Research Note
Enriched Rice Husk Biochar Minimizes Ammonia Loss from Applied Urea Fertilizer Under Waterlogged Environment
Gunavathy Selvarajh
1, Huck Ywih Ch’ng
1,2,*, Norhafizah Md Zain
1, Issariyaporn Damrongrak
31Faculty of Agro-Based Industry, Universiti Malaysia Kelantan Campus Jeli, 17600 Jeli, Kelantan
2Agro Techno Park, Universiti Malaysia Kelantan, 17600 Jeli Kelantan
3Agriculture Program, Science Technology and Agriculture, Rajabhat Yala University, Thailand
*Author for correspondence; Email: [email protected]; Tel.: +6017-8537510
Received: 04 August 2020/ Revised: 10 March 2021/ Accepted: 12 March 2021
A laboratory-scale closed dynamic air flow system was carried out to assess the effect of enriched rice husk biochar on ammonia volatilization, soil exchangeable ammonium, and available nitrate from the applied urea in comparison to urea alone under waterlogged conditions. Treatments with enriched rice husk biochar (5, 10, 15, and 20 t ha-1) significantly minimized ammonia volatilization, however, only biochar with rates of 5, 10, and 15 t ha-1 had significantly retained more soil exchangeable ammonium by 14% – 43%. Additionally, soil available nitrate was lower in all treatments except in T1 (urea alone). This indicates that enriched rice husk biochar minimizes ammonia volatilization, retains more ammonium, and slows down the conversion of ammonium to nitrate under a waterlogged environment. Mixing urea with rice husk biochar at rates of 5 t ha-1 and 10 t ha-1 offers a significant advantage over urea alone by minimizing ammonia volatilization by
> 33.8% over urea alone. The mixture successfully increased retention of ammonium ions and has the potential to minimize ammonia loss and increase nitrogen availability in the soil.
Key Words: ammonia volatilization, biochar, urea, acidic soil, ammonium, nitrate, tropics.
Abbreviations: AAS - atomic absorption spectrophotometer, CEC – cation exchange capacity, N – nitrogen, NH3 – ammonia, NH4+ - ammonium, NO3- - nitrate.
June 2021
INTRODUCTION
Nutrients such as P (Phosphorus), K (Potassium), and most especially N (Nitrogen) are essential for good and abundant plant growth. Agricultural fields need additional N fertilizer application either in the form of direct broadcasting or foliar spray in order to meet the plant nutrient requirements. However, the fertilizer applied in agricultural field tends to undergo rapid volatilization of ammonia (NH3) (He et al. 2002; Tang et al. 2018), specifically surface broadcasted urea fertilizer (Omar et al. 2010). Surface broadcasted urea fertilizer undergoes rapid NH3 volatilization upon contact with water due to the hydrolysis process (Fageria et al. 2010).
During the hydrolysis process, urease triggers the formation of NH3 loss and consequently, the conversion
of NH3 to ammonium ions (NH4+) speeds up. The retention of NH4+ in soil is poor (Ahmed et al. 2010) due to the factors of lesser availability of clay content and organic matter in soil. This will eventually impede the plant NH4+ uptake. In order to meet the crop demand of N and increase soil NH4+ ions, farmers apply excessive N fertilizer. However, additional input being applied is costly and continuous application more than the recommended dosage is not advisable since it creates environmental problems.
Hence, to overcome the loss of NH3 from the applied fertilizer, a sustainable and viable approach is needed.
Biochar made from agricultural wastes can be an alternative solution to minimize NH3 loss. Biochar is a biomass-derived charcoal that has been thermally decomposed under pyrolysis condition (Ghorbani and
Amirahmadi 2018). Generally, biochar can be produced in a small kiln or an advanced pyrolysis machine (Abrol and Sharma 2019). The small kiln method of biochar production can be easily adopted by farmers since it is cost-efficient and not time-consuming. Agricultural wastes such as rice husk which can be found in abundance with an annual production of > 926,886 tonnes (Mansor et al. 2018) can be converted into biochar.
