Preparation of Lignin-Based Slow-Release Nitrogen Fertilizer

Abstract

Slow-release nitrogen fertilizer technology is essential for sustainable agriculture, reducing field pollution and enhancing fertilizer efficiency. Lignin, a natural polymer derived from agricultural and forestry waste, offers unique benefits for slow-release fertilizers due to its biocompatibility, biodegradability and low cost. Unlike conventional biochar-based fertilizers that often rely on simple pyrolysis, this study employs hydrothermal activation to create a lignin-based slow-release nitrogen fertilizer (LSRF) with enhanced nutrient retention and controlled release capabilities. By incorporating porous carbon derived from industrial alkaline lignin, this LSRF not only improves soil fertility, but also reduces nitrogen loss and environmental contamination, addressing key limitations in existing fertilizer technologies. We studied the hydrothermal carbonization and chemical activation of IAL, optimizing the conditions for producing LSRF by adjusting the ratios of PC, IAL and urea. Using BET, SEM and FT-IR analyses, we characterized the PC, finding a high specific surface area of 1935.5 m²/g. A selected PC sample with 1923.51 m²/g surface area and 0.82 cm³/g pore volume and yield (37.59%) was combined with urea via extrusion granulation to create the LSRF product. Soil column leaching experiments showed that LSRF effectively controls nutrient release, reducing nitrogen loss and groundwater contamination, ensuring long-term crop nutrition. This research demonstrates LSRF’s potential in improving fertilizer efficiency and promoting sustainable agriculture globally.

1. Introduction

In recent years, the excessive use of traditional nitrogen fertilizers has led to low fertilizer utilization rates (generally 30%–50%) and various ecological and environmental issues, including soil acidification, greenhouse gas emissions, water eutrophication [1,2,3] and air pollution [4]. To address these issues, researchers have developed slow-release nitrogen fertilizers that prolong nutrient availability in soil, aiming to enhance fertilizer efficiency and reduce environmental impacts [5,6,7]. Carbon-based fertilizers are among the simplest slow-release fertilizers to produce and are suitable for industrial applications [8]. Biochar is a kind of matrix material with a high specific surface area and high porosity [9]. Carbon-based fertilizers are produced by loading traditional nitrogen fertilizers onto the porous structure and surface of biochar particles, enabling controlled nutrient release. However, wood fiber was usually used as raw material directly. The preparation method limited to the pyrolysis method is single. In addition, the number of surface functional groups and the porosity of the biochar materials is generally low, which limits the adsorption, loading and controlled-release ability of the biochar materials.

This study leverages the hydrothermal activation of biorefinery-derived alkaline lignin to produce porous carbon materials with a high surface area and abundant functional groups. Unlike traditional methods, this approach enhances the nitrogen retention and slow-release properties of the fertilizer, presenting a promising alternative for carbon-based fertilizers with improved efficiency [10,11,12,13]. NH₄⁺ ions were absorbed by biochar, and then the adsorbed carbon materials were mixed with a polymer matrix composed of a cotton pole, acrylic acid, 2-acrylamide-2-methy-lpropanesulfonic acid and bentonite, and a crosslinked reaction was carried out in a microwave synthesizer to produce a kind of biochar slow-release nitrogen fertilizer with a certain water retention capacity [8]. A series of biochar were prepared at different temperatures (T) via high T pyrolyzing method. The adsorption capacity of biochar for NH₄⁺, NO₃⁻ and NO₂⁻ was investigated in detail. The results show that the cation exchange capacity (CEC), porosity and the content of surface functional groups of biochar have great influence on its adsorption capacity. Compared with NO₃⁻ and NO₂⁻ ions, biochar has stronger adsorption capacity of NH₄⁺ ion [14]. In addition, some studies have found that biochar materials have a certain slow-release ability of nutrients [13], and can be used as soil modifier, biochar can improve the basic character and microecological environment of soil and improve the soil fertility and crop quality as well [15,16,17].

