The Sustainable Management of Nitrogen Fertilizers for Environmental Impact Mitigation by Biochar Applications to Soils: A Review from the Past Decade

Abstract

This review assesses biochar’s potential to mitigate nitrogen (N) losses when co-applied with N fertilizers, emphasizing mechanisms linked to its measurable physicochemical properties. The mitigation of ammonia (NH₃) volatilization shows variable effects from its cation exchange capacity (−21.7% to 20.4%) and specific surface area (SSA; −23.8% to 39.1%). However, the biochar pH (influencing mitigation from −45.0% to −9.0%) and application rate are key factors, with clayey soils exhibiting the greatest mitigation (−52.2%), potentially due to their high bulk density. High SSA biochar, often from high pyrolysis temperatures, reduces nitrate-N (NO₃⁻-N) leaching (up to −26.6%) by improving the soil’s water-holding capacity. A co-application with organic fertilizers shows a pronounced mitigation (up to −39.0%) due to a slower N release coupled with biochar adsorption. A high SSA also plays an important role in mitigating nitrous oxide (N₂O) emissions (up to −25.9%). A higher biochar C/N ratio promotes microbial N immobilization, contributing to N₂O reductions (+1.5% to −34.2%). Mitigation is greater in sandy/loamy soils (−18.7% to −7.9%) than in clayey soils, where emissions might increase (+18.0%). Overall, biochar applications demonstrate significant potential to mitigate N losses and improve N use efficiency, thereby supporting sustainable agriculture; however, its effectiveness is optimized when biochar properties (e.g., high SSA and appropriate C/N ratio) and application strategies are tailored to specific soil types and N sources.

1. Introduction

Nitrogen (N) fertilizer is a common fertilizer in modern agriculture and is used in large quantities to meet the increasing food demand of a growing world population [1]. Since the global spread of inorganic N fertilizer, the world population has continued to increase explosively along with the dramatic increase in the crop yield per acre of agricultural land, reaching a peak of about 9.7 billion in 2050 and 10.4 billion in 2080, and is projected to remain at that level until 2100 [2]. However, the intensive use of chemical N fertilizers, such as urea-, ammonium-N (NH₄⁺-N)-, and nitrate-N (NO₃⁻-N)-based fertilizers, has led to pressing environmental concerns. These include ammonia (NH₃) volatilization contributing to air pollution, nitrate (NO₃⁻-N) leaching causing groundwater contamination and eutrophication, and emissions of nitrous oxide (N₂O), a potent greenhouse gas with a global warming potential approximately 265–298 times that of carbon dioxide (CO₂) [3,4,5]. These N losses from the soil not only pose environmental risks but also reduce the N use efficiency (NUE) of crops [6], challenging the sustainability of agricultural systems.

Compared to chemical fertilizers, organic fertilizers have a lower environmental impact, prevent the destruction of soil microflora, and provide a continuous supply of nutrients over a long period [7,8,9]. However, organic fertilizers have some disadvantages, including a nutrient imbalance, difficulty in storage and handling, and limited availability of feedstock, and thus have not become as popular as chemical fertilizers [10,11]. Therefore, it is necessary to find more sustainable ways to improve crop NUE while mitigating N losses from the soil regardless of fertilizer type.

Biochar, a carbonaceous material produced from the pyrolysis of organic materials under anoxic conditions [12], has garnered increasing attention as a soil amendment with the potential to mitigate these N-related environmental issues. Previous studies indicate that biochar applications can reduce NH₃ and N₂O gas emissions and decrease NO₃⁻-N leaching [6,13,14], thereby improving soil fertility and potentially increasing crop yields when co-applied with fertilizers [15,16].

Despite these promising findings, a clear and comprehensive understanding of the mechanisms by which biochar mitigates various N losses, particularly focusing on the role of its specific physicochemical properties, remains an area requiring further synthesis, especially from research conducted over the past decade. An earlier review published about 10 years ago touched upon the exhaustive N loss mitigation effects of biochar but lacked a detailed data analysis, reporting mainly general tendencies. More recent reviews and meta-analyses have often concentrated on individual N loss pathways, such as NH₃ volatilization, NO₃⁻-N leaching, or N₂O emissions, rather than a simultaneous, integrated assessment. Consequently, there is a limited number of studies that comprehensively evaluate the effects of biochar on all three major N loss pathways when co-applied with N fertilizers and critically link these effects to a range of biochar’s inherent physicochemical properties based on recent quantitative data.

This review aims to address this knowledge gap by systematically evaluating the interaction between pristine biochar and N fertilizers across multiple N loss pathways (NH₃ volatilization, NO₃⁻-N leaching, and N₂O emissions), based on quantitative data from studies published between 2014 and 2023. The novelty of this study lies in its focused analysis of how biochar’s overall physicochemical characteristics—such as its pH, cation exchange capacity (CEC), specific surface area (SSA), total carbon (TC), total nitrogen (TN), C/N ratio, and ash content—influence its N loss mitigation performance. By examining these relationships, this research seeks to provide a new, integrated understanding of how different biochar feedstocks and pyrolysis conditions affect N retention and loss. Ultimately, this study offers insights into optimizing biochar applications for an improved N use efficiency and reduced environmental impacts, contributing to the development of more sustainable agricultural practices.

The subsequent sections of this review are structured as follows: Section 2 details the methodology for the data compilation and analysis. Section 3, ‘Results and Discussion’, forms the core of this paper. It begins with an analysis of biochar physicochemical properties based on feedstock types (Section 3.1). It then systematically evaluates the mitigation effects of biochar on NH₃ volatilization (Section 3.2), NO₃⁻-N leaching (Section 3.3), and N₂O emission (Section 3.4). Each of these subsections is further divided to discuss the influence of biochar’s intrinsic physicochemical properties first, followed by the impact of various experimental conditions. Principal component analyses (PCA) are also presented within these sections to elucidate complex interactions. Finally, Section 4 provides overall conclusions and suggests directions for future research.

