Response to Ammonia Emission Flux to Different pH Conditions under Biochar and Liquid Fertilizer Application

Abstract

For the ammonia generated in Korea, the contribution rate of livestock manure is high, and a large amount of ammonia (NH₃) is emitted into the atmosphere during the soil application process. Volatilization of NH₃ is affected by soil characteristics as well as manure characteristics, but the current inventory does not sufficiently reflect this. This study was conducted to confirm the change of the NH₃ emitted from liquid fertilizer (LF) due to soil pH and to evaluate the impacts of biochar (BC) on the suppression of NH₃ volatilization. Estimating the NH₃ emission flux using the chamber for 24 soils after LF treatment, it showed a tendency to increase exponentially as the pH in soil increased from 4 to 7. In addition, the parallel treatment of BC and LF increased the soil pH, thereby increasing the NH₃ emission flux. The rise of soil pH due to LF treatment is a temporary phenomenon that appears in the early stage, but since NH₃ volatilization is also highest at the beginning of LF application, the effect of soil characteristics on emission factor and its inventory should be considered when calculating the amount of NH₃ emissions. Therefore, follow-up studies such as subdividing and enhancing the NH₃ emission factor by soil characteristics and developing a reduction coefficient to certify the amount of emission reduction are needed.

1. Introduction

Particulate matter (PM) has a differing effect depending on its particle size, the exposure time, and the health status of the receptor. The smaller the particle, the more easily it is absorbed into human organs and the deeper it can reach down the respiratory tract, causing a fatally adverse effect on health [1,2]. In Korea, due to the common perception of the dangers and urgency of resolving PM crises, measures to improve atmosphere quality have been implemented and, as a result, amounts of both PM-10 (<10 μm) and PM-2.5 (<2.5 μm) are generally decreasing [3].

Coarse-sized particles (>2.5 μm) containing primary aerosols discharged directly from pollutants in a particulate form are mainly produced by mechanical processes on the surface. In contrast, PM-2.5 is caused by condensation and aggregation of substances in the atmosphere. Therefore, it is difficult to manage and control PM-2.5 [4]. The major constituents of PM-2.5 are ammonium (NH₄⁺), SO_X, NO_X, and organic carbon. Its small size and large surface area make it easy to adsorb various heavy metals and organic pollutants [5]. Among the precursors of PM-2.5, ammonia (NH₃) is one of the few base gases present on the earth that reacts readily with acid gases to form particulate inorganic ammonium salts [6].

According to the Clean Air Policy Support System (CAPSS) of the National Institute of Environmental Research (NIER) in Korea, ammonia emissions in 2017 amounted to 308,298 tons, up 2.3% compared to 2016, of which 79.3% was emitted from agriculture. The increase was attributed to an increase in livestock (pigs and chickens) [7]. In domestic inventory, livestock manure management accounts for 73.5% of total emissions. The livestock species’ contribution rate in manure management was in the order of pigs, chickens, cows, and dairy [8]. The current emissions are calculated as the product of the emission factor (kg head⁻¹) and activity level (number of head) for each livestock type, the emission factor was developed by the NIER in 2008 and categorized by age: less than 1 year, 1–2 years, more than 2 years for cows and piglets, growing pigs, finishing pigs, and sows for pigs [9]. For example, cattle over 2 years has an emission factor of 16.8 kg NH₃ head⁻¹, which consists of in the barn (8.8 kg-NH₃ head⁻¹), composting (2.0 kg-NH₃ head⁻¹), and soil spreading (6.0 kg-NH₃ head⁻¹). In the case of pigs, sow has an emission factor of 21.42 NH₃ head⁻¹, which consists of in the pig house (11.01 kg-NH₃ head⁻¹), composting facility (0.25 kg-NH₃ head⁻¹), livestock manure facility (0.07 kg-NH₃ head⁻¹), and soil spreading (10.10 kg-NH₃ head⁻¹) [9]. Looking at the domestic emission factors calculated through empirical experiments in the field scale, it seems that the stage in which a large amount of NH₃ is generated and discharged in the manure management sector is the stage of breeding facilities and the soil application stage. This trend can also be found in the NH₃ inventory of the US and EU-28 [8]. In other words, for accurate calculation of emissions, it is necessary to understand the NH₃ emission processes and the contribution of breeding facilities and soil spreading to NH₃, which have a high contribution to emissions. Of the two stages, this study focuses on the soil spreading stage.