Conversion of rice husk to biochar directly reduces wastage problem and air pollution resulting from landfill burning. Biochar has alkaline properties, is highly porous, and has a larger surface area. This larger surface area aids in anions and cations binding onto its exchange site (Laird et al. 2010). This binding capacity of biochar can minimize NH3 volatilization and increase soil NH4+
for a more efficient plant uptake.
Previous researchers demonstrated that biochar comes in different properties based on feedstock, charring condition, and activation. This agrees with Spokas et al. (2012) who also stressed the need for further research on biochar’s economic and agronomic benefits.
Additionally, there is also a scarcity of information on green feedstock biochar in amending soil fertility by preventing N loss either in rice, cash crops, or other agricultural fields. It is crucial to know the properties of rice husk biochar and its ability to retain nutrients.
Therefore, this study was carried out to determine the effect of mixing urea with enriched rice husk biochar on NH3 volatilization, soil exchangeable NH4+, and available nitrate (NO3-) contents, as compared to application of urea alone under a waterlogged condition.
MATERIALS AND METHODS
Soil Sampling, Preparation and Characterization The soil used in this study was Renggam sandy clay loam (Typic Pale ud ult). It was sampled at 0-30 cm from a piece of land at the Agro Techno Park in the Universiti Malaysia Kelantan Jeli Campus, Malaysia (5.6955 latitude and 101.8389 longitude) which has not been cultivated since 2007. The collected soil was air-dried, crushed, and sieved to pass through a 2 mm sieve for initial soil characterization. Soil pH was measured in a ratio of 1:10 (soil:water) using a digital pH meter (Peech 1965). Soil texture was determined using the hydrometer method (Bouyoucos 1962). Total organic matter content, ash content, and total organic carbon were determined using the loss-on ignition method (Tan 2005). The equations to determine ash content and total C were as follows:
Ash content =(Initial Weight of Sample - Final Weight of Sample)/(Initial Weight of Sample) × 100
Total C = Total organic matter × 0.58
Total N was determined using the Kjeldahl method (Bremner 1965). The double acid method described by Mehlich (1953) was used to extract soil available P and exchangeable cations (Ca, Mg, K, and Na), after which the cations were determined using an Atomic Absorption Spectrophotometer (AAS) (Analyst 800, Perkin Elmer, Norwalk, USA) while soil available P was determined using molybdenum blue method (Murphy and Riley 1962). The developed blue color was analyzed using a UV-VIS spectrophotometer (Thermo Scientific Genesys 20, USA) at 882 nm wavelengths. Soil CEC was determined by ammonium acetate leaching method (Cotteine 1980). The exchangeable acidity and exchangeable Al3+ were determined by the acid-base titration method described by Rowell (1994). The method described by Keeney and Nelson (1982) was used to extract exchangeable NH4+ and available NO3−, after which the ions were determined via steam distillation (Tan 2005).
Biochar Production, Activation and Characterization
Rice husk collected from Pasir Puteh Rice Mill, Malaysia, was used for biochar production. Two cylindrical kilns, a 200 L drum with removable chimney caps, and an airtight 110 L drum were constructed for biochar production. The rice husk was bulked inside the 110 L drum then closed and placed in the middle of the 200 L drum, where the fire was kindled starting from the bottom of the drum. The residence time was 4 hours with the temperature ranging from 300 - 400°C and left for cooling for 2 hours. The temperature inside the kiln was measured using Extech TM100 K/J (Single Input Thermometer, Waltham, Massachusetts, United States).