First, the consensus among the world’s scientists is that sustainable green energy should be developed [18]. Lignocellulosic biomass, a renewable resource abundant in nature and a by-product of the paper and biorefining industries [19], which has the potential to be a substitute for fossil fuels [20,21]. However, due to lignin’s complex structure [22] and low chemical reactivity, much of it is burned or discarded [23,24], limiting its application [22,25]. Lignin can be completely degraded into humus by soil microbes to improve the organic matter content and the fertility of soil [26,27]. Therefore, lignin is often used to prepare various slow-release fertilizers [1,28,29,30]. Urea is a synthetic fertilizer and is widely used among nitrogen fertilizers due to its high nitrogen content. It is also easily soluble in water, leading to groundwater pollution. Therefore, a common method to prepare controlled release fertilizers is to control the fertilizers’ solubility [31,32,33]. In addition, the rapid hydrolysis of urea in soil leads to the loss of a large amount of nitrogen (30%–50%), and just a small amount of this can, indeed, be absorbed by crops [34,35]. In addition, lignin can effectively inhibit the activity of the urea in the soil. Consequently, the urea can be kept in the soil for a longer time [36].

In this paper, PC was prepared by hydrothermal synthesis and activation from biorefinery technical lignin, and then, a series of lignin-based slow-release nitrogen fertilizers were prepared by extrusion granulation by compounding them with a certain amount of urea at high T and high pressure. To obtain the optimal carbonation conditions, we investigated the effects of carbonization T and carbonization time on the specific surface area, surface functional group content and yield of the obtained hydrothermal synthesis materials. The effects of different inputs of each component on the nutrient release of SRF during extrusion granulation were studied with a soil column leaching experiment.

2. Materials and Methods

2.1. Materials

Shandong Longlive Bio-technology Co., Ltd., Yucheng, China, supplied the industrial alkali lignin. The general production processes were as follows: (a) the hemicellulose in corn cob was degraded into Xylo oligosaccharide by hydrothermal pretreatment; (b) lignin was extracted from hydrothermal residue with dilute alkaline solution and alkali lignin was obtained; (c) finally, the residual cellulose-rich solid residue was subjected to prepare the biological ethanol after enzymolysis and fermentation. In addition, the chemical agents used for the study, including potassium hydroxide and urea, were all analytical grade and were all purchased from Sigma-Aldrich (Beijing, China).

2.2. Preparation of Carbon Material

The hydrothermal synthesis of lignin was conducted in a high-pressure reactor to produce hydrothermal carbon (HC):

An amount of 5 g of biorefinery-derived technical lignin were combined with 50 mL of deionized water, maintaining a solid–liquid ratio of 1:10 [37]. The mixture was thoroughly stirred to ensure homogeneity and then transferred into a sealed high-pressure reactor. The sealed reactor was placed in a preheated oven, and the reaction T and duration were set according to the parameters in Table 1. The optimal conditions aimed to enhance the development of a porous structure in the resulting carbon material. The reaction was carried out at the selected T for the designated time. Upon completion of the hydrothermal reaction, the reactor was carefully removed and allowed to cool to room T. The hydrothermal product was then collected, filtered and washed multiple times with deionized water to remove residual impurities. The filtered hydrothermal carbon was placed in a vacuum drying oven set at 80 °C and dried to a constant weight. This step ensured the removal of any remaining moisture, resulting in the formation of stable hydrothermal carbon (HC).

The activation of HC was carried out in a tube furnace. The prepared HC and KOH were mixed in 50 mL deionized water at a mass ratio of 1:2 (g/g). Then the mixture was dried in a drying oven at 80 °C and then put in a tube furnace to heat it up to 800 °C at the rate of 5 °C/min in nitrogen atmosphere for 2 h. Nitrogen gas was selected as the inert atmosphere during carbon activation to prevent oxidation of the carbon materials at high T s, ensuring the structural integrity of the porous carbon. This step is critical for maintaining the desired pore structure, which influences the nutrient adsorption capacity [38,39]. After the reaction, as shown in Table 1, the porous carbon material (AC₁–AC₉) was obtained by cooling room T in the tube furnace, taking out the samples, and then drying them to a constant weight in a vacuum drying box at 80 °C, after repeated washing and filtration. The optimal porous carbon materials will be selected from these nine materials and combined with urea.

