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Review

Biochar for Mitigating Nitrate Leaching in Agricultural Soils: Mechanisms, Challenges, and Future Directions

by
Lan Luo
1,
Jie Li
1,*,
Zihan Xing
1,
Tao Jing
2,
Xinrui Wang
1 and
Guilong Zhang
1,*
1
Agro-Environmental Protection Institute, Ministry of Agriculture and Rural Affairs, Tianjin 300191, China
2
Zhongnong Chuangda (Beijing) Environmental Protection Technology Co., Ltd., Beijing 100094, China
*
Authors to whom correspondence should be addressed.
Water 2025, 17(17), 2590; https://doi.org/10.3390/w17172590
Submission received: 23 July 2025 / Revised: 20 August 2025 / Accepted: 29 August 2025 / Published: 1 September 2025
(This article belongs to the Special Issue Advanced Research in Non-Point Source Pollution of Watersheds)
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Abstract

Nitrate leaching from agricultural soils is a major contributor to groundwater contamination and non-point source pollution. Controlling this loss remains challenging due to the complexity of soil–water–nutrient interactions under intensive farming practices. Biochar, a porous, carbon-rich material derived from biomass pyrolysis, has emerged as a promising amendment for nitrate mitigation. This review summarizes recent advances in understanding the roles of biochar in nitrate retention and transformation in soils, including both direct mechanisms—such as surface adsorption, ion exchange, and pore entrapment—and indirect mechanisms—such as enhanced microbial activity, soil structure improvement, and root system development. Field and laboratory evidence shows that biochar can reduce NO3−-N leaching by 15–70%, depending on its properties, soil conditions, and application context. However, inconsistencies in performance due to differences in biochar types, soil conditions, and environmental factors remain a major barrier to widespread adoption. This review also suggests current knowledge gaps and research needs, including long-term field validation, biochar material optimization, and integration of biochar into precision nutrient management. Overall, biochar presents a multifunctional strategy for reducing nitrate leaching and promoting sustainable nitrogen management in agroecosystems.

Graphical Abstract

1. Introduction

Nitrate nitrogen (NO3−-N) leaching from agricultural soils has emerged as a widespread and persistent environmental challenge, particularly in intensive agricultural systems [1,2]. Globally, it is estimated that 75 Tg of reactive nitrogen is lost from croplands annually, much of which is rapidly transformed into the highly mobile NO3− in soils [3,4]. This not only reduces the nitrogen use efficiency (NUE) but also causes nitrate accumulation in groundwater, leading to groundwater contamination, eutrophication, and public health risks, such as methemoglobinemia in infants [5,6,7]. Therefore, effective strategies to mitigate nitrate nitrogen loss while maintaining crop productivity are essential for achieving sustainable nitrogen management.
Among the various mitigation strategies, biochar, a carbonaceous material derived from the pyrolysis of biomass, has garnered significant attention for its multifunctional role in soil nutrient management [8]. Its high porosity, large specific surface area, and various surface functional groups allow biochar to interact with nitrogen ions via physical, chemical, and microbial pathways [9]. When applied to soils, biochar enhances the water-holding capacity and supports NO3−-N retention through mechanisms such as ion exchange, electrostatic attraction, and pore entrapment [10,11]. Field experiments by Liu et al. [11] demonstrated that applying 5% biochar reduced NO3−-N leaching by approximately 40%. Similarly, Sun et al. [12] found stronger nitrate immobilization under simulated rainfall. Cui et al. [13] further confirmed that biochar can stimulate microbial denitrification pathways by increasing the nirK gene abundance and reducing nitrate accumulation.
Despite its promising multifunctionality, biochar’s direct capacity to adsorb NO3−-N remains limited under many soil conditions, especially in neutral to alkaline environments [14]. Recent mechanistic research has revealed that while biochar can retain ammonium and organic nitrogen via cation exchange and surface complexation, its interaction with nitrate is considerably weaker due to inherent physicochemical constraints [15]. First, the negatively charged surfaces of most unmodified biochars lead to the electrostatic repulsion of NO3− ions, severely limiting their sorption potential [16,17]. Second, biochars typically exhibit low or negligible anion exchange capacities (AECs), in contrast to their well-documented cation exchange capacities (CECs), thereby failing to offer sufficient reactive sites for nitrate retention [18,19]. Moreover, the porous structure of biochar, though beneficial for moisture retention, lacks the micropore specificity required to physically immobilize mobile nitrate ions [20,21]. Additionally, competition from co-existing anions (e.g., Cl−, SO42−, HCO3−) in the soil solution can further inhibit NO3−-N adsorption by occupying limited sorption sites or altering the ionic balance [22,23]. These intrinsic limitations not only result in inconsistent NO3−-N mitigation across different soil types but also obscure the predictability of the biochar performance under field conditions. Consequently, the design and application of biochar for nitrate management must be informed by a mechanistic understanding of biochar–anion interactions, supported by modifications to enhance nitrate affinity or promote indirect biological pathways, such as denitrification facilitation [24,25]. Understanding these barriers is essential for translating biochar’s potential into scalable strategies for mitigating nitrogen losses in sustainable agricultural systems.
To address these challenges, recent studies have explored biochar modification techniques, including metal oxide doping (e.g., Fe, Mg, Ca) to improve anion affinity, co-application with organic ligands or bio-based polymers, and optimizing the pyrolysis conditions to tailor the surface chemistry [20,26,27]. Nevertheless, the long-term effectiveness and environmental safety of these approaches under diverse agro-ecosystems remain underexplored. Accordingly, this review aims to (1) clarify the mechanisms by which biochar adsorbs and immobilizes nitrate nitrogen in agricultural soils; (2) assess how biochar alters the soil physicochemical and microbial environment to regulate nitrogen cycling; and (3) evaluate its mitigation efficacy for nitrate leaching across varying soil textures, pH levels, and cropping systems, considering both the effective and ineffective biochar performance and the underlying mechanisms. By synthesizing these recent advances, this work provides a comprehensive foundation for designing precision-engineered biochar strategies for nitrate-leaching control and contributes to non-point source pollution mitigation in sustainable agriculture.

