Nano-Biochar Enhanced Adsorption of NO₃⁻-N and Its Role in Mitigating N₂O Emissions: Performance and Mechanisms

Abstract

Biochar (BC) demonstrates considerable potential for reducing nitrogen emissions. However, it frequently exhibits a limited capacity for the adsorption of NO₃⁻-N, thereby reducing its effectiveness in mitigating N₂O emissions. Nano-biochar (NBC) is attracting attention due to its higher surface energy, but there is a lack of information on enhancing NO₃⁻-N adsorption and reducing N₂O emissions. Accordingly, this study conducted batch adsorption experiments for NO₃⁻-N and simulated N₂O emissions experiments. The NO₃⁻-N adsorption experiments included two treatments: bulk BC and NBC; the N₂O emissions experiments involved three treatments: a no-biochar control, BC, and NBC. N₂O emissions experiments were incorporated into the soil at mass ratios of 0.3%, 0.6%, 1%, and 3%. The results demonstrate that NBC exhibits nearly twice the NO₃⁻-N adsorption capacity compared to bulk biochar (BC), with adsorption behavior best described by a physical adsorption model. The enhanced adsorption performance was primarily attributed to NBC’s significantly increased specific surface area, pore volume, abundance of surface acidic functional groups, and higher aromaticity, which collectively strengthened multiple sorption mechanisms, including physical adsorption, electrostatic interactions, π–π interactions, and apparent ion exchange. In addition, NBC application (0.3–3%) reduced cumulative N₂O emissions by 11.60–54.77%, outperforming BC (9.16–32.65%). NBC treatments also increased soil NH₄⁺-N and NO₃⁻-N concentrations by 2.4–8.2% and 7.3–59.0%, respectively, indicating improved inorganic N retention. Overall, NBC demonstrated superior efficacy over bulk BC in mitigating N₂O emissions and conserving soil nitrogen, highlighting its promise as a sustainable amendment for integrated nutrient management and greenhouse gas reduction in soil.

1. Introduction

Global agricultural intensification has significantly increased food production; however, the excessive application of nitrogen fertilizers has profoundly disrupted the global nitrogen cycle, resulting in severe environmental pollution [1]. Studies have shown that the nitrogen use efficiency in major agricultural countries, such as China, is below 50%, with substantial nitrogen losses to the environment primarily in the forms of nitrate (NO₃⁻-N) and gaseous nitrogen oxides [2]. Among these, NO₃⁻-N, due to its weak adsorption capacity and high mobility in soils, is the predominant form of nitrogen leaching in agricultural systems, often leading to groundwater contamination and the eutrophication of water bodies [3]. Meanwhile, nitrous oxide (N₂O), a byproduct of the soil nitrogen cycle, is a potent greenhouse gas with a global warming potential 298 times that of CO₂, and is one of the primary anthropogenic contributors to ozone layer depletion [4,5]. N₂O is mainly produced through nitrification and denitrification [6], making it a priority target for mitigation among nitrogen loss pathways in agricultural systems. Therefore, reducing NO₃⁻-N leaching and N₂O emissions is crucial for improving nitrogen use efficiency and alleviating environmental stress.

Acidic soils, due to their distinctive physicochemical properties, pose heightened risks in nitrogen migration and transformation processes. Low pH conditions inhibit nitrification and exacerbate the mobility of NO₃⁻-N [7,8]. Unlike ammonium (NH₄⁺), which binds strongly to soil colloids, NO₃⁻-N lacks effective adsorption sites and primarily resides in the soil solution, where its weak competitive adsorption renders it highly susceptible to leaching under rainfall or irrigation [9]. Therefore, nitrogen losses under acidic conditions are more complex, necessitating the development of integrated nitrogen management strategies that concurrently suppress nitrate mobility and N₂O emissions, thereby supporting the establishment of efficient and environmentally sustainable agricultural systems.

Biochar (BC) is a carbon-rich porous material produced by pyrolyzing agricultural waste at 300–700 °C under oxygen-limited or anaerobic conditions [10]. Owing to its well-developed pore structure and abundant surface functional groups, BC exhibits a high cation exchange capacity and excellent adsorption properties, making it widely used in improving crop yields, carbon sequestration, mitigation of greenhouse gas emissions, and remediation of heavy metal contamination [11,12,13]. In high-temperature metallurgical processes, BC is extensively applied in operations such as blast-furnace pulverized coal injection (PCI), direct reduced iron (DRI), and electric arc furnaces (EAFs), due to its high fixed carbon content (>80%) and low ash content (<5%). It can replace 20–40% of coke or coal in a PCI, thereby reducing CO₂ emissions by approximately 18–40% [14]. In DRI and EAF processes, BC can also substitute 10–50% of conventional reductants, while enhancing sulfur capture efficiency and improving the foaming stability of steelmaking slag. Industrial trials have demonstrated that the use of BC in EAFs enhances carburization efficiency, reduces electrode consumption, and improves energy utilization [15]. In recent years, BC has garnered considerable attention as a promising soil amendment for enhancing nitrogen retention and mitigating environmental nitrogen losses [16]. BC is a carbon-rich material produced through the pyrolysis of organic feedstocks, exhibiting diverse physicochemical properties conducive to the adsorption, slow release, and retention of soil nutrients [17,18].

Numerous studies have demonstrated that the application of BC can significantly reduce soil NO₃⁻-N leaching and N₂O emissions, thereby alleviating nitrogen pollution in agricultural systems [19,20,21]. A review by Liu et al. reported that bulk BC application reduced average soil N₂O emissions by 32% [21]. Moreover, biochar contributes to reduced nitrate leaching and enhanced nitrogen retention in soils [22]. Similarly, a meta-analysis of 88 studies by Borchard et al. revealed that biochar application decreased NO₃⁻-N leaching by an average of 13%, with reductions exceeding 26% in trials lasting over 30 days, and reduced N₂O emissions by an average of 38% [23]. Therefore, by reducing inorganic nitrogen losses from soil, biochar enhances nitrogen use efficiency and helps alleviate environmental pressure caused by NO₃⁻ contamination in aquatic ecosystems.