2. Materials and Methods

2.1. Data Compilation

Data on changes in NH₃ volatilization, NO₃⁻-N leaching, and N₂O emission by biochar application were collected by searching published literature on the Scopus and Google Scholar databases using combinations of keywords of “biochar” AND “ammonia” OR “NH₃” AND “volatilization” AND “nitrate” OR “NO₃⁻” AND “leaching” AND “nitrous oxide” OR “N₂O” AND “emission” in the title field and “soil” in the topic field for the period between 2014 and 2023 (for 10 years). The initial search on Scopus and Google Scholar databases yielded approximately 450 articles. After screening titles and abstracts for relevance to the keywords and scope, around 150 articles were selected for full-text review before applying the detailed inclusion/exclusion criteria outlined below. Among those, studies that satisfied the following conditions were finally included in this review: studies (i) included at least three replicates per treatment; (ii) contained paired control (fertilizer alone) and biochar application (fertilizer + biochar); (iii) reported cumulative NH₃ volatilization, NO₃⁻-N leaching, or N₂O emission data with experimental-unit area; (iv) conducted by pristine (i.e., unmodified) biochar without chemical and physical modifications; and (v) written in English. Conversely, studies were excluded if they: (a) were themselves review articles or meta-analyses; (b) did not provide quantitative, replicable data for both control (fertilizer alone) and biochar co-application treatments regarding cumulative N losses; (c) utilized biochar that had undergone significant chemical or physical modifications beyond typical pristine production (as this study focused on pristine biochar); (d) lacked clear reporting of essential experimental details such as biochar application rates or fundamental production conditions relevant to its properties; or (e) were not published in English. This focused selection aimed to enhance comparability and ensure the reliability of the synthesized data. Based on these inclusion/exclusion criteria, 20 studies for NH₃ volatilization (n = 101), 29 studies for NO₃⁻-N leaching (n = 102), and 23 studies for N₂O emission (n = 100) were selected. Here, “n” refers to the total number of individual data points (observations or treatment means from replicated experiments) extracted from the compiled studies for each respective N loss pathway. This study focused exclusively on pristine biochar. Modified biochar often possesses specific chemical or physical properties (e.g., altered surface functional groups or attached nanoparticles) that can complicate its effects on N loss. In contrast, pristine biochar retains the characteristics typically obtained under standard production conditions, making it more suitable for elucidating the fundamental relationship between biochar properties and soil N dynamics. By adopting this criterion, we aimed to enhance the comparability between studies and ensure that the interpretation of results remains consistent and reliable. Data were collected from tables and figures of these selected studies. The selected total of 72 studies were listed in

Table 1. Selected articles in data analysis.

Index	Experimental Types	References
NH3 volatilization	Incubation	[17,18,19,20,21,22,23]
	Column	[24,25,26]
	Pot	[27,28,29]
	Chamber	[30,31]
	Field	[32,33,34,35,36]
NO3−-N leaching	Incubation	[19,37]
	Column	[26,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52]
	Pot	[27,53,54,55,56]
	Field	[57,58,59,60,61,62]
N2O emission	Incubation	[63,64,65,66,67,68,69,70,71,72,73,74]
	Pot	[29,75]
	Column	[25,47]
	Field	[61,62,66,76,77,78,79,80]

.

When collecting data, if actual measured values were described in the text, those values were collected. When actual measured values were not described in the text, data were extracted directly from the figure using WebPlotDigitizer (version 4.7) (available at https://automeris.io/WebPlotDigitizer.html; accessed on 10 September 2024). This tool is widely utilized as an efficient and accurate method for extracting numerical data from published graphs and figures [81]. To ensure accuracy, we conducted the following validation steps: First, we used graphs with known numerical data and extracted the data using WebPlotDigitizer. The extracted values were then compared with the original data, confirming a margin of error within ±0.1%. Additionally, all data extraction processes were independently performed by two researchers, and their results were cross-checked to ensure precision. These measures enhance the reliability of the extracted data. Either the mean or experimental unit-level data were collected depending on data availability. In this case, biochar application rates per unit area were calculated using the area of the experimental vessel and the amount of biochar applied. Soil texture was largely classified into the following three categories: clayey (clay, silty clay, and sandy clay), loamy (silt, sandy clay loam, slay loam, silty clay loam, loam, and silty loam), and sandy (sand, loamy sand, and sandy loam) soils following the methodology from previous study [82].

2.2. Data Analysis

Changes in NH₃ volatilization, NO₃⁻-N leaching, and N₂O emission with biochar treatments to the corresponding control treatments (i.e., fertilizer alone) (referred to as “reduction index (R_index)” hereafter) were calculated with the following Equation (1):

$${\mathrm{R}}_{\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}}(\mathrm{\%})=\left[\left({\mathrm{V}}_{\mathrm{B}\mathrm{T}}-{\mathrm{V}}_{\mathrm{C}\mathrm{T}}\right)/{\mathrm{V}}_{\mathrm{C}\mathrm{T}}\right]\times 100$$

(1)

where V_BT represents NH₃ volatilization, NO₃⁻-N leaching, and N₂O emission in biochar treatment and V_CT represents that of fertilizer alone condition (control), respectively [82]. Positive or negative R_index values indicate that N loss is enhanced or mitigated by biochar soil application compared to control, respectively.