2. Bibliometric Insights into Trends of Biochar and Nitrate Nitrogen Research

The bibliometric analysis (Figure 1a,b) shows a clear and sustained global interest in the intersection of biochar and nitrate nitrogen over the past decade, as evidenced by the increasing number of publications. A total of 162 articles published in 2024 were analyzed, identifying 30 key research topics (Figure 1c). Most studies (42.6%) focus on nitrogen loss, nitrate leaching, and the effects of biochar on greenhouse gas emissions, ammonia oxidation, and crop yields. Keyword co-occurrence analysis revealed two dominant themes: the “immobilization” and “bioavailability” of nitrogen within the soil nitrogen cycle, highlighting biochar’s role in reducing nitrogen loss and improving fertilizer efficiency. Emerging links between “remediation” and “microbial diversity” also point to biochar’s potential in environmental restoration and biodiversity conservation (Figure 2).
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Figure 2. Keyword co-occurrence network analysis in biochar and nitrate nitrogen research (2013–2024), using VOSviewer (version 1.6.20). Circle size represents frequency of co-occurrence, with color indicating publication year (yellow for recent, blue for older). Key research themes include nitrogen immobilization, bioavailability, and environmental remediation.
Figure 2. Keyword co-occurrence network analysis in biochar and nitrate nitrogen research (2013–2024), using VOSviewer (version 1.6.20). Circle size represents frequency of co-occurrence, with color indicating publication year (yellow for recent, blue for older). Key research themes include nitrogen immobilization, bioavailability, and environmental remediation.
Water 17 02590 g002

3. Mechanisms of Biochar-Mediated Nitrate Retention: Direct, Indirect, and Feedback Pathways

Biochar retains nitrate through direct mechanisms such as physical adsorption and electrostatic interactions with surface functional groups and cations [28,29]. Indirectly, it modifies the soil pH, moisture, and redox conditions, thereby influencing microbial communities and nitrogen transformation processes, such as the dissimilatory nitrate reduction to ammonium (DNRA) [13,20,29]. Over time, biotic–abiotic feedbacks enhance these effects, stabilizing nitrate and reducing its environmental loss.