2.3. Composite of the Porous Carbon Material and Urea

The carbon material (AC₁–AC₉) and urea were fully mixed in a certain proportion (1:1, 1:2, 1:3, g/g), and then reacted at the T of 121 °C for 30 min in the autoclave (miniature high-pressure reactor SLM250, Beijing, China), so that urea loaded fully on the surface and pore structure of the carbon material at a high T vapor pressure, and then the compound of the carbon material and urea was dried to a constant weight in a drying oven at 60 °C and stored for use.

2.4. Preparation of Biochar-Based Slow-Release Nitrogen Fertilizer

First, a certain amount of biorefinery technical lignin was mixed with the prepared composite of carbon material and the urea (as shown in Table 2), and the proper amount of deionized water was added. Then, the mixture was extruded and granulated in the extruding granulating machine (Figure 1). The parameters were as follows: the pressure was 1 MPa; the T was 90 °C; and the time was 3 min. The resultant samples were taken from the mold and dried in a drying oven at 60 °C to obtain a lignin-based slow-release nitrogen fertilizer.

2.5. Characterization of Carbon Materials

The HC AC₁–AC₉ was detected by Fourier transform infrared (FT-IR, Thermo Scientific Nicolet 6700, Waltham, MA, USA). Scanning electron microscope (FEG Quanta 250, Thermo Scientific, Waltham, MA, USA) was performed to detect the sample before and after the loading of embedding urea particles. In this study, the specific surface area, pore volume and average pore size of the PC was measured by a BET surface area and pore size analyzer (Micrometrics ASAP 2460, Norcross, GA, USA). The obtained porous carbon material was subjected to an infrared spectrum analysis using a Bruker TENSOR37 infrared spectrometer (Bruker, Ann Arbor, MI, USA) by means of a potassium bromide tablet.

2.6. Soil Column Leaching Experiments

In this study, soil column leaching experiments were used to explore the nitrogen release regulation of LSRF in soil. The experiment methods were as follows. Firstly, topsoil samples (0–20 cm) were collected, removing the sundries. Then, the samples were screened (2 mm) after 5 days of natural air drying at room T and stored. After that, two layers of filter paper were placed at the bottom of each soil column. Weighing the soil sample of 2 kg and filling it in the soil column device according to the bulk density of 1.3 g/cm³ (the inner diameter was 10 cm and the height was 40 cm). The remaining 1 kg soil sample was fully mixed with a certain amount of fertilizer and filled the upper layer of the soil column device according to the bulk density of 1.3 g/cm³ (Figure 2). The amount of fertilizer applied in all test groups was determined according to the soil standard of 100mg N/kg, as follows: CK (no fertilizer); urea (0.66 g urea); SRF₁ (2.31g SRF₁); SRF₂ (1.98 g SRF₂); SRF₃ (1.32 g SRF₃); SRF₄ (1.65 g SRF₄); SRF₅ (1.21 g SRF₅); SRF₆ (1.54 g SRF₆). Moreover, in order to reduce disturbance to the soil during the leaching process, the upper layer of each soil column was covered with a layer of 2 cm quartz sand. After the soil column was filled, the soil water content reached about 70% of the field water content by adding water to each soil column. The soil columns were cultured at room T and 500 mL deionized water was added to each soil column at the 1st, 4th, 7th, 14th, 21st and 28th days of the experiment, respectively, collecting the leachate. Finally, the leachate was filtered, its volume was recorded and it was stored in a refrigerator at 24 °C, while waiting for the sample to be measured. This was repeated 3 times for each treatment.