Although bulk BC shows promising potential in nitrogen retention and emissions reduction, its effectiveness is sometimes constrained by inherent material properties, such as limited surface reactivity, excessive alkalinity, and the requirement for high application rates [24]. In recent years, NBC has attracted increasing attention [25]. NBC is typically defined as biochar particles with a diameter less than 100 nm; however, some studies extend the upper limit to 500 nm depending on measurement methods and application purposes [26]. NBC is typically produced by mechanically grinding or ball milling bulk BC [27]. Ball milling employs high-energy mechanical forces to pulverize biochar into ultrafine powder, substantially increasing its specific surface area and pore structure in the process. Studies have demonstrated that ball milling significantly enhances the microporous and external surface areas of biochar, thereby improving its adsorption capacity for pollutants and nutrients [28].

Xiao et al. found that ball-milled bone-derived biochar exhibited increased adsorption capacities for Cd²⁺, Cu²⁺, and Pb²⁺ by 93.91%, 75.56%, and 64.61%, respectively [29]. Ma et al. reported that ball milling increased the specific surface area and pore volume of biochar by 22- and 15-fold, respectively, and enhanced the maximum adsorption capacity for Cd to three times that of unmodified biochar [30]. Additionally, ball milling significantly exposes internal functional groups and increases the abundance of oxygen-containing moieties, such as carboxyl and phenolic hydroxyl groups, thereby enhancing surface energy and chemical reactivity [31,32]. Lyu et al. confirmed that ball-milled biochar exhibited a several-fold increase in maximum adsorption capacity for the organic contaminant methylene blue, indicating enhanced pollutant immobilization potential [33]. NBC has also shown promising potential in enhancing soil nutrient retention. In the Loess Plateau, Chen et al. found that applying 0.3–1% NBC reduced hillslope runoff by 39.7–74.4% and NO₃⁻-N leaching by 13.6–59.8% [34]. Furthermore, Kong et al. demonstrated that biochar composited with nanoscale zero-valent iron in constructed wetlands significantly improved NO₃⁻ adsorption and suppressed N₂O emissions [19]. NBC’s advantages in nitrogen regulation lie not only in its superior adsorption capacity, but also in its controllable influence on soil environmental conditions. The introduction of additional acidic functional groups through nanostructuring typically results in lower pH values for NBC compared to bulk BC [35], thereby avoiding the intensified NH₃ volatilization commonly associated with excessive alkalinity [36]. Sun et al. reported that ball-milled straw biochar increased NH₄⁺-N adsorption by nearly 90% while reducing cumulative soil NH₃ volatilization by approximately 42%, markedly outperforming conventional treatments [37]. Thus, through the synergistic optimization of surface structure, functional group characteristics, and reactivity, NBC is considered a highly promising material for regulating soil nitrogen dynamics.

Although NBC possesses numerous advantageous properties, such as a high specific surface area, abundant functional groups, and favorable interfacial reactivity, its role in the soil nitrogen cycle remains insufficiently studied. The current research on NBC primarily focuses on its synthesis methods, physicochemical properties, and pollutant removal performance in aqueous systems, whereas its potential to regulate nitrogen transformation and migration in soil environments has received limited attention [38,39,40,41,42]. To address this research gap, this study for the first time synthesizes nano-biochar from rice straw using a one-step ball-milling method and investigates its adsorption behavior toward nitrate nitrogen (NO₃⁻-N) and its influence on nitrous oxide (N₂O) emissions. This study is centered around the following two scientific questions:

• RQ1—Can rice straw-derived NBC significantly enhance NO₃⁻-N adsorption under acidic conditions, and what are the underlying adsorption mechanisms?
• RQ2—How does NBC application affect N₂O emissions in simulated soils, and is this mitigation effect driven by improved microscale surface properties?

To address these questions, a series of batch isothermal–kinetic adsorption experiments and simulated soil incubation tests were conducted to elucidate the key mechanisms of NBC-mediated nitrogen transformation, thereby providing theoretical support for its application in soil nitrogen management and agricultural greenhouse gas mitigation.

2. Experimental Materials and Methods

2.1. Fabrication and Characterization of Nano-Biochar

Bulk biochar (BC) was synthesized from rice straw through limited-oxygen pyrolysis at 450 °C for 2 h in a muffle furnace, yielding particles with an average diameter ranging from 1.48 to 1.72 μm. Nano-biochar (NBC) was produced by grinding the bulk biochar using a planetary ball mill at 540 rpm for 6 h. This milling process resulted in a particle size distribution between 295 and 342 nm. The ball mill operated by alternating between forward grinding for 30 min and a 10 min pause, followed by reverse grinding for another 30 min. The grinding balls used were of 3 mm, 5 mm, and 8 mm diameters, with a ball-to-powder ratio of 5:1.

The physicochemical properties of bulk BC and NBC before and after NO₃⁻-N adsorption were characterized using the following methods. The specific surface area and porosity were determined using a fully automated surface area and porosity analyzer, Micromeritics ASAP 2460 (Micromeritics Instrument Corp., Norcross, GA, USA). Morphological analysis and EDS spectra of both bulk BC and NBC were obtained via scanning electron microscopy, ZEISS Sigma 300 (Carl Zeiss Microscopy GmbH, Oberkochen, Germany). Particle size distribution and zeta potential were measured with a nanoparticle size and zeta potential analyzer, Zetasizer Nano ZS90 (Malvern Panalytical Ltd., Malvern, UK). The distribution of surface functional groups was evaluated using Fourier transform infrared spectroscopy, Nicolet Nexus 470 (Thermo Scientific/Nicolet Corp., Madison, WI, USA), and the concentration of surface acidic functional groups was quantified by the Boehm titration method.