3.1. Direct Mechanisms: Physical Adsorption and Electrochemical Interactions

Biochar can directly mitigate nitrate leaching through physical adsorption, surface electrostatic attraction, and cation-mediated ion bridging, particularly in the early stages after application and in soils with low CECs or organic matter contents (Figure 3). These mechanisms serve as the first line of defense against nitrate mobility in biochar-amended systems.
One of biochar’s most prominent physical properties is its high specific surface area (SSA) and porous structure, which enable temporary nitrate retention through pore-filling and steric entrapment. Although nitrate (NO3−) exhibits weak adsorption due to its negative charge and low polarizability, meso- and microporous structures provide transient anchoring sites, particularly in low-ionic-strength conditions [23,30]. For example, pinewood-derived biochar prepared at 500 °C with an SSA of 300 m2/g achieved a maximum NO3−-N adsorption capacity of 22 mg/g, suggesting physical adsorption potential under optimized pore configurations [31]. However, due to electrostatic repulsion between negatively charged biochar surfaces and nitrate ions, physical adsorption alone is generally insufficient for sustained nitrate immobilization.
The electrochemical interaction mechanisms are closely related to the surface charge of the biochar, which is influenced by its point of zero charge (pHPZC). When the soil pH is lower than the biochar’s pHPZC, the surface becomes protonated, facilitating electrostatic attraction between positively charged sites and NO3− [28,29,32]. In contrast, in alkaline soils where pH > pHPZC, electrostatic repulsion dominates, weakening nitrate binding [33]. To overcome this limitation, surface modification techniques, such as metal doping (e.g., Mg2+, Fe3+), have been developed to introduce additional positively charged active sites [32,34]. For instance, Mg/Fe dual-modified biochar (MgFe@BC) significantly improved the NO3− retention, with enhanced electrostatic attraction and the formation of stable Fe–NO3− complexes [8]. Sathishkumar et al. [35] reported that Fe-doped biochar reached a maximum nitrate adsorption capacity of 45 mg/g, attributed to the combined effects of surface redox cycling and stable ternary complexation.
Ion bridging represents another electrostatic retention pathway, especially in calcium- and magnesium-rich environments. In this process, divalent or trivalent cations (e.g., Ca2+, Mg2+, Fe3+) serve as electrostatic intermediaries, forming ternary complexes, such as –COO−–Ca2+–NO3−, which stabilize nitrate at the biochar–soil interface [34,36]. This mechanism is particularly effective in calcareous soils and saline–alkali environments, where the abundance of free cations facilitates nitrate stabilization. Haider et al. [22] further demonstrated that carbonate-supported ion bridging significantly enhances nitrate retention in biochars derived from alkaline feedstocks.
Additionally, ligand exchange and functional group binding can contribute to NO3− retention. Though nitrate is a weak ligand and seldom forms strong inner-sphere complexes with biochar surfaces, outer-sphere interactions via exchange with hydroxide, phosphate, or water molecules at oxygen-containing sites (e.g., −OH, −COOH, −C=O) may allow for moderate affinity under certain ionic conditions [37,38]. Nitrogen-doped biochars containing amides or pyridinic N structures can further promote polar–polar interaction and enhance nitrate binding [36,37,39]. Surface oxidation pretreatments, such as HNO3 acid modification, KMnO4 oxidation, or steam activation, significantly increase the abundance and accessibility of surface functional groups, expanding the spectrum of potential nitrate-binding sites [39,40]. However, such enhancements are often transient and depend on the pH, redox potential, and competing anion concentrations.
In brief, these direct mechanisms primarily affect the initial immobilization of nitrate after biochar incorporation. Nevertheless, due to their declining effectiveness over time, it is essential to consider indirect pathways and microbial feedbacks, which are elaborated in the following sections.
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Figure 3. Mechanism of nitrate nitrogen adsorption by biochar [30,31,32,33,34,36,37,38,39]. Notes: ↑ indicates an increase; ↓ indicates a decrease.
Figure 3. Mechanism of nitrate nitrogen adsorption by biochar [30,31,32,33,34,36,37,38,39]. Notes: ↑ indicates an increase; ↓ indicates a decrease.
Water 17 02590 g003