The concentrations of ammonium nitrogen and nitrate nitrogen in the leachate were determined by an AA3 flow analyzer (Bran Luebbe, Hamburg, Germany). The averages of three repetitions were selected as the final values. The contents of ammonium nitrogen, nitrate nitrogen and total nitrogen in the leachate were calculated according to the following formula:

(1)${\mathrm{N}\mathrm{H}}_4^{+}-\mathrm{N}={\mathsf{\alpha }}_{\mathrm{p}\mathrm{p}\mathrm{m}}\times {10}^{-3}\times \mathrm{d}\times \mathrm{V}$

(2)${\mathrm{N}}_3^{-}-\mathrm{N}={\mathsf{\alpha }}_{\mathrm{p}\mathrm{p}\mathrm{m}}\times {10}^{-3}\times \mathrm{d}\times \mathrm{V}$

(3)$\mathrm{N}\mathrm{H}={\mathrm{N}\mathrm{H}}_4^{+}-\mathrm{N}+{\mathrm{N}\mathrm{O}}_3^{-}-\mathrm{N}$

where ${\mathsf{\alpha }}_{\mathrm{ppm}}$ is the nitrogen content determined by the flow analyzer (mg/L), d is the sample dilution multiple, V is the volume of filtrate collected (mL), TIN is the total inorganic nitrogen content.

3. Results and Discussion

3.1. Effect of Carbonation on the Functional Groups of the Carbon Materials

The IAL was hydrothermally carbonized in a high-pressure reactor under diverse conditions with different proportions of reactants (Table 1). Generally, a lignin macromolecule contains various functional groups, like methoxyl, hydroxyl and a few terminal aldehyde groups. These groups can form weak chemical bonds or electrostatic interactions with ammonium and nitrate ions, helping to hold these nutrients within the matrix. As soil moisture and microbial activity affect these interactions, they gradually release nitrogen, matching plant nutrient uptake more effectively than conventional fertilizers. The functional group characteristics of the products were analyzed by FT-IR. From the infrared spectra in Figure 3, the peak at 1622 cm⁻¹ is the result of the shift of the carbonyl group to a low frequency due to the formation of hydrogen bonds and the effects of the conjugated aromatic ring, as well as the stretching vibration of C=O. In addition, the existence of a carboxylic acid structure can be further confirmed by two strong and wide bending vibration absorption peaks (O-H) in the regions of 1400 cm⁻¹ and 891 cm⁻¹. The double peaks at 2986 cm⁻¹ indicate the stretching vibration of the C-H bond. The peak at 1050 cm⁻¹ illustrates C-O stretching in the aliphatic ether [8,40], and the peak at 1250 cm⁻¹ (C-O) also proves this. Furthermore, there is a peak of O-H bond vibration at 3650 cm⁻¹, and it is inferred that the peak at 1230 cm⁻¹ (C-O) proves the existence of aliphatic ether functional groups. It could be seen that there are many oxygen-containing functional groups in carbonized IAL, which results in the ameliorative hydrophilicity of lignin. In the later experiments, these functional groups provided many active sites, which provided a good foundation for the superiority of the materials.

3.2. Effect of Carbonation Conditions on Basic Physical Properties of PC

Specific surface area, pore volume and average pore size are important physical properties of PC, which directly affect the adsorption and loading capacity of carbon materials to soil nutrients [14,41]. To obtain the optimal hydrothermal synthesis conditions, the effects of hydrothermal synthesis T and time on the specific surface area, the pore volume and average pore size of carbon materials are studied in this chapter (Table 3).

At a hydrothermal synthesis T of 200 °C, the specific surface area of the carbon material increased to 1800.15 m²/g as the reaction time was extended from 10 to 14 h. This effect is likely due to the gradual decomposition and reorganization of the lignin structure, leading to more accessible pore structures. However, when the T reached 220 °C, an excessive synthesis time caused a decrease in specific surface area. This reduction may be attributed to pore collapse or the condensation of functional groups, which reduces the material’s porosity. These changes directly influence the nitrogen adsorption capacity, as a higher specific surface area generally provides more sites for nutrient retention, while reduced porosity limits the nutrient holding capacity. When the T rose to 240 °C, the prolongation of carbonation time leads to a significant decrease in the specific surface area of carbon materials (from 1935.25 to 1474.22 m²/g). Higher specific surface areas and pore volumes provide more adsorption sites for nitrogen compounds, enhancing nutrient retention within the PC matrix and slowing release rates. This structural feature effectively creates a controlled diffusion barrier, reducing the rate of nitrogen leaching into the soil.