2.2. Batch Adsorption Experiments

(1)

Adsorption experiments with different biochar dosages. A 1000 mg L⁻¹ stock solution of KNO₃ was prepared with an initial pH of 5.0. A total of 50 mL of this solution was mixed with varying amounts of bulk BC and NBC (0.2, 0.4, 0.6, 0.8, 1.0, 2.0, and 3.0 g). The mixture was placed in a thermostatic water bath at 25 °C and agitated at a shaking speed of 200 rpm for 4 h. After the adsorption process, the NO₃⁻-N concentration in the supernatant was analyzed to determine the adsorption efficiency.

(2)

Adsorption experiments at different initial solution pH levels. A 1000 mg L⁻¹ KNO₃ solution (50 mL) was prepared and mixed with 1.0 g of bulk BC and NBC. The pH of the solution was adjusted to 5, 6, 7, 8, 9, and 10 using NaOH and HCl. The mixture was placed in a constant-temperature water bath (25 °C) and agitated at 200 rpm for 4 h. After the reaction, the NO₃⁻-N concentration in the supernatant was measured.

(3)

Adsorption experiments at different initial nitrogen concentrations. A series of KNO₃ solutions with concentrations of 100, 300, 600, 900, 1200, 2000, and 3000 mg L⁻¹ were prepared. For each concentration, 50 mL of KNO₃ solution with an initial pH adjusted to 5.0 was mixed with 1.0 g of bulk BC or NBC. All mixtures were placed in a constant-temperature water bath at 25 °C and stirred at 200 rpm for 4 h. The same treatment procedure (mixing time, temperature, and stirring rate) was applied to all samples. After the adsorption process, the NO₃⁻-N concentration in the supernatant was measured.

(4)

Adsorption experiments at different contact times. A total of 1.0 g of bulk BC or NBC was added to 50 mL of 1000 mg L⁻¹ KNO₃ solution (pH adjusted to 5.0). The mixture was stirred at 200 rpm in a 25 °C water bath for various durations: 10, 20, 40, 60, 90, 120, 150, 180, 210, and 240 min. The pH was maintained at 5.0 throughout the experiment. All treatments followed identical procedures. After the designated contact time, the NO₃⁻-N concentration in the supernatant was analyzed.

Each experimental condition was conducted in triplicate. After adsorption, the suspensions were filtered through a 0.45 μm PES membrane, and 10 mL of the filtrate was diluted to 50 mL with deionized water. Subsequently, 1 mL of hydrochloric acid solution (prepared by mixing 100 mL concentrated HCl with 900 mL distilled water) and 0.1 mL of sulfamic acid solution (prepared by dissolving 8 g of sulfamic acid in 1000 mL of distilled water) were added. After thorough mixing, the absorbance at 220 nm and 275 nm was measured using a UV–visible spectrophotometer, Shimadzu UV-3600 (Shimadzu Corporation, Kyoto, Japan), and the NO₃⁻-N concentration was calculated using the equation A = A₂₂₀ − 2A₂₇₅ [43].

The adsorption capacity was determined using Equation (1):

$$q_e={\displaystyle \frac{V}{M}}\left(C_0-C_e\right)$$

(1)

where $q_e$ represents the amount of adsorbate adsorbed at equilibrium (mg L⁻¹), $C_0$ is the initial concentration of the adsorbate (mg L⁻¹), $C_e$ is the concentration of the adsorbate at equilibrium (mg L⁻¹), $M$ is the mass of the adsorbent (g), and $V$ is the volume of the solution (L).

2.3. N₂O Emissions Experiment

The N₂O emissions experiment was conducted on 6 February 2024, in a temperature-controlled laboratory. The ambient temperature was maintained at (25 ± 1) °C, with a relative humidity of approximately 60%, and all incubations were carried out in the dark to eliminate light interference. A two-factor, randomized complete block design was applied, including two types of BC and NBC and four application rates (0.3%, 0.6%, 1%, and 3%, w/w), with three replicates per treatment. The treatments included no biochar (CK); BC at 0.3%, 0.6%, 1%, and 3% (BC_0.3, BC_0.6, BC₁, BC₃); and NBC at the same rates (NBC_0.3, NBC_0.6, NBC₁, NBC₃). For each treatment, 210 g of the biochar–soil mixture was packed into a cylindrical soil column (5 cm diameter × 15 cm height). After uniformly adding 75 mL of distilled water, the columns were sealed with plastic film and incubated at 25 °C in the dark for 7 days to restore microbial activity. Following pre-incubation, a 0.2% urea solution (based on soil dry mass) was applied to each column. To ensure adequate aeration, five small holes were pierced into the plastic film. The columns were then incubated for an additional 24 h at 30 °C in darkness. Before gas sampling, the air in the syringes and gas bags was expelled, and the headspace inside each incubation bottle was flushed twice with filtered indoor air to ensure atmospheric consistency.

2.4. Sampling and Measurement

2.4.1. N₂O Emissions Measurement

Nitrous oxide (N₂O) was collected using a gas chromatograph. Gas samples were taken at specific intervals following urea application, on days 1, 3, 6, 9, 13, 17, 21, and 25, with sampling occurring daily at 8:30 AM. Each culture bottle was connected to a vacuum pump, and after evacuating the flask, air was introduced. The internal gas was mixed by extracting it three times using a 25 mL syringe connected to a three-way valve. The collected gas was immediately analyzed for concentration using a gas chromatograph. Sampling intervals were set at 0, 15, and 30 min.