Later, the pile of biochar sample was spread out for cooling. After this, the enrichment of biochar was carried out by soaking with 5% chicken slurry for 7 days which later was dried and stored in a big container for further use. The enrichment of biochar with chicken slurry was crucial to further increase the nutrient content, alter the surface area, and increase the pore size. The non-enriched and enriched biochar were analyzed for pH (Peech 1965), CEC, and total N (Bremner 1965). The single dry ashing method (Tan 2005) was used to extract nutrients from rice husk biochar for analysis of Ca, Mg, Na, P, and K using an AAS (Analyst 800, Perkin Elmer, Norwalk, USA), while total P content was determined using the molybdenum blue method (Murphy and Riley 1962), after which the blue color developed was analyzed using a UV-VIS Spectrophotometer (Thermo Scientific Genesys 20, USA) (Murphy and Riley 1962). Total C was determined using
the loss on ignition method (Tan 2005). Additionally, microanalysis through Scanning Electron Microscopy- attached with Energy Dispersive X-ray Spectroscopy analysis (SEM-EDX JEOL JSM- 6400) was carried out to analyze the surface morphology of non-enriched and enriched rice husk biochar. The chicken slurry used in this study had been characterised for pH, total organic matter, total C, total N, total P, total K, total Ca, total Mg, total Na, and total Fe following the method described for rice husk characterization.
Ammonia Volatilization Incubation Study The ammonia volatilization incubation study was conducted for a period of 28 days. For the laboratory- scale NH3 volatilization study, 175 kg ha-1 urea and different rates of enriched rice husk biochar (5, 10, 15, and 20 t ha-1) were scaled down, with the actual amount of the treatments evaluated listed in Table 1.
Soil, urea, and enriched biochar were mixed well before being placed into a 250 mL conical flask, after which water was added to create a waterlogged condition. Throughout the study, the water level was maintained at 3 cm above the soil. The system was set to be a closed dynamic air flow system and the NH3 loss from urea was measured daily (Siva et al. 1999; Ahmed et al. 2006a, 2006b) (Fig. 1). The system came with an exchange chamber which consisted of a 250 mL conical flask containing soil mixture and a 250 mL conical flask containing 75 mL of boric acid which was stoppered and fit with inlet/outlet pipes. The inlet of the chamber containing the water and soil-biochar mixture was connected to an aquarium air pump and the outlet was connected to a conical flask containing the boric acid solution. Air was passed through the chambers at a rate of 2.75 L−1 min−1 chamber−1. This setup was done to create soil aeration and trap NH3 loss via the volatilization
process. The released NH3 was captured in the trapping solution containing 75 mL of boric acid with a color indicator. The boric acid indicator trapped in the incubation chambers was replaced every 24 h and back titrated with 0.01 M HCl to estimate the NH3 released.
Measurement was continued until the loss declined to 1%
of the N added with urea (Ahmed et al. 2008). After the ammonia volatilization was evaluated, the soil samples were used for p H, exchangeable NH4+, and available NO3− determinations.
Statistical Analysis
The treatments were arranged in a completely randomized design with three replicates. An independent t-test was conducted using SPSS software version 24.0 (SPSS Inc., US) to compare the significant difference between non-enriched biochar and enriched biochar. The effect of different rates of enriched rice husk biochar addition on all the treatments was subjected to one-way analysis of variance (ANOVA). Statistical analysis for all the data was performed using SPSS software version 24.0 (SPSS Inc., US). Significant differences among treatments were separated by Tukey’s HSD test and considered significant at p ≤ 0.05.
RESULTS AND DISCUSSION
The selected chemical properties of soil used are summarized in Table 2. The texture of the soil used in this study was sandy clay loam. The soil is naturally acidic (pH 5.5) and has very low content of N (0.07 %), NH4+ (89 mg kg-1), and NO3- (30 mg kg-1). Additionally, acidic soil slows down the mineralization process which causes lower N content in the soil. The soil available P is 0.385 mg kg-1 and the causal factor to low P is high exchangeable Al (1.14 cmolc kg-1), exchangeable Fe (0.091 cmolc kg-1), and acidity (0.7 cmolc kg-1) in the soil.
Generally, all tropical soils in Malaysia have relatively low soil available P due to P fixation by Al and Fe.