3.2. Indirect Mechanisms: Soil Property Alteration

Besides direct adsorption, biochar can mitigate nitrate leaching through indirect pathways by altering the soil physical and chemical properties, including the nutrient retention capacity, water dynamics, and structural stability (Figure 4). The systemic modulation becomes especially important under long-term or field-scale conditions, where nitrate behavior is governed more by system-level soil processes than by surface adsorption alone [23,34,41].
A key pathway is the enhancement of the cation exchange capacity (CEC). Due to its persistent surface charge and stable aromatic structure, biochar increases the soil’s ability to retain positively charged ammonium (NH4+)—a key precursor to nitrate [41,42]. By improving NH4+ retention, biochar indirectly suppresses its conversion to nitrate via nitrification, thereby lowering nitrate accumulation. This process is particularly valuable in coarse-textured or degraded soils with low intrinsic CECs [39,43]. Moreover, biochar increases the soil water-holding capacity and modifies the hydrodynamic behavior. Its porous framework contributes to better moisture retention, which slows infiltration and prolongs nitrogen residence time, allowing for greater crop uptake or microbial immobilization [44,45]. Moreover, biochar can reduce preferential flow pathways and modify hydraulic conductivity, effectively minimizing the rapid nitrate movement following irrigation or rainfall events [42,45].
In addition, biochar plays a role in reinforcing the soil structural stability, particularly by promoting aggregate formation and reducing the bulk density. Stable soil aggregates limit macro-pore connectivity and improve the physical entrapment of water and solutes, creating a more diffusive transport environment that reduces nitrate leaching [46,47]. These physical changes complement rather than duplicate the direct-sorption mechanisms.
Biochar modulates the soil pH by shaping the chemical environment in which nitrogen transformations occur [10,19,32]. In acidic soils, pH increases induced by biochar can reduce aluminum toxicity, enhance root nutrient uptake, and shift microbial communities toward pathways that are less conducive to rapid nitrification [19,20]. However, in alkaline soils, this buffering effect may be minimal or even adverse [19,25] and thus requires site-specific consideration.
In summary, the indirect mechanisms through which biochar mitigates nitrate leaching operate by modifying the surrounding soil environment rather than by binding nitrate directly. By buffering the pH, improving the nutrient retention capacity, enhancing the water availability, and restructuring the soil matrix, biochar reduces the vulnerability of nitrogen to loss pathways. These indirect mechanisms are especially critical in field settings and overextended timeframes, where changes in the soil function accumulate and outweigh the short-lived effects of surface sorption.