2.4.2. Soil NH₄⁺-N and Soil NO₃⁻-N Measurements

Soil samples were collected at intervals of 1, 3, 5, 7, 9, 11, 13, 17, 21, and 25 days following nitrogen fertilization. The mineralized nitrogen in the soil was quantified using the potassium chloride extraction method, followed by spectrophotometric analysis. The concentration of mineralized nitrogen in the extract was determined using an ultraviolet spectrophotometer (UV3600, Shimadzu, Japan).

2.5. Data Analysis

2.5.1. Nitrogen Adsorption Models

(1)

Adsorption isotherm models

The Langmuir and Freundlich isotherm models were applied to simulate the data obtained from experiment (3). The equations of the models are as follows:

$$q_e={\displaystyle \frac{q_{\max }{K}_L{C}_e}{1+K_L{C}_e}}$$

(2)

$$q_e=K_F{C}_e^{1/n}$$

(3)

where $q_e$ is the unit adsorption amount at equilibrium (mg g⁻¹), $q_{\max }$ is the maximum adsorption amount (mg g⁻¹), $C_e$ is the concentration of the solution at equilibrium (mg L⁻¹), $K_L$ is the Langmuir model parameter, $K_F$ is the Freundlich adsorption constant (L mg⁻¹), and $n$ is the Freundlich model parameter.

(2)

Adsorption kinetic models

The pseudo-first-order and pseudo-second-order models were employed to simulate the data obtained from experiment (4). The equations of the models are as follows:

$$q_t=q_e(1-e^{{-K}_{1}t})$$

(4)

$$q_t={\displaystyle \frac{K_2{q}_e^{2}t}{1+K_2{q}_{e}t}}$$

(5)

where $q_t$ is the unit adsorption amount at time $t$ (mg g⁻¹), $t$ is the adsorption time (min), $q_e$ is the unit adsorption amount at equilibrium (mg g⁻¹), $K_1$ is the adsorption rate constant for the pseudo-first-order model (g (mg·min)⁻¹), and $K_2$ is the adsorption rate constant for the pseudo-second-order model (g (mg·min)⁻¹).

2.5.2. N₂O Emissions Fluxes

The daily N₂O emissions flux was calculated using Equation (6):

$$f_{N_{2}O-N}=\rho h\times {\displaystyle \frac{dC}{dt}}\times {\displaystyle \frac{273}{273+T}}$$

(6)

where $f_{N_{2}O-N}$ represents the N₂O flux (μg h⁻¹ m⁻²), ρ is the standard density of N₂O (1.8 kg m⁻³), h represents the chamber height (m), T is the air temperature inside the chamber (°C), and dC/dt is the rate of change in the N₂O concentration during the sampling period (μg h⁻¹ m⁻²).

Cumulative N₂O emissions were determined using Equation (7):

$$T_{N_{2}O-N}=\sum _{i=1}^{n-1}[{\displaystyle \frac{f_i+f_{i+1}}{2}}\times d\times 24\times { 10}^{-5}]$$

(7)

where $T_{N_{2}O-N}$ represents the cumulative N₂O emissions between two consecutive sampling intervals (kg ha⁻¹), ${ f}_i$ and $f_{i+1}$ are the N₂O fluxes measured on two adjacent sampling days (kg ha⁻¹ d⁻¹), $d$ is the time gap between these events (days), and n is the total number of sampling periods.

2.5.3. Statistical Analysis

A two-way analysis of variance (ANOVA) was conducted to evaluate the effects of biochar (BC) type, addition rate, and their interaction. All statistical analyses were performed using SPSS software version 25.0. The mean values for each treatment were based on three independent replicates. Tukey’s honestly significant difference (HSD) test was applied to determine significant differences between treatments at a significance level of p < 0.05. Graphical representations and data fitting were carried out using Origin 2019 (OriginLab, Northampton, MA, USA).

3. Results

3.1. Characterization of Bulk Biochar and Nano-Biochar

The surface morphology of bulk biochar (BC) typically exhibits a layered and flaky structure, with tubular and porous internal features (Figure 1a). The surface roughness of nano-biochar (NBC) is significantly increased, exhibiting a porous morphology with aggregated and irregularly shaped particles (Figure 1c). A noticeable difference is observed between the surfaces of the two types of biochar after adsorption, with white crystalline formations of nitrate nitrogen evident on both (Figure 1e,g). EDS spectra show that the NBC surface contains a greater variety of elemental compositions compared to BC (Figure 1b,f), and a more substantial reduction in elemental content is observed after adsorption (Figure 1d,h). Compared to BC, NBC’s specific surface area (S_BET), micropore surface area (S_micr), micropore volume (V_micro), and total pore volume (V_total) increased by 49.27%, 188.66%, 188.24%, and 36%, respectively.

The FTIR spectra reveal surface chemical changes in NBC and BC before and after adsorption (Figure 2). Before adsorption, both NBC and BC spectra show typical functional group features, including -OH, C=O, C=C, and C-O-C. Specifically, the peak at 797 cm⁻¹ corresponds to the out-of-plane bending vibration of aromatic C-H, 1039 cm⁻¹ represents the stretching vibration of C-O-C ester groups, 1455 cm⁻¹ is attributed to the bending vibration of alkyl C-H, 1600 cm⁻¹ to the stretching vibration of aromatic C=C, 1716 cm⁻¹ corresponds to the contraction vibration of carbonyl C=O, 2917 cm⁻¹ indicates the stretching vibration of aliphatic C-H, and 3626 cm⁻¹ reflects the sharp absorption stretching vibration of hydroxyl O-H. These features suggest that the biochar surfaces possess abundant oxygen-containing functional groups and aromatic structures.