The reaction T and time of hydrothermal synthesis had a great influence on the specific surface area of the obtained carbon materials, which was related to the condition that the T and time determined the reaction degree of hydrothermal synthesis to a certain extent. When the T was low, the hydrothermal synthesis reaction of lignin was slow. Therefore, prolonging the reaction time was helpful to improve the reaction degree and increase the content of oxygen-containing functional groups on the surface of HC. The specific surface area of activated carbon materials would gradually increase. With the increase in T, the reaction became extremely violent. At this time, the prolongation of reaction time would lead to an excessive degree of hydrothermal synthesis of lignin, and the dehydration and condensation of some active oxygen-containing functional groups on the surface of the material. Therefore, the specific surface area of activated carbon material would decrease significantly as the reaction time was prolonged [10,11,41,42,43]. In addition, the pore volume and specific surface area of all carbon materials had similar changes, within a range of 0.65–0.91 cm³/g. The average pore size of carbon materials did not change with the reaction T and time of hydrothermal synthesis, mainly distributed in the range between 1.66 nm and 1.86 nm, which indicates that the change of reaction T and time only affects the specific surface area and total pore volume of activated carbon materials but has little effect on the average pore size. The image Figure 4a shows that the surface of carbon material is reticulated and porous, so it has a high specific surface area [44]. In Figure 4b, we find that the carbon material surface is uniformly covered with a layer of white material, which is the urea embedded in the carbon material gap in a sheet-like uniform condition. This shows that the carbon material has been successfully mixed with nitrogen after the reaction in the autoclave, and the aim of increasing the nitrogen content of fertilizer has been achieved. In the later soil column leaching experiment, the fertilizer mixed with IAL also had a distinct slow-release effect. The insights gained from SEM and FTIR analyses underscore the material’s potential in sustainable agriculture [45]. By minimizing nutrient loss and providing a prolonged nutrient release, this lignin-based fertilizer can improve fertilizer use efficiency, reduce environmental pollution (such as groundwater contamination from nitrogen leaching) and support long-term soil health [46].

3.3. Effect of Carbonation Conditions on Yield of PC

From Table 3, it can be found that, when the T was low (200 °C), the yield of carbon material increased slightly with the prolongation of reaction time. This increase was due to the fact that, as the reaction time was prolonged, the hydrothermal synthesis of lignin became more thorough. When the hydrothermal synthesis T was 220 °C and 240 °C, the yield of carbon material decreased obviously with the prolongation of reaction time. Compared with AC₄–AC₆, the yield of AC₇–AC₉ decreased obviously, which may be due to the intense reaction caused by the increase in T, so more liquid products and gas products were produced, while the yield of solid products (HC) decreased significantly [10,47]. In conclusion, a reaction time of 12 h and a reaction T of 220 °C were identified as the optimal conditions for lignin hydrothermal synthesis in this study. The porous carbon materials (AC₅) produced under these conditions exhibited a relatively high surface area (1923.51 m²/g), pore volume (0.82 cm³/g) and yield (37.59%).