Exchangeable K, Ca, Mg, and Na were found to be low in the soil (Table 2). This might be due to the incapability of the soil to retain nutrients effectively because of its low CEC (5.4 cmolc kg-1). The CEC readily influences the nutrient availability and soil pH. Having a low soil CEC Table 1. Treatments evaluated in the ammonia volatilization incubation study.
Treatments Description
T0 100 g soil only
T1 100 g soil + 0.7 gurea
T2 100 g soil + 0.7 gurea + 2.8 grice husk biochar T3 100 g soil + 0.7 gurea + 5.5 g rice husk biochar T4 100 g soil + 0.7 gurea + 8.3 g rice husk biochar T5 100 g soil + 0.7 gurea + 11.1 grice husk biochar
Fig. 1. Experimental setup for the closed dynamic air flow system.
accelerates the deterioration of soil pH and becomes vulnerable to low nutrient holding capacity.
The surface morphological characteristics of rice husk biochar before and after enrichment are shown in Fig. 2 and Fig. 3, respectively. The non-enriched rice husk biochar was porous and has a larger surface area (Fig. 2).
After enrichment, most of the empty pores of rice husk biochar were compacted with nutrients and there were some pores that were still free (Fig. 3). The free pores are crucial for further adsorption of nutrients in the soil. The biochar porosity and surface area are influenced by the heating rate, temperature, and burning hours (Zabaniotou et al. 2008). The rice husk biochar showed a porous structure and a large surface area (Lua et al. 2004) which is useful in binding ions (Schmidt et al. 2015).
According to Deenik et al. (2010) and Spokas et al. (2011), biochar produced at a temperature more than 500°C
contributed to the N immobilization which could inhibit plant growth. In addition, the enriched rice husk biochar also had a slightly higher CEC which was 66.6 cmol kg-1 (Table 3). Major et al. (2010) stated that CEC, surface area, and nutrient content are interconnected factors in improving soil fertility. With this property, enriched rice husk biochar has a higher affinity to adsorb nutrients and release them slowly. The enriched rice husk biochar total N and exchangeable Ca and Na absorption increased significantly compared to before enrichment (Table 3).
There is a huge increase (37%) in the content of available P after enrichment which might be due to the higher total P content in chicken slurry (Table 4). This clearly shows that enriched rice husk biochar has a huge capability in capturing and retaining nutrients. The pH of rice husk biochar was alkaline (9.1) and highly favorable to increase and modify the acidity of the soil which will reduce the practice of liming (Butterly et al. 2009). This Table 2. Selected soil chemical properties.
Property Value Obtained
pH 5.500
EC (dS m-1) 0.022
Soil organic matter (%) 6.240
Total Carbon (%) 3.620
Ash content (%) 6.400
Cation Exchange Capacity (CEC)
(cmolc kg-1) 5.400
Ammonium (mg kg-1) 89.000
Nitrate (mg kg-1) 30.000
Total N (%) 0.070
Available P (mg kg-1) 0.385
Exchangeable K (cmolc kg-1) 0.084
Exchangeable Ca (cmolc kg-1) 0.100
Exchangeable Mg (cmolc kg-1) 0.082
Exchangeable Na (cmolc kg-1) 0.024
Exchangeable Fe (cmolc kg-1) 0.091
Exchangeable acidity (cmolc kg-1) 0.700
Exchangeable Al (cmolc kg-1) 1.140
Fig. 2. Rice husk biochar surface before enrichment at 750x magnification under SEM.
Fig. 3. Rice husk biochar surface after enrichment at 550x magnification under SEM. The arrow indicates empty pores as well as the pores compacted with nutrients.
Table 3. Selected chemical properties of rice husk biochar before and after enrichment.