Differences in R_index among biochar pyrolysis temperature, biochar physicochemical properties (pH, CEC, specific surface area (SSA), total C (TC), total N (TN), C-to-N ratio (C/N ratio), and ash content), biochar application rate to soil, experimental types, fertilizer types, and soil type (texture and initial pH) were assessed by one-way analysis of variance (ANOVA) using the statistical software Statistica 6.1 (StatSoft. Inc., Tulsa, OK, USA). A Tukey honestly significant difference (HSD) analysis was performed for multiple comparisons of the treatment effects. Statistical significances were determined at p < 0.05. Principal component analysis (PCA) was performed to investigate the important components in the large data set, and the parameters for biochar physicochemical properties were introduced as the analysis variables in the PCA. Correlations among biochar physicochemical properties were computed with Pearson’s two-tailed test at p < 0.05. Pearson correlation analysis was distinguished by biochar feedstock, and it was performed for lignocellulosic feedstock (n = 92) and non-lignocellulosic feedstock (n = 31). The PCA and Pearson correlation analysis and graph creation were both conducted by Origin software (OriginPro 2024).

3. Results and Discussion

3.1. Biochar Physicochemical Properties

The physicochemical properties of biochar have been found to be highly dependent on the feedstock type and pyrolysis temperature [83]. In this study, biochar produced from lignocellulosic (e.g., wood, agricultural residues, herbaceous plants, etc.) and non-lignocellulosic feedstock (e.g., animal manure, municipal waste, microbial biomass, etc.) was broadly classified and evaluated.

Firstly, for lignocellulosic feedstock, the pyrolysis temperature of the biochar showed a significant positive correlation with the pH (r = 0.36; p < 0.001), SSA (r = 0.51; p < 0.001), and C/N ratio (r = 0.30; p < 0.01; Figure 1). As the pyrolysis temperature of the biochar increases, the pH increases due to the enrichment of alkaline components (K, Ca, Mg, etc.), especially carbonates, and the decomposition of surface acidic functional groups on biochar [84]. The SSA and porosity of the biochar have also been found to vary significantly with the pyrolysis temperature, probably due to the decomposition and/or volatilization of organic matter and the formation of micropores [85]. In addition, the aromaticity of C in biochar increases with increasing pyrolysis temperatures, while N decreases with volatilization, resulting in an increase in the C/N ratio [86,87]. Similarly, for biochar derived from non-lignocellulosic feedstock, the pyrolysis temperature was very strongly and significantly positively correlated with the pH (r = 0.69; p < 0.001) and C/N ratio (r = 0.48; p < 0.01; Figure 2). However, differently from the case of the lignocellulosic feedstock, the positive correlation between the pyrolysis temperature and SSA (r = 0.44) was not significant. This suggests that, compared to lignocellulosic feedstocks, the structure in non-lignocellulosic feedstock is fragile without structured cell walls and is heterogeneous with a high ash content and that pores do not simply develop as the pyrolysis temperature increases [88]. This could be supported by the non-significant but positive correlation between the pyrolysis temperature and ash content (r = 0.54). A negative correlation was observed between the pyrolysis temperature and TN (r = −0.40; p < 0.05). This significance was because biochar made from non-lignocellulosic feedstock, such as animal manure, generally contains a high amount of N compared with that of lignocellulosic feedstock, and the N reduction amount is proportionally high with the increasing pyrolysis temperature.

In lignocellulosic feedstock biochar (Figure 1), highly significant positive and negative correlations were found between the C/N ratio and TC (r = 0.47; p < 0.001) and the C/N ratio and TN (r = −0.54; p < 0.001). On the other hand, for non-lignocellulosic feedstock biochar (Figure 2), there was no correlation between the C/N ratio and TC (r = −0.011) and there was a significant negative correlation between the C/N ratio and TN (r = −0.53; p < 0.01). This result suggested that in lignocellulosic biochar the increase in C and decrease in N directly affect the C/N ratio, while in non-lignocellulosic biochar the increase or decrease in C is not relevant, and the decrease in N contributes significantly to the C/N ratio. Therefore, although there was a significant positive correlation between the pyrolysis temperature and C/N ratio for both biochar feedstock types (r = 0.30 to 0.48; p < 0.01; Figure 1 and Figure 2), the causes of the change in the C/N ratio were different.

3.2. Mitigation of Ammonia Volatilization

3.2.1. Effect of Biochar Physicochemical Properties

It is well known that the decrease in the NH₃ volatilization was related to the change in soil physicochemical properties and microbial activities [34]. From the results, the biochar pyrolysis temperature increased from low to high and the R_index also increased (−34.4% to −9.2%; Figure 3a), indicating that the effect of biochar on the NH₃ volatilization mitigation was weakened. This is probably due to the influence of the biochar pH. In general, NH₃ volatilization would easily occur with soil pH increases [89]. Biochar has a high pH and when applied to soil, a liming effect (i.e., increasing soil pH) would immediately occur. In fact, in this study, it was shown that biochar with a high pH increased the R_index, which was a similar trend to the pyrolysis temperature (Figure 3b). In this study, there was a significant positive correlation between the biochar pyrolysis temperature and pH (Figure 1 and Figure 2), clearly suggesting that the biochar pH was closely related to the increasing NH₃ volatilization. More specifically, the biochar pH is highly related to the ash content in biochar [90]. In this study, the NH₃ volatilization was significantly enhanced when the biochar ash content was more than 16% (25.1%), compared to less than 15% (−52.2%; Figure 3h), suggesting that the effect of the pH change was substantial for the NH₃ volatilization.