After adsorption, a clear band appears at approximately 1384 cm⁻¹, corresponding to the asymmetric stretching vibration of NO₃⁻, confirming successful nitrate uptake onto the NBC surface, with a significantly stronger intensity observed for NBC—underscoring its superior nitrate adsorption performance [44]. Moreover, the -OH stretching vibration peak at 3626 cm⁻¹ significantly weakens, indicating a reduction in the availability of surface hydroxyl groups after adsorption. Additionally, the absorption peaks for C=O and C=C also show a decrease, suggesting interactions between the adsorbed molecules and these functional groups. After adsorption, the spectral changes in NBC are more pronounced than in BC, particularly the enhanced stretching vibration peak for C-O-C, indicating more complex chemical reactions occurring on the NBC surface. These changes suggest that NBC exhibits higher adsorption efficiency, which may be closely related to its greater surface reactivity and richer functional groups.

As shown in

Table 1. Physiochemical properties of bulk biochar and nano-biochar.

Samples	SBET (m2 g−1)	Smicro (m2 g−1)	Vtotal (cm3 g−1)	Vmicro (cm3 g−1)	Acidic Functional Group (mmoL g−1)			Alkaline Functional Groups (mmoL g−1)	pH	pHzpc
					Carboxyl	Lactone Group	Phenolic Hydroxyl Group
Bulk biochar	19.060	8.200	0.025	0.003	0.187	0.269	0.592	0.931	10.090	6.730
Nano-biochar	28.450	23.670	0.034	0.010	0.226	0.315	0.719	0.851	9.550	6.440

S_BET, S_micro, V_total, and V_micro indicate surface area, micropore area, total pore volume, and micropore volume; pHzpc indicates the zero point charge of bulk BC and NBC.

, compared to BC, NBC exhibits a marked increase in oxygen-containing functional groups on its surface. The concentrations of carboxyl, lactone, and phenolic hydroxyl groups increased by approximately 20.8%, 17.1%, and 21.4%, respectively. In contrast, the content of basic functional groups decreased by about 8.5%, resulting in a 5.4% decrease in pH. Additionally, the zero point charge (pH_zpc) of NBC (6.44) is lower than that of BC (6.73), further confirming that ball milling significantly enriched the surface of NBC with oxygen-containing functional groups.

3.2. Effects of Biochar Dosage and pH of the Solution

As shown in Figure 3, both the biochar dosage and solution pH significantly affect the adsorption performance of NO₃⁻-N. In the dosage gradient experiment (Figure 3a), as the biochar dosage increased, the unit adsorption capacity of both BC and NBC initially increased and then stabilized, indicating that a moderate increase in biochar dosage helps to enhance the removal efficiency of NO₃⁻-N. Notably, NBC consistently exhibited a higher adsorption capacity than BC at all dosage levels, with a particularly rapid increase observed in the range of 0.5–1.5 g, which may be attributed to its higher specific surface area and the greater density of oxygen-containing functional groups, leading to a larger number of available adsorption sites.

Under different pH conditions (Figure 3b), the NO₃⁻-N adsorption capacity of both biochars significantly decreased as the pH increased, suggesting that acidic conditions favor the adsorption of NO₃⁻-N. Specifically, at pH = 5, NBC reached its peak adsorption capacity, whereas in alkaline conditions (pH = 8–10), the adsorption ability sharply declined. This phenomenon may be related to changes in the surface charge at a high pH and the electrostatic repulsion caused by the anionic form of NO₃⁻-N.

3.3. Adsorption Isotherms

Figure 3c and

Table 2. Fitting parameters of adsorption isotherms and adsorption kinetics.

Samples	Langmuir			Freundlich			Pseudo-First Order			Pseudo-Second Order
	qmax	KL	R2	KF	n	R2	qe	K1	R2	qe	K2	R2
Bulk biochar	3.691	0.001	0.909	0.101	2.317	0.770	2.883	0.014	0.974	3.915	0.003	0.960
Nano-biochar	8.235	0.001	0.964	0.172	2.165	0.874	5.789	0.011	0.997	8.067	0.001	0.994

present the adsorption isotherms of NO₃⁻-N and the model fitting parameters for BC and NBC. The results show that the adsorption data for both biochars can be described by the Langmuir and Freundlich models, with the Langmuir model providing a better fit, as indicated by the higher R² values, compared to the Freundlich model. This suggests that the adsorption of NO₃⁻-N on the surfaces of BC and NBC follows a monolayer adsorption mechanism.

Regarding the maximum adsorption capacity (Q_max), NBC shows an adsorption amount of 8.235 mg g⁻¹, which is significantly higher than BC’s 3.691 mg g⁻¹. These results suggest that the optimization of surface structure and functional groups in NBC has likely enhanced its adsorption capacity for NO₃⁻-N.

3.4. Adsorption Kinetics

Figure 3d and Table 2 present the kinetic fitting results for NO₃⁻-N adsorption by different biochars. The fitting results indicate that the adsorption behavior of both biochars can be described by both pseudo-first-order and pseudo-second-order kinetic models, with the pseudo-first-order model providing a better fit, as indicated by the higher R² values compared to the pseudo-first-order model. This suggests that the adsorption process of NO₃⁻-N is primarily governed by physical adsorption.

In terms of adsorption rate and capacity, NBC’s equilibrium adsorption capacity (Q_e) is 2.883 mg g⁻¹, which is higher than BC’s 5.789 mg g⁻¹. Although the pseudo-first-order rate constant (K₁) of BC is slightly higher than that of NBC, this could be attributed to the more abundant microporous structure of NBC, which may introduce internal diffusion resistance and temporarily hinder the initial adsorption rate. Nevertheless, NBC still exhibits faster overall adsorption kinetics and a significantly higher adsorption capacity.