Sommer and Hutchings [10] summarized the factors that affect the volatilization of NH₃ discharged from compost and liquid manure sprayed on agricultural land in four primary categories. The first is the concentration of NH₃ on the surface of the compost and liquid manure (the pH of the manure and the NH₄⁺ concentration in the manure, the atmospheric temperature, the volatilization rate, etc.). The second is the transition of NH₃ from the surface to the atmosphere (height of the crop, surface roughness, wind speed, amount of light, etc.). The third is the area to which the compost and liquid manure is applied (total amount sprayed, the method of spraying, and the contamination of crop by compost and liquid manure, etc.). The fourth is the time exposed to the atmosphere (spraying time, tillage, infiltration rate, evaporation rate, etc.). In general, it is known that the volatilization of NH₄⁺ to NH₃ increases as the concentration of total ammoniacal nitrogen (TAN) in compost and liquid manure increases; the pH also increases [11]. It also increases with higher wind speed and ventilation rate [12]. When liquid manure is applied to soil, several soil characteristics such as the number of pores and infiltration rate, cation exchange capacity (CEC), temperature, soil moisture, affect the volatilization of NH₃ [10]. To reduce the volatilization of NH₃ after spraying, methods such as microbial agents or acidic substances being added to the liquid manure, injection into the soil rather than surface spraying, or a method of prespraying an adsorbent onto the soil, are being carried out in many studies [6,9,10,13,14]. Among the various types of absorbents, BC is known to have excellent adsorption capacity due to its porous structure, so it is used in various fields for the purpose of adsorbing and removing pollutants from the wastewater and stabilizing pollutants in the soil. Still, the alkaline properties of BC also inevitably affect the environmental medium’s pH [15]. Eventually, the change of the environmental medium’s pH once again affects the volatilization of NH₃, suggesting that even if the same amount of liquid manure is applied to the soil, the emission amount and emission factor may vary according to the characteristics of the soil. However, as described above, the NH₃ inventory does not sufficiently reflect the soil factors that affect the emission factor of the compost/liquid application process.

Therefore, the purpose of this study is to examine the effect of soil pH on NH₃ emissions and to evaluate the effect of BC treatment on soil pH and NH₃ volatilization after LF application, focusing on the pH that has an immediate impact on NH₃ volatilization.

2. Materials and Methods

2.1. Sampling and Analysis of Soil and Liquid Fertilizer

Soils were chosen in the agricultural land of an affiliated farm located in Namyangju, Gyeonggi-do in Korea. Through the preliminary investigation, soils having various pH ranges were collected (total 24 points). At the time of field collection, a 10 mm sieve was used for primary filtration, and after air drying, a 2 mm sieve was used as a sample for soil chemistry analysis. Soil pH and electrical conductivity (EC) were measured using an electrode after adding distilled water at a ratio of 1:5 and stirring for an hour (Thermo Orion 920A, Thermo Fisher Scientific, Waltham, MA, USA). The organic matter content of the soil was quantified by heating at 400 °C for 16 h using a loss-on-ignition (LOI) method [16]. The 1 M CH₃COONH₄ extraction method was used to measure soil CEC [17]. The liquid fertilizer to be used in the experiment was obtained through a resource facility located in Hongcheon-gun, Gangwon-do in Korea. After sample collection, until immediately before analysis, it was stored in a freezer (−20°C) in order to minimize the effect of microorganisms. The pH of the liquid fertilizer was measured using an electrode (Thermo Orion 920A, Thermo Fisher Scientific, Waltham, MA, USA), and the total nitrogen (TN) and total ammonia nitrogen (TAN) in the liquid fertilizer were distilled using Kjeldahl (KjelDigester K-446, BÜCHI, Flawil, Switzerland). In addition, it was quantified using a color development method [17].

2.2. Estimation of NH₃ Emission Flux Using a Dynamic Chamber-Capture System

To calculate the amount of NH₃ after the application of liquid fertilizer to the soil, a dynamic chamber-capture system (DCS) manufactured and improved in a previous study was used (Figure 1). In brief, this system is able to control the flow rate by installing a rotameter at the front and rear ends based on a closed chamber located in a constant temperature water bath (20 °C) and treats the air entering the chamber using activated carbon and distilled water [12]. The soil passed through a 10 mm sieve was put into the chamber to a height of 5 cm, and then treated with the liquid fertilizer at a ratio of 200 kg-N ha⁻¹ in consideration of the nitrogen concentration in the liquid fertilizer [volume of liquid fertilizer (L) = 200 kg ha⁻¹ × chamber surface area (ha) ÷ nitrogen concentration in liquid fertilizer (mg L⁻¹)] × 10⁶ mg kg⁻¹]. A ratio of 200 kg-N ha⁻¹ is commonly used when conducting nitrogen experiment in domestic paddy soil [18,19,20]. The soil and liquid fertilizer were treated in the chamber, and the ammonia volatilized and discharged from the liquid fertilizer was evacuated through a vacuum pump located at the end of the system, and the discharged ammonia was arranged to pass through a 5% boric acid solution, and the whole amount was collected. The NH₃ collected for 1 h after the liquid fertilizer treatment was colored with bromocresol green and methyl red mixed indicators, and then quantified using 0.01 N or 0.1 N sulfuric acid solution. The NH₃ that was emitted and collected for a given time and quantified was calculated as an emission flux (mg m⁻² h⁻¹) using the chamber area and discharge time.