3.4. Nitrogen Release of Carbon-Based Fertilizer in Soil

3.4.1. Effect of Porous Carbon Material Usage on Nitrogen Release of SRFs

The nitrogen release of all SRFs in soil was investigated by a soil column leaching experiment. Figure 5 shows the nitrogen accumulation leaching line diagram of the control-group (Urea) and the lignin-based slow-release fertilizer test group (SRF₁, SRF₂ and SRF₃) with different carbon contents. It can be seen from the leaching line diagram of the control-group that when urea is applied to the soil, the nitrogen in urea is rapidly decomposed and transformed into ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃⁻-N) under the action of soil microorganisms and various enzymes, so that most of the nutrients will be released within 28 days after urea was applied into the soil. Compared to the control group, the cumulative release of NH₄⁺-N in the carbon-based sustained-release fertilizer group (SRF₁, SRF₂ and SRF₃) decreased by 20.26%–47.49%, 73.62%–83.52%, 74.21%–85.08%, 65.33%–71.60%, 47.56%–58.05% and 24.78%–37.94%, respectively (Figure 5a); the cumulative release of NO₃⁻-N decreased by 9.99%–38.89%, 24.89%–50.35%, 31.73%–51.97%, 27.48%–47.93%, 32.27%–51.14% and 21.63%–41.22%, respectively (Figure 5b); and the cumulative release of total nitrogen decreased by 12.10%–39.37%, 43.73%–61.26%, 50.17%–64.92%, 46.26%–59.68%, 40.63%–54.92% and 23.40%–38.95%, respectively (Figure 5c), at the 1st, 4th, 7th, 14th, 21^st and 28th days of the experiment. Compared to urea, SRF₁, SRF₂ and SRF₃ have good sustained-release effects. Furthermore, Figure 5a reveals that there is no significant difference in the cumulative leaching amount of NH₄⁺-N between SRF₁, SRF₂ and SRF₃ at the 1st, 4th, 7th, 14th, 21st and 28th days of the experiment. Figure 5b shows that the cumulative leaching amount of NO₃⁻-N between SRF₁, SRF₂ and SRF₃ is obviously different in the corresponding time, and with the increase in the use of PC in the carbon-based fertilizer, the cumulative leaching amount of NO₃⁻-N decreases gradually (SRF₁ < SRF₂ < SRF₃), which is basically consistent with the change in the cumulative leaching amount of total nitrogen in Figure 5c. These findings indicate that increasing the content of porous carbon (PC) in the lignin-based fertilizer significantly enhances the sustained-release effect. This improvement can be attributed to the high specific surface area and abundant pore structure of PC, which provide more sites for nutrient adsorption and retention. When nitrogen compounds are loaded onto these sites, they are gradually released into the soil solution as water penetrates the pores. This controlled release not only extends the nutrient availability but also minimizes nutrient leaching, reducing nitrogen loss and potential groundwater contamination. Moreover, the interaction between lignin functional groups and nitrogen ions might further stabilize the nitrogen, enhancing retention within the soil matrix.

3.4.2. Effect of Lignin Usage on Nitrogen Release of SRFs

Figure 6 shows the nitrogen accumulation leaching curve of the control group (Urea) and lignin-based slow-release fertilizer test group (SRF₁, SRF₄ and SRF₅) with different lignin dosages in soil. It can be seen from Figure 6 that, compared with the urea control group, the cumulative leaching amount of nitrogen in the lignin-based slow-release fertilizer test group (SRF₁, SRF₄ and SRF₅) decreased significantly within 1, 4, 7, 14, 21 and 28 days, and the cumulative leaching amount of NH₄⁺-N decreased by 11.59%–77.37%, 56.04%–82.97%, 63.80%–84.13%, 60.13%–71.59%, 48.87%–58.05% and 26.59%–37.94%, respectively (Figure 6a); the cumulative release of NO₃⁻-N decreased by 8.37%–34.74%, 36.45%–50.35%, 39.17%–51.97%, 33.76%–47.93%, 38.59%–51.14% and 28.67%–40.22%, respectively (Figure 6b); and the total nitrogen accumulation decreased by 24.92%–42.14%, 44.03%–61.26%, 49.86%–64.92%, 46.84%–59.68%, 45.74%–54.92% and 32.16%–38.95%, respectively (Figure 6c). This result shows that, compared to urea, SRF₁, SRF₄ and SRF₅ all have good sustained-release effects. In addition, from Figure 6, we can also find that the cumulative leaching amounts of ammonium nitrogen, nitrate nitrogen and total nitrogen among the three experimental groups SRF₁, SRF₄ and SRF₅ are not much different, which indicates that changing the amount of lignin in the preparation process of a carbon-based fertilizer will not significantly affect the nitrogen release law of the carbon-based fertilizer.

Additionally, the presence of lignin as a binder and its impact on nitrogen release should be considered. Lignin not only contributes to the structural integrity of the granulated fertilizer, but also potentially inhibits rapid urea hydrolysis by soil enzymes, prolonging nitrogen availability in the soil [48]. This interaction underscores the dual role of lignin in improving nutrient release control and enhancing soil health through gradual organic matter decomposition [49].