3.5. Nitrous Oxide Emissions

The changes in N₂O emissions flux are shown in Figure 4a. Influenced by fertilization and soil drying, several peaks in N₂O emissions flux were observed throughout the experimental period, with the main peak being significantly higher than the others. Compared to the untreated biochar (CK), the inhibition effect on N₂O flux peaks for BC added at 0.3% to 3% ranged from 7.53% to 47.69%, while NBC exhibited an inhibition effect ranging from 12.29% to 58.37%. The inhibition effect was more pronounced with higher application ratios. At the same application ratio, NBC exhibited a greater suppression effect on N₂O emissions flux.

The changes in cumulative N₂O emissions are shown in Figure 4b. Compared to the untreated biochar (CK), 0.3% to 3% BC and 0.3% to 3% NBC reduced the cumulative N₂O emissions from 9.16% to 32.65% and 11.60% to 54.77%, respectively. At the same application ratio, NBC exhibited a greater suppression effect on cumulative N₂O emissions. Notably, the addition of 3% NBC resulted in the greatest reduction in cumulative N₂O emissions. Furthermore, no significant difference was observed between 0.6% NBC and 3% BC, indicating that even lower application ratios of NBC can effectively suppress N₂O emissions.

3.6. Soil NH₄⁺-N and Soil NO₃⁻-N

Based on the results shown in Figure 5a,b, the soil NH₄⁺-N concentration increased rapidly after nitrogen application, reaching its peak on day 5, and then began to decline and stabilize. The application of BC and NBC significantly increased the soil NH₄⁺-N concentration compared to CK, with NBC showing the most prominent effect. Figure 6a,b show that the soil NO₃⁻-N concentration began to increase rapidly on day 10 after nitrogen application, and the application of BC and NBC significantly enhanced the accumulation of NO₃⁻-N. NBC application exhibited a stronger effect on NO₃⁻-N concentration, particularly at higher concentrations (1% and 3%), where NBC significantly increased NO₃⁻-N content. As shown in Figure 7a, the application of NBC at concentrations ranging from 0.3% to 3% increased the average soil NH₄⁺-N concentration by 2.7–5.7% and 2.4–8.2%, respectively, indicating that NBC had a marked promoting effect on NH₄⁺-N in the early stages. As shown in Figure 7b, the application of NBC at concentrations from 0.3% to 3% increased the average soil NO₃⁻-N concentration by 9.8–54.2% and 7.3–59.0%, respectively.

These results suggest that NBC not only performs well in enhancing NH₄⁺-N concentrations but also effectively promotes NO₃⁻-N accumulation, especially in the later stages of the experiment. The significant effect of NBC may be related to its higher surface activity and stronger nitrogen adsorption capacity, which enhance its nitrogen slow-release effect in the soil, thereby optimizing the nitrogen transformation process.

4. Discussion

4.1. Adsorption Mechanism

The adsorption capacity of NBC is influenced by factors such as specific surface area, pore structure, surface functional groups, electronegativity, and the presence of metal cations [39,45]. Its adsorption mechanisms include physical adsorption, electrostatic attraction, metal ion exchange, and π–π interactions [11], which often operate synergistically [46]. Under acidic conditions in particular, the effects of pore structure and surface functional groups become more pronounced, enhancing the adsorption efficiency of NBC for NO₃⁻.

The specific surface area and pore structure of biochar play a critical role in determining its adsorption performance. A larger surface area and more complex pore architecture provide additional adsorption sites, thereby enhancing its overall sorption capacity [47]. In particular, the presence of micropores and mesopores facilitates the accommodation of more adsorbate molecules and promotes interactions between the surface and target compounds [48]. Bian et al. noted that the abundant pore structure of biochar enables adsorbates to diffuse through channels and bind to active sites within the micropores [49]. This study showed that ball-milled NBC exhibited significantly increased specific surface area, micropore area, micropore volume, and total pore volume (Table 1). These improvements are attributed to the ball-milling process, which not only reduces particle size and increases external surface area, but also opens the internal pore network, thereby expanding the internal surface area [33,50]. Lyu et al. also reported that ball milling markedly increased the pore volume of biochar, further supporting its effectiveness in expanding the pore network [51]. However, Takaya et al. cautioned that an increase in surface area does not always correspond to improved adsorption, suggesting that while surface area and pore structure are important, they are not the sole determinants of adsorption performance [52].

The ionization behavior of biochar surface functional groups under varying pH conditions governs its adsorption capacity for NO₃⁻-N [53]. Acidic functional groups dissociate under alkaline conditions, releasing H⁺ and generating negative surface charges that enhance cation adsorption. Conversely, in acidic environments, these groups tend to retain H⁺, rendering the surface positively charged and thus favorable for electrostatic attraction of anions, such as NO₃⁻ [54]. Tian et al. demonstrated that nitrogen-doped NBC possesses basic nitrogen-containing groups capable of protonation, thereby acquiring positive charges and enhancing its affinity for negatively charged species [55]. This process is primarily driven by electrostatic interactions, and its efficiency is highly dependent on solution pH. When the pH is below the point of zero charge (pHpzc) of the biochar, the surface is positively charged and tends to adsorb anions; in contrast, at pH values above the pHpzc, the surface becomes negatively charged, favoring cation adsorption [56]. In this study, the experimental solution had a pH of 5, which is lower than the pHpzc of the applied biochar (Table 1); therefore, the biochar surface was positively charged, facilitating the adsorption of negatively charged NO₃⁻.