2.3. Effect of Biochar Treatment on Soil pH and NH₃ Emission Flux

Spent coffee grounds, raw materials of the biochar used in the experiment were collected from a nearby coffee shop. After drying of the grounds, pyrolysis was conducted at 400 °C for one hour. The prepared coffee ground char (SCGC) was homogenized through a 0.5 mm sieve. Based on the results of the previous experiment, two soils were selected out of 24 soils, and treated with biochar at 1%, 2%, and 3% levels, respectively; then, water was added to the level of 50% of the water holding capacity for 1 week. After aging, the liquid fertilizer was applied, and the NH₃ emission flux was measured using the same chamber system and method. The pH and EC of the soil after biochar and liquid fertilizer treatment were measured using the same method described in Section 2.1.

2.4. Statistical Analysis

All experiments were performed in three repetitions, and the results are expressed as average values. Significance between treatment groups was tested using the Tukey test after one-way ANOVA using a statistical analysis program (SAS 9.2, SAS Institute Inc., Cary, NC, USA).

3. Results and Discussion

3.1. Chemical Characteristics of Soils and Liquid Fertilizer

The basic chemical properties of the 24 site soils used in this experiment are shown in

Table 1. Chemical properties of examined soils (n = 24).

	pH	EC 1	LOI 2	CEC 3
Average	5.92	0.114	4.94	10.95
Standard deviation	0.66	0.109	0.30	1.28
Maximum	6.80	0.510	5.58	13.09
Minimum	4.37	0.043	4.35	8.25
Median	6.19	0.075	4.91	10.79

¹ Electrical conductivity (ds m⁻¹). ² Loss-on-ignition (%). ³ Cation exchange capacity (cmol kg⁻¹).

. The average pH of the soil is 5.92, showing a weak acidity trend and electrical conductivity (EC) of 0.114 ds m⁻¹. The average loss-on-ignition (LOI) and CEC are 4.94% and 10.95 cmol kg⁻¹, respectively. There are no specific issues that could be problematic in the study procedure, and the standard deviation of soil pH is wide, while other chemical properties show relatively low deviation. This indicates that suitable soils were selected to ascertain the meaningful effect of soil pH on NH₃ volatilization. The pH of the LF used in the experiment is 8.9; the EC is 23.2 ds m⁻¹; and the TN and TAN are 2004 and 1598 mg L⁻¹, respectively.

3.2. Effect of Soil pH on NH₃ Emission Flux

The NH₃ emission trend is highest immediately after application of the LF and showed a tendency to decrease gradually over time [12]. Therefore, in this study, the emitted NH₃ per unit area was calculated by collecting the NH₃ emitted during the initial hour, with the highest emitted flow rate after LF was applied to the soil. As a result, as the soil pH increases, the NH₃ emission flux tends to increase (Figure 2). Although a total of 24 soils were treated with the same fertilizer at the same ratio, the emission flux values show a wide range, from a minimum of 12.38 mg m⁻² h⁻¹ to a maximum of 137.35 mg m⁻² h⁻¹. In a study conducted under the same conditions using granite weathered soil (GWS) rather than agricultural soil, an emission flux was calculated as 115.34 mg m⁻² h⁻¹ [12]. At that time, the pH of GWS was 6.9 and the LOI was 0.8%. In this study, the highest pH among the investigated soils is 6.80, the highest LOI is 5.0%, and the emission flux is calculated as 105.52 mg m⁻² h⁻¹. It is judged that the large difference in organic matter does not have a significant effect on the NH₃ emission flux. On the other hand, while soil pH increases from 4.44 to 6.47, the NH₃ emission flux increases more than ten times, from 12.38 mg m⁻² h⁻¹ to 130.27 mg m⁻² h⁻¹. The volatilization of NH₄⁺ into NH₃ is based on Henry’s constant at the surface of liquid and gas and proceeds by diffusion, and volatilization is promoted as the pH of the solution increases [21]. Considering that the NH₃ emission flux is 855.5 mg m⁻² h⁻¹ when only LF is put in the chamber without any solid medium [12], soil media appears to play a role in reducing NH₃ volatilization. Since the ability to buffer the effect of increasing the pH of the soil–LF mixed medium improves as the pH of the soil is lowered, it is considered that the lower the soil pH, the more effectively the NH₃ emission flux is decreased. In the end, the results indicate that the characteristics of fertilizer and compost/LF are important in the volatilization of nitrogen originating from fertilizer to NH₃, but it is also greatly influenced by the characteristics of the agricultural soil that is applied. Not only soil pH, but also CEC and infiltration depth are known to be factors that significantly affect NH₃ volatilization [22]; however, in this study, the effect of pH is very high and the CEC difference between soils is not significant. Due to the characteristics of the chamber experiment, it is difficult to confirm a significant effect because there is no difference in the penetration depth.