3.3. Feedback Mechanisms: Microbial Mediation and Root Interactions

Beyond physical and chemical processes, biochar exerts long-term control over nitrate dynamics by biological feedbacks. These include microbial community shifts, changes in functional gene expression, and root–rhizosphere interactions, all of which influence nitrogen transformation pathways and the spatial–temporal distribution of nitrate within the soil–plant continuum (Figure 4). These biotic feedbacks are often slower to emerge than direct-sorption or soil property changes, but their effects are more sustained and adaptive to environmental fluctuation.
One of the most important feedback mechanisms is the biochar-induced restructuring of microbial communities [48]. Biochar provides a porous habitat with diverse surface chemistries that favor the colonization and persistence of nitrifiers, denitrifiers, and other nitrogen-transforming microorganisms [49,50]. In particular, biochar amendment can stimulate denitrifier abundance under suboxic or anaerobic microsites created within aggregates, increasing the potential for nitrate to be converted to gaseous forms, such as N2 or N2O [46,51]. Several studies have reported the upregulated expression of functional genes such as nirS, nosZ, and nrfA in biochar-treated soils, indicating a shift toward denitrification and DNRA pathways [48,52]. These microbial changes are highly dependent on the biochar properties (e.g., redox-active surfaces, labile carbon content) and soil moisture conditions, underscoring the role of biochar as a biogeochemical catalyst rather than a static sorbent [45,46,47].
In addition to microbial effects, biochar also interacts with plant root systems, altering the nitrate distribution and uptake within the rhizosphere. By reducing mechanical impedance and improving the bulk soil structure, biochar promotes root proliferation and a finer root architecture, which expands the effective rhizosphere volume and increases nitrate interception [11,53]. A meta-analysis across multiple crops indicated that biochar application increased the root mass by 32%, the volume by 29%, the surface area by 39%, and the length by 52%, leading to enhanced root–soil contact and resource absorption [54]. These morphological improvements are further supported by improved water availability and pH buffering, both of which promote nitrate transporter activity and reduce nutrient stress signaling [32,55].
Furthermore, biochar can influence the root exudate composition, indirectly restructuring the rhizosphere microbiome and reinforcing microbial-driven nitrogen cycling [20,35]. The increased mycorrhizal colonization rates and rhizosphere enzyme activity observed under biochar treatment further support its role in root–microbe interactions [48,56]. In this way, biochar contributes to a more coupled plant–microbe system, where nitrate is more efficiently retained and utilized rather than lost. These biotic feedbacks are dynamic and context-dependent, emerging over weeks to months after biochar application and varying across cropping systems and soil types [22,23]. Unlike direct-sorption mechanisms, which tend to saturate quickly, biological feedbacks self-regulate, adapting to nutrient inputs, redox conditions, and seasonal changes [55,57]. When properly supported by biochar design and nutrient management strategies, these feedbacks can shift the soil nitrogen cycle toward higher retention efficiency and lower loss pathways, enhancing long-term sustainability [23,58].
The microbial- and plant-mediated feedback mechanisms induced by biochar represent a third, biologically integrated layer of nitrate retention. While not immediately observable, these responses build over time and reinforce the physical and chemical changes induced by biochar. Their inclusion is essential in any holistic assessment of biochar’s effectiveness in nitrate-leaching mitigation under sustainable soil–plant–microbe systems. Future research should explore the synergistic effects of biochar on plant–root interactions and microbial activity for enhanced nitrate mitigation.
However, it is important to acknowledge the limitations of controlled-condition studies, which often fail to fully capture the complexity of and variability in real-world environments. While these mechanisms are significant in controlled settings, the true effectiveness of biochar in nitrate-leaching mitigation can only be accurately assessed through large-scale, system-level evaluations that account for environmental variables, such as the soil type, climate, and agricultural practices. This transition to real-world evaluations is necessary to further understand biochar’s long-term impact and scalability.
Water 17 02590 g004
Figure 4. Biochar’s nitrate retention mechanisms: root, microbial, and soil interactions [46,47,48,55,56,57]. Notes: ↑ indicates an increase; ↓ indicates a decrease.
Figure 4. Biochar’s nitrate retention mechanisms: root, microbial, and soil interactions [46,47,48,55,56,57]. Notes: ↑ indicates an increase; ↓ indicates a decrease.
Water 17 02590 g004

4. Variability of Biochar for Nitrate Control in Diverse Agroecosystems

The effectiveness of biochar in mitigating nitrate leaching varies markedly across agroecosystems, shaped by the interplay of the soil type, climatic conditions, crop systems, and application strategies [59,60,61]. In many contexts, biochar has demonstrated the capacity to reduce nitrate losses by improving the nitrogen retention, modulating the soil pH, stimulating microbial activity, and optimizing nitrogen release patterns [61,62]. However, these effects are highly dependent on contextual conditions, and it cannot be assumed that they are universal.

4.1. The Significant Inhibitory Effect of Biochar on Nitrate Leaching and Its Mechanism

Numerous studies have demonstrated that the effectiveness of biochar at mitigating nitrate leaching is strongly influenced by the soil type, cropping system, and application strategy, as summarized in Table 1. For instance, in acidic red soils, biochar reduces nitrate mobility primarily by increasing the soil pH [32,53]. Applications typically raise the pH by 0.5–1.5 units, thereby lowering the nitrate solubility and leaching risk. In sandy soils, biochar enhances water retention and improves the soil structure, reducing nitrate loss caused by excessive irrigation. One study observed an approximately 30% increase in the water retention in sandy soil following biochar application [25]. In loam soils, biochar promotes nitrogen retention by increasing the soil organic matter and stimulating microbial activity, leading to improved nitrogen transformation and reduced leaching [10,42,45]. Organic matter levels have been shown to rise by an average of 15% in such cases.
The application method also plays a critical role. Integrating biochar with fertilizers or organic amendments, especially through split applications, enhances the nitrogen use efficiency and lowers nitrate leaching [42,44]. Repeated small doses reduced the nitrate loss by 20%, while single high-dose applications could lead to nitrogen over-adsorption and short-term crop nitrogen deficiency [44].
Overall, biochar mitigates nitrate leaching through its multifaceted influence on the soil chemical, physical, and biological properties. Its performance is shaped by the application strategy and site-specific conditions such as the soil pH, texture, and climate. Notably, biochar tends to be more effective in acidic soils and cooler climates, where it raises the pH (e.g., from 5.5 to ~7.0), stabilizes the soil structure, and improves the N cycling [63]. In such environments, nitrate-leaching reductions of up to 30% have been reported [64].