Variations in solution pH not only influence the distribution of surface charges on the adsorbent, but also affect the adsorption process through the protonation and deprotonation of surface functional groups [57]. Under all tested pH conditions, the adsorption capacity of NBC for NO₃⁻-N was significantly higher than that of bulk BC, indicating that its enhanced performance is closely related to a higher density of surface positive charges and more abundant functional groups. However, as the pH increased, the adsorption capacity decreased markedly, primarily due to the enhanced electrostatic repulsion resulting from the deprotonation of surface functional groups [58]. Following ball milling, the pH of biochar decreased, and the surface exhibited a higher abundance of acidic functional groups, with a significantly increased degree of dissociation [59]. However, in acidic environments, the high concentration of H⁺ suppresses the dissociation of these groups, resulting in a more positively charged NBC surface that facilitates electrostatic adsorption of negatively charged NO₃⁻-N. Similarly, Wei et al. found that ball-milled CuO-modified biochar exhibited an improved adsorption performance, highlighting the critical role of electrostatic interactions in the removal of anionic pollutants [60]. The results of this study confirm that under low-pH conditions, the NBC surface is positively charged, which enhances the adsorption of negatively charged NO₃⁻-N.

According to SAYA et al., π–π interactions are considered one of the key mechanisms involved in the adsorption process of biochar [61]. The number and structure of aromatic rings significantly influence π–π interactions [62]. A higher abundance of aromatic rings increases the electron-donating capacity of biochar, thereby strengthening its interaction with adsorbates [63]. In this study, the FTIR spectrum displayed a characteristic peak around 1600 cm⁻¹ corresponding to the stretching vibrations of C=C bonds in aromatic rings [29], indicating the presence of abundant aromatic structures in the biochar (Figure 2). Notably, NBC exhibited a higher aromatic ring content, and ball milling further increased the number of aromatic structures and π-electron availability, thereby enhancing its adsorption capacity for NO₃⁻. The π–π interaction mechanism primarily occurs through the interaction between π-electrons in aromatic rings and those in anionic species [64]. Under acidic conditions, the electron density of NO₃⁻ is relatively low, making it more prone to interact with the π-electron cloud of aromatic rings in biochar and form stable π–π interactions [65]. Moreover, under acidic conditions, the positively charged surface of biochar further promotes electrostatic attraction with negatively charged NO₃⁻ ions, and the synergy between electrostatic and π–π interactions enhances overall adsorption efficiency [66]. Additionally, Zhang et al. reported that π–π interactions between anions and the π-conjugated aromatic structures on the biochar surface play a vital role in the adsorption process [67]. Such interactions are not limited to anion adsorption but may also modify the electronic structure of the biochar surface, thereby enhancing its adsorption capacity for a wider range of contaminants. Therefore, the enriched π-conjugated aromatic structures in NBC and their π–π interactions with NO₃⁻ are considered one of the critical mechanisms driving its adsorption behavior.

Studies have shown that ion exchange plays a critical role in the adsorption of pollutants by biochar, particularly in relation to the charge characteristics of surface functional groups and the synergistic effects of metal cations [68]. In acidic environments, the high concentration of H⁺ suppresses the dissociation of acidic functional groups, resulting in a net positive surface charge. This shift in surface charge facilitates ion exchange with negatively charged NO₃⁻. The observed attenuation of the -OH stretching peak in the FTIR spectrum following NO₃⁻ adsorption (Figure 2) may serve as evidence of this interaction. The efficiency of ion exchange is influenced not only by the type and abundance of acidic functional groups, but also by the presence of surface metal cations, such as Ca²⁺ and Mg²⁺ [69]. These metal ions can form coordination complexes with surface functional groups, providing potential exchange sites that enhance the adsorption capacity for NO₃⁻ [70]. The EDS spectra in Figure 1 illustrate changes in surface metal composition between the two types of biochar, with a more pronounced decrease in metal elements observed on NBC, suggesting a possible ion exchange between NO₃⁻ and surface-bound cations. Based on combined FTIR and EDS results, the adsorption of NO₃⁻ by NBC is presumed to involve apparent ion exchange mechanisms, potentially mediated by interactions with protonated sites or surface metal cations.

At the initial stage of the experiment, the adsorption capacity of biochar for NO₃⁻-N increased significantly with rising nitrogen concentrations in the solution. This trend is attributed to the greater concentration gradient between the solution and the biochar surface at higher nitrogen concentrations, which enhances the adsorption driving force and facilitates NO₃⁻-N uptake [71]. However, as the nitrogen concentration increases further, the adsorption sites on the biochar surface become gradually saturated, resulting in a plateau in adsorption capacity and indicating that the reaction is limited by the number of available binding sites [72]. The Langmuir isotherm model accurately describes the adsorption behavior of both bulk biochar and NBC, suggesting that the process is primarily governed by a monolayer with a heterogeneous distribution of adsorption sites [73]. In the Freundlich isotherm model, the 1/n values for both types of biochar ranged between 0 and 1 (Table 2), indicating the favorable adsorption of NO₃⁻-N [74]. The good fit of the pseudo-first-order kinetic model suggests that the adsorption process is primarily governed by physisorption mechanisms, such as external diffusion and weak intermolecular forces. This also implies that adsorption occurs through a complex, multi-stage pathway before ultimately reaching equilibrium [75].

Overall, the adsorption of NO₃⁻-N by biochar under acidic conditions is constrained by the number of surface adsorption sites and the solution concentration, and is predominantly governed by chemisorption. Due to its greater number of adsorption sites and higher binding stability, NBC exhibits a superior adsorption performance compared to bulk biochar. The dominant adsorption mechanisms include physical adsorption, electrostatic interactions, π–π interactions, and apparent ion exchange (Figure 8).

4.2. Nitrous Oxide Emissions Reduction Mechanism

This study confirmed that nano-biochar (NBC) demonstrates significant advantages in reducing soil N₂O emissions and retaining inorganic nitrogen content. Compared to the control (CK), NBC reduced N₂O emissions by 11.60–54.77% at equivalent application rates, which was substantially more effective than the reduction achieved by the BC treatment group (7.53–47.69%) (Figure 4). These findings align with the review by Liu et al., which reported that bulk BC can reduce soil N₂O emissions by an average of 32% [21]; notably, the NBC treatment in this study demonstrated an even greater mitigation effect.