Property Rice husk biochar Enriched rice husk biochar
pH (water) 8.0a 9.1b
Cation exchange
capacity (cmol kg-1) 65.5a 66.6b
Total Nitrogen (%) 0.28a 0.33b
Total P (mg kg-1) 10.4a 14.3b
Total Ca (mg kg-1) 86.1a 1048b
Total Mg (mg kg-1) 734.8a 508b
Total K (mg kg-1) 5686a 4925b
Total Na (mg kg-1) 121.4a 256b
Means between rows with different letter(s) indicate significant differ- ence between treatments by Independent T-test at p ≤ 0.05.
gives an early and clear indication that enriched rice husk biochar packed with nutrient contents can act as one of the organic amendments to reduce the liming process, increase soil fertility, decrease NH3 volatilization, and enhance plant growth. Nutrient adsorbed onto the exchange sites of rice husk biochar during the chicken slurry soaking process indicates that it has a capability to adsorb nutrients from organic sources. This property of rice husk biochar can further adsorb nutrients from the soil as well as from applied chemical fertilizer. Mandal et al. (2016) stated that the adsorbed nutrients on biochar can be gradually released and become available to plants.
The daily volatilization rate of NH3 from urea fertilizer over a 28-day incubation period is presented in Fig. 4A. The NH3 loss in T0 was nil and, in T1, the loss started on the 3rd day. The NH3 loss in T2 and T3 started on the 5th day, while T4 and T5 started on the 4th day. It had been proven that the NH3 loss can be delayed for 3 - 6 days during an incubation study by using organic amendments (Omar et al. 2010). The NH3 loss under T1 peaked on the 5th day, while in T2 and T4 NH3 loss peaked on the 12th and 13th day, respectively. The T3 have shown irregular increase and decrease of NH3 loss. The fluctuation might have occurred due to the reaction between urea, soil, and enriched rice husk biochar in forming NH4+ over NH3. The loss of NH3 increased and reduced gradually up to the 29th day until the added urea
ceased up to 1%. The addition of enriched rice husk biochar delayed the NH3 loss successfully for more than 5 days which might be due to the increased adsorption of NH4+ ions over NH3 onto the surface of biochar. The cumulative losses of NH3 from each experiment are summarized in Figure 4B. This finding is consistent with that of daily NH3 volatilization. It is clear that NH3 losses were lesser in treatments amended with enriched rice husk biochar as compared to the control. The cumulative ammonia loss in T1 is the highest which is 44.52%.
Treatments T2 and T3 showed a least cumulative NH3 loss over the rest of the treatments.
The treatments with enriched rice husk biochar (T2, T3, T4, and T5) had significantly minimized NH3 loss compared to urea alone (T1) (Table 5). Noticeably, T2 and T3 minimized NH3 loss significantly over T1. In this study, the application of 5 t ha-1 and 10 t ha-1 of enriched rice husk biochar (T2 and T3) were more effective than 15 t ha-1 and 20 t ha-1, possibly due to the equal adsorption capacity of biochar regardless of higher or lower application rate. This is because application of biochar at 5 t ha-1 and 10 t ha-1 minimized NH3 loss by approximately > 33.8% over T1 even more effectively than 15 t ha-1 and 20 t ha-1. The reduction in NH3 volatilization directly relates to the increased amount of NH4+ retained in treatments amended with 5 t ha-1 and 10 t ha-1 enriched risk husk biochar (Table 6), preventing rapid conversion of NH4+ to NH3 even at a higher pH level (Table 5). This shows that a lower application rate of biochar can act as one of the cost-effective options because a lesser amount is needed to minimize NH3 loss from the agricultural field. Gale et al. (2016) also stated that a lower application rate of biochar does not inhibit plant growth. Instead, the decrease in NH3 loss was due to the capability of enriched rice husk biochar to modify the soil environment. Lehmann and Joseph (2015) stated that application of biochar alters the soil nature such as structure, surface area, and pore size which will improve soil fertility and increase nutrient uptake. Cayuela et al.
Table 4. Selected chemical properties of chicken slurry.