A previous study concluded that biochar applications increased the soil CEC, leading to a mitigated NH₃ volatilization because of the increased NH₄⁺-N adsorption (i.e., retention) capacity compared to soil without biochar [17]. In addition, another previous study also reported that the biochar SSA was probably responsible for the NH₄⁺-N adsorption and mitigating the NH₃ volatilization [22]. Based on this knowledge, we hypothesized that biochar with a high CEC and SSA would improve the soil N retention capacity through adsorption and could decrease the NH₃ volatilization. However, in this study, the biochar CEC (Figure 3c) showed a negative R_index only below 20 cmol₊ kg⁻¹ (−21.7%), while biochar with a high CEC (above 21 cmol₊ kg⁻¹) conversely enhanced the NH₃ volatilization (2.3% to 20.4%). The biochar’s SSA (Figure 3d) was −23.8% and −7.7% below 51 m² g⁻¹ and above 101 m² g⁻¹, respectively, while a SSA of 51–100 m² g⁻¹ substantially enhanced the NH₃ volatilization (39.1%). These results showed different tendencies from our hypothesis. These results probably suggested that soil applications of biochar with a high CEC and SSA would not always contribute to the NH₃ volatilization mitigation because the soil pH could increase due to the high pH of the biochar. There may be an inexplicable and subtle equilibrium between the biochar CEC and SSA, the soil pH, and the NH₃ volatilization upon biochar applications to the soil. According to the boosted regression tree (BRT) analysis from a previous study [91], the soil pH (18.3%) and biochar pH (16.4%) were the first and second most important explanatory variables for the NH₃ volatilization response to biochar applications, respectively. Indeed, several previous studies have observed that a rapid increase in the soil pH after a biochar application enhanced the NH₃ volatilization [19,24,31]. However, a previous study also reported that biochar characteristics (TC, SSA, and CEC) and the biochar application rate are equally important variables for NH₃ volatilization [91].

The biochar C/N ratio and R_index results (Figure 3g) for biochar showed that it mitigated NH₃ volatilization in all ranges, but the effect weakened as the C/N ratio increased (−22.3% to −12.0%). This was possibly because of the decreased nitrifying microbial activity. From a previous study, an increase in the soil C/N ratio inhibits nitrification [92]. A previous study reported that under N fertilization, nitrification rates, ammonia-oxidizing bacteria (AOB), and ammonia-oxidizing archaea (AOA) gene copy numbers responded negatively to biochar applications [93]. They concluded that the biochar application suppressed the nitrification in soil by altering the AOB and AOA community structure and decreased the relative abundance of key functional ammonia-oxidizers, such as Nitrosospira, Nitrosomonas, and Nitrosopumilus. Therefore, it was considered that the application of biochar with a high C/N ratio in this study weakened the mitigation effect for the NH₃ volatilization due to the prolonged NH₄⁺-N residence time by inhibiting nitrification. In addition, a previous study documented that the application of biochar with a high C/N ratio (i.e., produced by a high pyrolysis temperature) in soil could increase the soil pH and simultaneously increase the NH₃ volatilization [21].

Increasing the biochar application rate (Figure 3i) reduced the NH₃ volatilization up to a moderate level of application (≦50 t ha⁻¹; −13.3% to −27.8%), but conversely enhanced the NH₃ volatilization at amounts above a certain level (51 t ha⁻¹≦; 7.6%). A previous study reported that increasing the biochar application rate from 10 to 60 t ha⁻¹ in paddy fields significantly enhanced the NH₃ volatilization [24]. They concluded this phenomenon was mainly due to the increased pH of the surface floodwater and soil and the suppressed nitrification process induced by the biochar application. Another possible mechanism for the increased NH₃ volatilization with increasing biochar application rates could also be attributed to the improved gas exchange capability of the soil due to the drastic reduction in the soil bulk density (BD) and the concomitant increase in the pore connectivity caused by a large amount of biochar applications [28,94]. This study suggested that a biochar application of up to 50 t ha⁻¹ is suitable for the mitigation of NH₃ volatilization because it prevents excessive soil pH increases and soil BD decreases.

For the PCA, the Pearson correlation coefficient between the NH₃ volatilization R_index and PC1 was 0.328 (p < 0.001). Thus, the PCA results were applied with PC1 and PC2 as the x-axis and y-axis, respectively (Figure 4). In this scores plot (Figure 4a), data on the R_index in NH₃ volatilization due to biochar applications were shown as points. When PC1 was a positive value, the data indicate an increasing NH₃ volatilization due to the biochar application, while when PC1 was a negative value, the data indicate a decrease in the NH₃ volatilization due to the biochar application. From the results of the vectors plot (Figure 4b), PC1 was related to the biochar ash content, C/N ratio, pyrolysis temperature, pH, CEC, SSA, and, slightly, to the biochar application rate. These arrows indicated the contributions of different variables to the main components, and the vector length corresponded to the importance of the variables. The scores plot result (Figure 4a) showed that PC1 and PC2 tended to have a high R_index in the positive value areas (i.e., the upper-right corner of the graph). It was shown that the biochar C/N ratio, CEC, pyrolysis temperature, and pH were particularly important vectors for this range (Figure 4b). This result indicated that these factors may have negatively influenced the NH₃ volatilization mitigation. On the other hand, the TN showed negative values in PC1 and a completely opposite direction to the ash content. This result suggested that the biochar TN may have had a positive effect on the NH₃ volatilization mitigation. These results were almost consistent with Figure 3.