4. Conclusions

This study successfully developed a high specific surface area, high pore volume, and functional group-rich porous carbon materials derived from commercial lignin through hydrothermal synthesis and activation, demonstrating significant potential for its use in slow-release nitrogen fertilizers (SRFs). Experimental results showed that, compared to conventional urea, the lignin-based slow-release nitrogen fertilizer (LSRF) significantly reduced the cumulative leaching of ammonium nitrogen, nitrate nitrogen and total nitrogen in soil, indicating an effective slow-release behavior. The hydrothermal synthesis method employed in this study utilizes low-cost and abundant lignin resources and, under the optimal synthesis condition of 220 °C, simplifies the process for large-scale production. This approach is not only efficient but also compatible with existing biomass processing technologies, showing strong potential for industrialization. Lignin-based SRFs possess environmentally friendly and cost-effective properties, with the potential to outperform traditional nitrogen fertilizers by reducing the environmental burden associated with nitrogen leaching.

Author Contributions

Conceptualization, Y.Z. and G.J.; methodology, Y.Z.; software, Y.Z.; validation, Y.Z. and G.J.; formal analysis, G.J.; investigation, Y.Z.; resources, D.S.; data curation, G.J.; writing—original draft preparation, Y.Z.; writing—review and editing, J.W.; visualization, D.S.; supervision, J.W.; project administration, D.S.; funding acquisition, D.S. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data are contained within the article.

Acknowledgments

The authors thank the three anonymous reviewers for their helpful comments on this article.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Preparation of lignin-based slow-release nitrogen fertilizer.

Figure 2. The diagrammatic picture of equipment used in the soil column leaching experiment.

Figure 3. FT-IR spectra of HC (AC1–AC9). (a) AC1–AC3; (b) AC4–AC6; (c) AC7–AC9.

Figure 4. (a) SEM images of activated carbon AC5 and (b) activated carbon after urea loading in autoclave.

Figure 5. The nitrogen release behavior of biochar-based slow-release fertilizer with different usage amounts of activated carbon. (a) Cumulative leaching amount of NH4+-N (b) Cumulative leaching amount of NO3−-N (c) Cumulative leaching amount of total nitrogen.

Figure 6. The nitrogen release behavior of biochar-based slow-release fertilizer with different usage amount of lignin. (a) Cumulative leaching amount of NH4+-N (b) Cumulative leaching amount of NO3−-N (c) Cumulative leaching amount of total nitrogen.

Table 1

Preparation parameters and activation conditions of activated carbon materials under different hydrothermal synthesis temperature and time conditions.

Sample	Hydrothermal Synthesis			Activation of KOH
	T (°C)	Time (h)	Solid/Liquid (g/mL)
AC1	200	10	1:10	800 °C, 2 h,HC/KOH = 1:2 (g/g)
AC2	200	12	1:10
AC3	200	14	1:10
AC4	220	10	1:10
AC5	220	12	1:10
AC6	220	14	1:10
AC7	240	10	1:10
AC8	240	12	1:10
AC9	240	14	1:10

Table 2

Usage amount of each component in the LSRF.

Sample	Usage Amount of Each Component in the Biochar-Based Fertilizer (g)
	Urea	PC Material	Bio-Technical Lignin
SRF1	3	3	4.5
SRF2	3	3	3
SRF3	3	1.5	1.5
SRF4	3	1.5	3
SRF5	3	1	1.5
SRF6	3	1	3

Table 3

Physical properties and yields of different activated carbon materials.

Sample	Specific Surface Area (m2/g)	Pore Volume (cm3/g)	Average Pore Size (nm)	Yield a (%)
AC1	1673.62	0.65	1.67	25.93
AC2	1709.37	0.72	1.69	27.45
AC3	1800.15	0.77	1.71	28.41
AC4	1740.83	0.75	1.68	38.48
AC5	1923.51	0.82	1.69	37.59
AC6	1861.37	0.73	1.66	35.22
AC7	1935.25	0.91	1.86	29.37
AC8	1930.32	0.82	1.69	26.43
AC9	1474.22	0.68	1.83	24.52

^a Yield refers to the yield of PC obtained by hydrothermal synthesis and activation of lignin.

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