3.3. Effect of Biochar on Soil pH and NH₃ Emission Flux

Based on the experimental results previously obtained, two acidic soils (pH 4.80 and 5.34) out of 24 soils were selected. After the application of BC, the change in pH was confirmed (Figure 3). As a result, the pH of the soil increased due to BC treatment and a significant increase was found for 3% BC treatment compared to the control in both soils. Although it varies depending on the raw material, in general, BC increases in alkalinity when applied into the environment due to its characteristics of having structural groups, functional groups, structural alkalis, carbonates, inorganic alkalis, and dissolved organic alkalis [15]. The increase in soil pH due to an increase in alkalinity can be confirmed through several previous studies, and its effect is known to be superior in acidic soils compared to neutral and alkaline soils [23]. Therefore, despite the relatively short aging period of one week, the effect of increasing the pH by BC treatment could be confirmed in both acidic soils.

After the LF was applied, and the amount of NH₃ emitted for an hour was calculated, the pH of the soil was also measured. As a result, the pH of the soil significantly increased in all soils and BC treatments (Figure 3). Looking at the extent of the increase in soil pH, in the case of soil (Figure 3a), the soil pH increases from 4.80 to 6.57 when the LF is applied, but when the BC is treated at the level of 3%, the soil pH during LF treatment is 4.92 to 7.54 compared to the control, indicating a greater pH increase. This trend can be confirmed in the same manner in soil (Figure 3b), suggesting that the effect of increasing soil pH by simultaneous treatment of LF and BC is immediate, and that they affect each other. In the early stages of treatment, the effect of increasing soil pH is much greater with LF treatment rather than that of BC. However, the rapid increase in soil pH due to LF treatment is due to the supply of a high concentration of NH₃, which is a general phenomenon that occurs during LF treatment [10], and it then returns to its original state through the infiltration and nitrification process [24]. Nevertheless, the temporary synergistic effect of increasing pH when BC and LF are simultaneously treated will need to be observed with interest in the future. As the soil pH increased significantly due to the simultaneous treatment of BC and LF, an increase in the NH₃ emission flux from the LF is expected. NH₃ emission flux is increased in both soils (Figure 3). In the case of soil (a), the NH₃ emission flux increased as the BC treatment increased, showing 77.22 mg m⁻² h⁻¹ in the control and 138.14 mg m⁻² h⁻¹ in the 3% treatment. In the case of soil (b), the NH₃ emission flux increased as the BC treatment increased, with 89.01 mg m⁻² h⁻¹ in the control and 150.32 mg m⁻² h⁻¹ in the 3% treatment. Sha et al. [25] performed a meta-analysis based on 144 observations from 41 references, and the effect of reducing NH₃ volatilization by BC treatment proceeds only under very limited conditions. It was reported that NH₃ volatilization increased by BC application. On the other hand, Mandal et al. [26] announced the results of a reduction in volatilization of NH₃ by up to 70% when urea, ammonium phosphate, and poultry fertilizer were added to five types of soil and demonstrated that the adsorption, immobilization, and nitrification of NH₄⁺ are significant mechanisms of the reduction in NH₃ volatilization by BC. Subsequently, Mandal et al. [27] reported an increase in wheat growth as NH₃ volatilization decreased to around 40%, and nitrogen loss decreased when BC was treated up to 2% in pH 7.8 soil. In summary, NH₃ volatilization and emission reduction in soil by BC proceeds under some restrictive conditions, and NH₃ volatilization is determined by the complex action of the liming effect among the properties of BC and nitrogen fertilizers as well as soil pH [25].

4. Conclusions

The results of estimating the NH₃ emission flux using the chamber for 24 soils after LF showed a tendency to increase exponentially as the soil pH increased from 4 to 7 ranges. In addition, the parallel treatment of BC and LF increased the soil pH, thereby increasing the NH₃ emission flux. The increase of soil pH due to LF treatment is a temporary phenomenon that appears in the early stage, but since NH₃ volatilization is also at its highest at the beginning of LF application, the effect of soil characteristics on emission factor and its inventory should be considered when calculating the amount of NH₃ emission. Therefore, follow-up studies such as subdividing and enhancing the NH₃ emission factor by soil characteristics and developing a reduction coefficient to certify the amount of emission reduction are needed.