3.2.2. Effect of Various Experimental Conditions

The effects of various experimental conditions on the NH₃ volatilization from biochar applications were investigated (Figure 5). The experimental types (Figure 5a) showed the NH₃ volatilization-mitigating effects in the incubation (−31.5%), chamber (−20.7%), and field (−8.4%), while the column (30.8%) and pot (29.0%) showed NH₃ volatilization-enhancing effects. Although the overall NH₃ volatilization mitigation effect (whole) was shown at −16.1%, these results indicate that the effect of biochar applications on NH₃ volatilization varies greatly depending on the experimental conditions. Therefore, it cannot be generalized that the application of biochar causes NH₃ volatilization mitigations at all times.

The co-application of the organic fertilizer and biochar was more effective in mitigating NH₃ volatilization (−33.3%) than that of the chemical fertilizer (−14.1%; Figure 5b). This may be because the chemical fertilizers in this study had a higher number of urea samples (n = 79 out of 88 for chemical fertilizer). Urea initially increases the soil pH around granules due to the formation of carbonate during hydrolysis, following the equation of [95]:

$$\mathrm{C}\mathrm{O}{\left({\mathrm{N}\mathrm{H}}_2\right)}_2+2{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{N}\mathrm{H}}_4^{+}+{\mathrm{C}\mathrm{O}}_3^{2-}$$

(2)

In addition, because the activity of urease, a soil enzyme for the hydrolysis of urea, increases when the soil pH is alkaline [96], the increased pH caused by the biochar may have accelerated hydrolysis. Therefore, urea was considered less effective in mitigating the NH₃ volatilization by biochar due to its tendency to easily and rapidly generate NH₃ after fertilization.

The NH₃ volatilization R_index results for different soil textures showed significant differences among soil types (Figure 5c). The maximum NH₃ volatilization mitigation effect of −52.2% was observed on clayey soils and smaller effects of −26.6% on sandy soils and −12.7% on loamy soils. This may be attributed to the change in the magnitude of the reduction in the soil BD due to the biochar application. As described above, when the soil BD is reduced, the soil gas exchange capability increases. The NH₃ volatilization mitigation effect of biochar may have been highly influenced by the soil’s original BD, assuming that the biochar application changes the soil BD in similar magnitudes in different textured soils. Therefore, the R_index in clayey soils was more pronounced because of their higher original BD and cohesive structure compared to loamy and sandy soils. A similar result was observed in a previous study [91].

R_index results with a different initial soil pH did not change significantly (Figure 5d). The R_index was −11.0% for acidic soils (below pH 6.9), while it was −15.5% for neutral to alkaline soils (above pH 7.0). As discussed earlier, it was shown that a greater NH₃ volatilization would be observed in soils with more biochar applications since the biochar pH was the primary factor in enhancing the NH₃ volatilization, but in this study, the results showed insignificant R_index values between acidic and alkaline soils. Therefore, the initial soil pH may not be an important factor, suggesting that the NH₃ volatilization mitigation effect was maintained even in alkaline soils.

3.3. Mitigation of Nitrate Leaching

3.3.1. Effect of Biochar Physicochemical Properties

When biochar was applied to soil, the R_index for the NO₃⁻-N leaching was generally negative regardless of physicochemical properties (Figure 6). In general, the amount of NO₃⁻-N leaching depends mainly on the water-holding capacity (WHC) of the soil, with a high WHC resulting low NO₃⁻-N leaching [97]. It is known that biochar applications to soil can significantly improve the soil WHC, but this soil WHC improvement effect varies greatly depending on biochar properties, such as the particle size, SSA, and porosity, with a smaller particle size and a larger SSA being more effective [98]. Therefore, it has been reported that biochar pyrolyzed at high temperatures resulted in a higher SSA and porosity than that at lower temperatures, which improved the soil WHC after the application to the soil [99]. In this study, R_index results for different biochar pyrolysis temperatures (−19.6% to −26.9%; Figure 6a) and SSAs (−17.6% to −22.6%; Figure 6d) showed a trend of a decreasing NO₃⁻-N leaching with increasing values in each parameter, even though there were no significant differences. Furthermore, the biochar SSA probably contributed to the adsorption and retention of NO₃⁻-N, which may have reduced the mobility of NO₃⁻-N in soil [100].

Due to changes in the biochar pH (Figure 6b), the R_index showed negative values (−10.0% to −28.2%) under all conditions, but particular trends and significant differences were not observed. Several previous studies have shown that biochar could have increased the soil pH and subsequent microbial colonization, thereby enhancing the soil microbial and enzyme activity and affecting soil N dynamics [62,96]. However, this study suggested that the change in the biochar pH was not an important factor for NO₃⁻-N leaching. In addition, there was no significant change in the R_index with differences in the biochar ash content (−11.6% to −12.3%; Figure 6h), which is closely related to the change in the biochar pH.

Due to the change in the biochar CEC, the R_index showed some differences ranging from −18.9% to −27.3%, but these were also insignificant (Figure 6c). Previous studies have suggested that biochar could adsorb NO₃⁻-N through pore filling, not through chemical adsorption, such as electrostatic interactions and ion exchange [101,102]. In general, biochar has a negative surface charge and is repelled by NO₃⁻-N in soil [103], suggesting that the effect of the biochar CEC may not be related to NO₃⁻-N leaching.

Among the biochar physicochemical properties in this study, only the biochar TC significantly affected the mitigation of NO₃⁻-N leaching, with the highest TC showing the highest mitigation effect (−18.4% to −33.8%; Figure 6e). In addition, although it was insignificant, a decreasing trend in the R_index was observed with increasing biochar C/N ratios (−15.4% to −24.9%; Figure 6g). These results may have attributed to the high C content of biochar creating soil aggregates. A previous study assessed the C content of different sizes of aggregates (>1000, 250–1000, 100–250, 53–100, and 25–53 µm) in soils affected by the application of biochar to clarify its potential and mechanism for mitigating NO₃⁻-N leaching from soils [37]. Their results showed that the biochar application increased the C content of soil aggregates between 53 and 1000 µm in size, which corresponded to the mesopores that could hold the available water in the soil. This was associated with a significant increase in the WHC (average increase of 9%) compared to the control, which may have affected the mitigation of the NO₃⁻-N leaching.

