly due to most of the N are in the form of NH₄⁺. In the present study, the delayed NH₃ volatilization in the soil with RSS-coated urea (CU1, CU2, CU3, and CU4) application was due to the lower rate of hydrolysis. A lower rate of urea hydrolysis leads to a higher accumulation of soil exchangeable NH₄⁺ and available NO₃⁻ in soil, suggesting that a coating of RSS might have effects on the slow release of soil hydrolysable organic N [51].

5. Conclusions

Urea coated with RSS effectively minimizes NH₃ loss and improves the retention of soil exchangeable NH₄⁺ and available NO₃⁻ compared with uncoated urea, particularly in CU1 (2% of RSS-coated urea). The RSS formulated in CU1, which was based on a 2% concentration of RSS, showed the most promising effects in mitigating NH₃ volatilization and the retention of soil exchangeable NH₄⁺ and available NO₃⁻ among all RSS formulations (CU2, CU3, CU4, and CU5). Urea could be coated with RSS at a 2% concentration to minimize NH₃ volatilization and enhanced the urea–N efficiency in such a way that urea could be released in a controlled but synchronized manner to meet the N requirements for crop uptake. The RSS-coated urea could be an alternative for farmers due to its controlled release of N properties and low-cost. Field planting using rice as a test crop to solidify the effectiveness of RSS-coated urea in improving N retention from urea is still ongoing.