Property Value Obtained
pH 7.52
Total organic matter (%) 80.70
Total Carbon (%) 46.80
Total N (%) 4.65
Total P (%) 0.296
Total K (%) 12.76
Total Ca (%) 4.40
Total Mg (%) 0.28
Total Na (%) 0.50
Total Fe (%) 0.16
A B
Fig. 4. (A) Daily and (B) Cumulative ammonia loss for every treatment over 28 days of incubation under waterlogged condition.
(2013) and Ding et al. (2016) stated that biochar adsorbs N which is easily volatilised and releases it slowly. This will lead to sustainable N management in the agricultural field, where there is no requirement to add urea fertilizer beyond the standard application rate to meet the demand of plant uptake.
Rice husk biochar can naturally lower the acidity of soil and can reduce the necessity of liming. Treatments added with enriched rice husk biochar had increased the pH of soil (Table 5). The soil pH increased due to the proton exchange in between the enriched rice husk biochar and the soil. Soil pH needs utmost consideration because crop plants’ tolerance to acidity varies and plant nutrients need different optimal pH ranges in order to be successfully utilized by the respective plants (Goulding 2016). Biochar contains organic matter and its addition increased soil pH by proton exchange which, as a whole, may improve the growth performance of the plants (Abrol and Sharma 2019). It is readily known that NH3 volatilization speeds up in soil with higher pH. The soil pH (5.5) used in this study was found to delay the NH3 loss and increase the formation of NH4+ ion because urea hydrolyzes slowly in acidic soil (Fan and Mackenzie 1993). The biochar pH is higher and alkaline and thus might increase the NH3 loss. However, contrastingly, enriched rice husk biochar minimized the losses due to high sorption capacity. This agreed with a
study conducted by Mandal et al. (2016) which found that application of high pH biochar (> pH 9) decreases NH3 volatilization due to the larger surface area and surface functional group. Studies have also shown that pH increase due to the biochar application is not high enough to enhance volatilization of NH3 (Kelly 2015). Dougherty (2016) stated that NH3 volatilization was reduced in soil amended with biochar because NH3 adsorbed at the oxygen containing the surface functional group. Chen et al. (2013) also stated that alkaline biochar application with a high adsorption capacity maintains soil pH and increases adsorption of NH4+ ions over NH3.
Biochar had efficiently adsorbed nutrients from the soil. Treatment T2 and T3 had retained the highest amount of NH4+ over T1, followed by T4 and T5 (Table 6). Egene et al. (2018) stated that biochar adsorbs NH4+ to its surface because biochar is negatively charged.
Higher retention of NH4+ could also occur because of a higher enriched rice husk biochar CEC value (66.6 cmol/
kg) which adsorbs the ions and releases them slowly.
This agreed with a study conducted by Omar et al.
(2010). These suggest that the inclusion of enriched rice husk biochar had improved the presence of NH4+ in the soil.
CONCLUSIONS
The result of this study suggests that application of urea with enriched rice husk biochar at rate of 5 t ha-1 and 10 t ha-1 offers a significant advantage over urea alone. The enriched biochar mixture has effectively retained more NH4+ and NO3- ions in the soil by minimizing conversion to NH3 even at increased soil pH levels. This leads to significant reduction in NH3 released to the atmosphere.
Addition of enriched rice husk biochar retained more inorganic N in the soil. Eventually, this will lead to a sustainable N management in rice production and will also prevent greenhouse NH3 gas emission to the environment. Currently, the results of the laboratory incubation are being evaluated in an ongoing field experiment. The research will undergo several cycles of field trials in different soil types to further assess the effectiveness of the enriched rice husk biochar in minimizing NH3 volatilization from applied urea fertilizer.
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Table 5. Total ammonia loss and soil pH from incubation study under waterlogged condition.
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Table 6. Effectiveness of enriched rice husk biochar in retaining ammonium and nitrate in the soil
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Columns represent the mean values ± standard error.
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