Another possible mechanism is a decrease in the soil BD, which was not included in the data set in this study. According to a previous study [61], the soil BD was lower in the biochar-applied soils (1.27–1.28 g cm⁻³) than that in the control (1.33 g cm⁻³). They discussed that the low BD possibly increased the soil WHC. Indeed, in this study, as the biochar application rate increased, the R_index showed a decreasing trend, which was especially significant at application rates above 51 t ha⁻¹ (Figure 6i). Therefore, it was suggested that the decrease in the soil BD improved the soil water retention and was effective in mitigating NO₃⁻-N leaching.

Other important perspectives on the effects of the soil microbial activity to mitigate NO₃⁻-N leaching were also suggested by a previous study [27]. They used a functional gene-specific qPCR (to investigate soil nitrifying bacteria) to elucidate the mechanism of the biochar NO₃⁻-N leaching mitigation by the co-application of digestate and biochar. Their results showed that in all soil layers, the amount of soil AOB was markedly reduced in the biochar-applied soil compared to the control, showing that NO₃⁻-N leaching was mitigated by the delayed nitrification due to a reduction in the soil nitrifying bacteria.

The PCA results showed a Pearson correlation coefficient of −0.468 (p < 0.001) between the R_index of the NO₃⁻-N leaching and PC3. Thus, the PCA results were applied with PC1 and PC3 as the x-axis and y-axis, respectively (Figure 7). This scores plot (Figure 7a) indicates R_index data regarding NO₃⁻-N leaching by biochar applications. In this case, due to the negative Pearson’s correlation (r = −0.468), the data points with positive values of PC3 indicate mitigated NO₃⁻-N leaching due to biochar applications, and conversely, the data points with negative PC3 values indicate enhanced NO₃⁻-N leaching. The results of the vectors plot (Figure 7b) showed that PC3 was strongly associated with the SSA and the application rate of the biochar. The results of the scores plot (Figure 7a) showed that PC1 and PC3 tended to have a lower R_index in the positive value range (i.e., the upper-right corner of the graph). Thus, the biochar application rate was a particularly important vector in this range (Figure 7b). The results suggested that the SSA and application rate of the biochar were negatively correlated with NO₃⁻-N leaching, indicating the promotion of mitigation.

3.3.2. Effect of Various Experimental Conditions

According to a previous study, results obtained under field conditions are often different from laboratory studies [104]. Results of the change in the R_index by different experimental types (Figure 8a) showed no significant differences among incubation, column, and field experiments (−10.7% to −21.5%), but a significant mitigation was only observed in the pot experiment (−39.4%). This difference in this study was probably because of variations in soil homogeneity and the influence of the root system. Firstly, the homogeneity of the soil can be obtained in pot experiments probably due to a homogeneous mixing of soil and biochar in the pots compared to other experiments, as well as the ease of moisture management. Secondly, regarding the effect of the root system in pot experiments, plant roots can likely be evenly distributed in the entire pots, and the effect of the biochar application on plant growth and NO₃⁻-N absorption by plant roots may be pronounced in pot experiments. Therefore, the NO₃⁻-N leaching mitigation is probably more pronounced in the pot than in other experiments. In fact, the results of a previous meta-analysis showed that the mean rate of change in crop productivity after biochar applications compared to a control was almost three times higher in pot than in field experiments, suggesting that the biochar NO₃⁻-N leaching mitigation effect may be more easily manifested in pot experiments [105].

The difference in fertilizer types (Figure 8b) showed a significant mitigation effect in organic fertilizers (−38.7%) than that in chemical fertilizers (−17.6%). Organic fertilizers undergo a microbial decomposition reaction in the soil that gradually releases inorganic N. Because this process is slow-acting, NO₃⁻-N does not increase rapidly in the soil which allows biochar to adsorb and retain NO₃⁻-N for a long period [106]. On the other hand, chemical fertilizers supply inorganic N directly and immediately, which causes a rapid increase in the NO₃⁻-N concentration and makes it difficult for biochar to adsorb NO₃⁻-N adequately.

For different soil textures, results (Figure 8c) showed that biochar applications resulted in an R_index of −17.6%, −19.2%, and −23.4% for clayey, loamy, and sandy soils, respectively. However, there were no significant differences among all soil textures. In general, sandy soils have a lower water- and nutrient-holding capacity than other soil types due to their physical characteristics [107]. Previous studies have suggested that biochar was more effective in mitigating NO₃⁻-N leaching when it was applied to sandy soils than to clayey soils, and this study showed a similar trend.

Differences in the initial soil pH had no significant influences on the R_index, yet negative R_index values were observed in both soils’ pH (−24.8% to −16.5%; Figure 8d). This means that biochar maintains a mitigation effect on NO₃⁻-N leaching regardless of the initial soil pH range, indicating that the soil pH may not be important to consider with biochar applications to soil.