The high mitigation efficiency of NBC may be attributed to its unique physicochemical properties, including a larger specific surface area, well-developed pore structure, higher aromaticity, and a greater abundance of acidic functional groups. These structural features endow NBC with a superior adsorption capacity and significantly influence nitrogen speciation and mobility in soils. Specifically, NBC provides more binding sites for NH₄⁺ and NO₃⁻, thereby reducing the availability of substrates for nitrifying and denitrifying microorganisms, and ultimately suppressing N₂O production. Experimental data show that following NBC application, soil NH₄⁺-N concentrations increased by 2.4% to 8.2%, while NO₃⁻-N concentrations increased by 7.3% to 59.0%, both significantly higher than those observed for bulk biochar (Figure 7), further supporting its superior performance in inorganic nitrogen retention. Nguyen et al. noted that biochar application can markedly reduce the availability of reactive nitrogen in soils, thereby suppressing N₂O emissions [76]. Similarly, Chen et al. reported that applying acid-modified biochar in paddy fields significantly decreased N₂O emissions by 18–34.4%, accompanied by a notable accumulation of NH₄⁺-N and NO₃⁻-N [77]. These results collectively suggest that NBC exerts significant greenhouse gas mitigation potential by regulating the dynamic transformation of inorganic nitrogen. However, some studies have reported that excessive concentrations of inorganic nitrogen may instead stimulate nitrification and denitrification, thereby enhancing N₂O emissions [78,79]. In this context, NBC—owing to its excellent nitrogen adsorption capacity—can prevent excessive accumulation of inorganic nitrogen while limiting substrate availability, thereby effectively controlling N₂O formation. This “nitrogen immobilization and substrate control” mechanism provides theoretical support for the observed mitigation effect.

In addition to physical adsorption, the pH-regulating capacity of NBC is also a critical contributor to its mitigation effect. Due to its high contents of ash, carbonates, and residual organic acids, NBC application can elevate soil pH, thereby enhancing the activity of nitrous oxide reductase (N₂OR) and promoting the complete reduction of N₂O to N₂ [80,81]. Moreover, NBC improves soil aggregate structure and pore aeration, weakening anaerobic conditions during denitrification and further suppressing N₂O formation [82]. Under alternate wetting and drying (AWD) conditions, biochar treatments are often affected by the release of labile organic carbon from microbial death, which can trigger N₂O emissions peaks [83]. However, this study found that NBC maintained a stable mitigation performance under AWD conditions. Notably, on day 17, the 3% NBC treatment exhibited a high N₂O suppression rate, indicating strong regulatory capacity and environmental adaptability under fluctuating soil moisture regimes.

In summary, compared to BC, NBC exhibits enhanced multifunctionality in both N₂O emissions reduction and inorganic nitrogen retention, along with advantages such as lower application rates, faster response times, and more comprehensive mechanisms of action. Its multifaceted mechanisms—comprising substrate adsorption, pH regulation, soil structural improvement, and microbial metabolic suppression—highlight its broad potential for application in agricultural greenhouse gas mitigation and nutrient management.

4.3. Application Potential of Nano-Biochar

Moreover, NBC can reduce nitrogen leaching [84]. Rashid et al. (2023) reported that the co-application of NBC containing 10% of the nitrogen application rate with vermicompost reduced ammonia emissions by 43% [85]. This suggests that NBC not only reduces nitrogen loss through fixation, but also significantly enhances nitrogen use efficiency in crops. Previous studies have shown that plants treated with NBC exhibit improved growth vigor and stress resistance, including increased plant height, leaf area, and yield [85,86]. Most life cycle assessments (LCAs) are conducted with a primary focus on environmental impacts [87]. According to a life cycle assessment (LCA) by Sahoo et al. (2023), using forestry residues as feedstock and applying biochar locally via portable pyrolysis units can yield a net greenhouse gas reduction of approximately 0.89–2.6 t CO₂ eq⁻¹ per ton, even without considering the dosage advantages of nanonization [88]. Upon further size reduction to the nanoscale, NBC exhibits significantly enhanced surface area and reactivity, allowing application rates to be reduced from one-third to one-fifth of conventional biochar. This reduction effectively minimizes total application volume, land use, and transport-related emissions while maintaining or even enhancing environmental performance. Although nanonization incurs additional energy consumption, its contribution to the total carbon footprint of the final product is controlled in the range of 15–25% [89], which is lower than that of most metal oxide nanomaterials, indicating favorable environmental and economic adaptability. Therefore, the application of NBC in agricultural soils offers dual benefits of enhanced crop productivity and environmental performance, providing vital support for future soil management and ecological improvement.

5. Conclusions

NBC demonstrated a substantially superior capacity for NO₃⁻-N adsorption under acidic soil conditions compared to traditional bulk BC, achieving nearly a two-fold increase in sorption performance. This enhancement can be primarily ascribed to NBC’s enlarged specific surface area and pore volume, the abundance of surface acidic functional groups, and elevated aromaticity, which collectively reinforce adsorption processes through multiple mechanisms, including physical adsorption, electrostatic interactions, π–π interactions, and apparent ion exchange. In addition to enhancing inorganic nitrogen retention, NBC significantly suppresses N₂O emissions, potentially due to mechanisms including substrate immobilization, pH buffering, and improved soil aggregate stability. Compared to conventional BC, NBC demonstrates superior performance in both nitrogen retention and greenhouse gas mitigation. However, its long-term stability and underlying mechanisms require further validation through field experiments.