3.4. Mitigation of Nitrous Oxide Emission

3.4.1. Effect of Biochar Physicochemical Properties

The R_index of N₂O from biochar soil applications was negative under almost all properties, indicating that biochar was effective in mitigating N₂O emissions (Figure 9). There was a mitigating trend in N₂O emissions with increasing biochar pyrolysis temperatures (−6.1% to −23.6%), but those differences were not significant (Figure 9a). Previous studies have suggested that the mitigation in N₂O emissions from biochar was due to the trapping of N₂O gas in the biochar pore structure [76,108]. Therefore, it was expected that the SSA was an important parameter for the N₂O emission mitigation, which is closely related to the biochar pore structure. The results of this study showed that biochar applications increased N₂O emissions when the biochar’s SSA was less than 50 m² g⁻¹ (4.1%), while a significant decrease was observed when the biochar’s SSA was more than 51 m² g⁻¹ (−25.9%), but this was not significant (Figure 9d).

Other possible mechanisms include the improved aeration of biochar-applied soils, which increases the oxygen availability and affects redox conditions in the soil and mitigates N₂O emissions by limiting the denitrifying bacteria and enzyme activity [70,76]. Soil aeration is closely related to the soil BD, and a biochar soil application generally improves aeration by significantly reducing the soil BD [109]. Therefore, it was expected that N₂O emissions would be mitigated as the biochar application rate increased. However, this study could not observe a clear trend or significant difference between the R_index and the biochar application rate, but the highest mitigation was observed for biochar applications of more than 51 t ha⁻¹ (−20.7%; Figure 9i).

Previous studies have shown a negative correlation between N₂O emissions and the soil pH [72]. The increase in the soil pH inhibits the reductase activity responsible for the conversion of NO₃⁻ and NO₂⁻ to N₂O and stimulates the N₂O reductase activity that catalyzes the reaction of N₂O to N₂ [110,111,112]. In addition to enzymes, it was also found that the application of biochar significantly increased the number of N₂O-reducing bacteria (e.g., nosZ denitrifiers), which promoted and accelerated the reduction of N₂O to N₂, resulting in mitigated N₂O emissions [113]. However, in this study, the N₂O emission mitigation effect weakened with an increasing biochar pH, although there was no significant difference (−24.7% to −6.2%; Figure 9b). This conflict results from the previous studies’ suggestion that an increased soil pH was not the primary mechanism for N₂O emission mitigation. As mentioned above, since the pyrolysis temperature was strongly positively correlated with the biochar pH (Figure 1 and Figure 2), the R_index of the pyrolysis temperature is expected to show similar results as that of the biochar pH. However, the R_index of these results showed an opposite tendency (Figure 9a,b). This controversy of the biochar liming effect may be explained by other biochar parameters, such as the TC (Figure 9e), TN (Figure 9f), and C/N ratio (Figure 9g). The R_index at the TC enhanced N₂O emissions below 500 g kg⁻¹ (3.5%) but significantly decreased above 501 g kg⁻¹ (−28.3% to −33.7%; Figure 9e). On the contrary, the R_index showed an increasing trend with the increasing TN (−30.8% to −6.8%; Figure 9f). Along with these changes, the R_index showed a clear decreasing trend as the C/N ratio increased (1.5% to −34.2%; Figure 9g). These are probably due to the promoted microbial immobilization and denitrification process in soil. When the soil C/N ratio is high, soil microorganisms uptake soil N, and N mineralization is suppressed in the soil. The resulting suppression of nitrification and denitrification processes may have mitigated N₂O emissions to some extent. It was also suggested that the high C/N ratio of biochar may have promoted a complete denitrification reaction for the remaining NO₃⁻-N since it acts as a source of organic C needed for denitrification. Based on these reactions, it was suggested that the R_index of the biochar pyrolysis temperature was more influenced by the results of the SSA (Figure 9d) and C/N ratio (Figure 9g) of biochar, which also correlated positively with the pyrolysis temperature (Figure 1 and Figure 2), rather than the biochar pH (Figure 9b).

PCA results showed a Pearson correlation coefficient of −0.235 (p < 0.05) between the N₂O emissions R_index and PC1. Thus, the PCA results were applied with PC1 and PC2 as the x-axis and y-axis, respectively (Figure 10). This scores plot (Figure 10a) provides R_index data points for the N₂O mitigation by biochar applications. In this case, based on the negative Pearson’s correlation, the data will show more mitigations in N₂O emissions due to biochar applications for positive PC1 values. Conversely, the data will show more points of increased N₂O emissions due to the biochar application for negative PC1 values. From the results of the scores plot (Figure 10a), it was observed that there was no clear division by the PC2 value, but two major distinctions were made by the PC1 value. The results of the vectors plot (Figure 10b) revealed strong relationships between the TC and SSA for biochar at positive values of PC1. Therefore, the TC and SSA of biochar were shown to be particularly important factors for the mitigation of N₂O emissions. Furthermore, the CEC, C/N ratio, and pyrolysis temperature were also suggested to be comparable mitigation factors subsequently. Contrastingly, the biochar ash content, TN, and pH were found to be factors that promoted N₂O emissions. These results are consistent with Figure 9.

3.4.2. Effect of Various Experimental Conditions

The variation in the R_index by experimental types revealed that all experimental types were effective in mitigating N₂O emissions, as the R_index was negative (−3.4% to −59.2%), but the differences were not significant (Figure 11a). The overall R_index (whole) was −18.2%, indicating that the effectiveness of the biochar was certain.

The co-application of the biochar and chemical fertilizer resulted in an R_index of −9.5%, while the co-application with the organic fertilizer resulted in −29.5% (Figure 11b), suggesting that the combination of the organic fertilizer and biochar was more effective in mitigating N₂O emissions, although this result was not significantly different. This is probably mainly due to the changes in nitrifying and denitrifying bacteria in the soil. A previous study found that the co-application of biochar and organic fertilizers to saline alkaline soils mitigated N₂O emissions due to reduced copy numbers of nirS and nirK (NO₂⁻-N reductase) and the increased copy number of nosZ (N₂O reductase) compared to the control [114]. Another possible mechanism is changes in the soil BD. For example, organic fertilizers, such as compost, have the effect of reducing the soil BD after the application compared to chemical fertilizers [115]. This suggests that the synergistic effect of fertilizers’ co-application with biochar may have changed the soil to a more aerobic condition and mitigated N₂O emissions.

R_index results for different soil textures (Figure 11c) showed that clayey soils enhanced N₂O emissions by 18.0%, while loamy and sandy soils mitigated emissions by −18.7% and −7.9%, respectively. The reason for the lack of a N₂O emission mitigation in clayey soils may have been due to the increased availability of the soil NH₄⁺-N caused by the interaction of biochar in clayey soils. In general, clayey soils have a high WHC due to their high CEC and specific pore volume, which allows water and nutrients to be retained in the soil for a long period [116]. In addition, biochar can adsorb soil NH₄⁺-N and improve its availability after the soil application [117]. These interactions may have increased the soil NH₄⁺-N concentration and provided an adequate N source directly to the microbial nitrification process, which may have resulted in the production of more N₂O during the nitrification process (i.e., nitrification-derived N₂O emission) in the biochar application [25,118]. Another possible reason is differences in soil physical structures. In general, the BD in clayey soils is high, and even with the application of biochar, the BD was less likely to decrease, suggesting that anaerobic conditions in clayey soils may have been maintained and masked the effect of the biochar N₂O emission mitigation.

N₂O reductase has been found to be inhibited when the soil pH is less than 6.1 to 6.8 [47]. The results of the R_index with changes in the initial soil pH (Figure 11d) did not show remarkable changes (−5.9 to −18.1%) and were not significantly different. This is probably due to the N₂O reductase activity enhanced by the increased soil pH caused by the biochar application to acidic soils. Therefore, the results suggested that the initial soil pH does not need to be considered as much when attempting to mitigate N₂O emissions using biochar.

4. Conclusions

This review systematically assessed the impact of a pristine biochar co-application with N fertilizers on mitigating NH₃ volatilization, NO₃⁻-N leaching, and N₂O emissions, drawing on quantitative data from studies published in the past decade.

For NH₃ volatilization, the biochar pH proved to be a dominant factor, with mitigation effects varying widely (from a −45.0% to −9.0% reduction based on pH); a high ash content (≧16%) significantly exacerbated volatilization (a 25.1% increase compared to the −52.2% mitigation with ≦15% ash). Moderate application rates (≦50 t ha⁻¹) were generally most effective for NH₃ mitigation (−13.3% to −27.8%), and the greatest reduction was observed in clayey soils (−52.2%).

Regarding NO₃⁻-N leaching, biochar produced at higher pyrolysis temperatures, often exhibiting a higher SSA, demonstrated an enhanced mitigation (up to a −26.6% reduction), primarily attributed to the improved soil water-holding capacity. Increasing the biochar application rate, particularly above 51 t ha⁻¹, significantly reduced NO₃⁻-N leaching. The co-application with organic fertilizers was notably more effective (up to −39.0% mitigation) than with chemical fertilizers, likely due to the slower N release allowing for enhanced biochar adsorption and retention. Pot experiments, where soil homogeneity and root system interactions are pronounced, showed particularly strong mitigation effects (−39.4%).

The biochar application was also effective in mitigating N₂O emissions. A higher SSA was linked to greater N₂O trapping within biochar pores and thus a more pronounced emission mitigation (up to 25.9% with an SSA ≧51 m² g⁻¹). Furthermore, increasing the biochar TC (e.g., ≧501 g kg⁻¹ leading to a −28.3% to −33.7% mitigation) and C/N ratio (e.g., ≧101 leading to −34.2% mitigation) significantly mitigated N₂O emissions, likely due to a promoted microbial N immobilization and a more complete denitrification process. However, the effect varied with soil textures; increased N₂O emissions were observed in clayey soils (+18.0%), while mitigation occurred in loamy (−18.7%) and sandy (−7.9%) soils, pointing to complex interactions with the soil NH₄⁺-N availability and physical structure.

Overall, this comprehensive review confirms that the biochar soil application is a promising technology for reducing N losses from agricultural soils and mitigating associated environmental impacts, thereby potentially enhancing NUE for crops and contributing to the development of sustainable agricultural systems. However, the findings critically underscore that the effectiveness of biochar is not universal. It depends significantly on a nuanced understanding and strategic matching of biochar’s physicochemical properties (e.g., SSA, pH, C/N ratio, and ash content) and application rates to specific soil types, fertilizer types, and the targeted N loss pathways.

To optimize biochar’s practical application and fully harness its potential, future research should prioritize several specific avenues: (i) Conducting long-term field studies across diverse agroecosystems to validate laboratory and short-term findings and to assess the persistence of biochar’s N mitigation effects over time. (ii) Undertaking deeper investigations into the microbial mechanisms, including functional gene shifts (e.g., nosZ denitrifiers) and community dynamics, that are driven by specific biochar properties under various N fertilization regimes. (iii) Developing predictive models based on an integrated understanding of biochar characteristics, soil types, climates, and management practices to guide customized biochar applications for targeted N loss mitigation. (iv) Exploring the synergistic effects of biochar when combined with other soil amendments or advanced agricultural management practices (e.g., controlled-release fertilizers and nitrification inhibitors) to further enhance N use efficiency and overall environmental benefits. Such focused research is crucial for translating the potential of biochar into reliable and effective solutions for sustainable N management in agriculture.