4.2. Cases and Mechanisms of Effective vs. Ineffective Effects of Biochar on Nitrate Leaching

Despite its promise, it is increasingly evident that biochar does not universally reduce nitrate leaching, and in some cases, it may exert neutral or even adverse effects [65,66]. These ineffective or adverse outcomes are particularly pronounced in saline–alkaline soils, degraded continuous cropping systems, contaminated farmland, and agricultural fields with high residual nitrogen accumulation (Table 1). Such inconsistencies arise from interacting chemical, physical, biological, and temporal mechanisms, which can offset biochar’s expected benefits.
Firstly, excessive increases in the soil pH due to biochar application can compromise the soil buffering capacity and disrupt nutrient availability. Salem et al. [65] reported that in saline–alkaline soils, biochar application raised the pH from 8.2 to 9.0, inhibiting nitrogen uptake, and resulted in a 15% rise in nitrate leaching relative to the control. Secondly, biochars derived from manure or crop residues at high pyrolysis temperatures often contain high ash and salt contents, increasing the soil electrical conductivity (EC) and possibly inducing ionic stress [56,57,58]. Murtaza et al. [66] observed a 40% increase in the soil EC following the application of biochar with EC > 4 dS·m−1, surpassing crop tolerance thresholds and impairing microbial nitrogen cycling. Thirdly, many biochars possess inherently low anion exchange capacities, limiting their ability to retain nitrate [49,50].
Clough et al. [52] found that hardwood biochar produced at 500 °C exhibited a negligible nitrate adsorption capacity. Under simulated rainfall conditions (40 mm h−1), nitrate leaching was 23% higher than in non-amended soils, likely due to insufficient retention under rapid percolation [61,66]. Additionally, asynchronous nitrogen release from biochar-amended soils relative to the crop nitrogen demand can lead to nitrate accumulation and subsequent leaching [60]. Zhang et al. [67] reported that in a maize cropping system, the topsoil nitrate concentrations were 31% higher in the early growth stage following biochar treatment, indicating a misalignment between mineralization and plant uptake.
In some cases, biochar may suppress microbial activity essential for nitrate reduction. Ali et al. [68] demonstrated that a high-dose biochar application (40 t·ha−1) reduced the microbial biomass carbon by 18% and the denitrification enzyme activity by 22%, thereby limiting microbial nitrate transformation pathways. In addition, the aging and degradation of biochar may reduce its nitrate retention capacity over time. Ren et al. [69] observed a >50% decline in nitrate sorption after two years of field aging, associated with the loss of surface oxygen-containing functional groups. Furthermore, the presence of competing anions like SO42− and PO43− can further inhibit the nitrate binding efficiency through sorption site competition [70].
In summary, the limited or negative effects of biochar on nitrate leaching are governed by a complex interplay of chemical (e.g., elevated pH, ionic stress), physical (e.g., low sorption capacity), biological (e.g., microbial inhibition), and temporal (e.g., release–uptake mismatch) factors. These findings underscore the need for site-specific biochar formulations and management strategies tailored to soil conditions, cropping systems, and environmental constraints. Future studies should investigate the site-specific factors that influence biochar’s effectiveness in nitrate-leaching mitigation. Additionally, research should explore how biochar formulations can be optimized to address the chemical, physical, and biological challenges that limit its performance in certain conditions.
Table 1. Summary of research cases, primary mechanisms, and underlying causes of biochar’s effects on nitrate mitigation and control under specific conditions.
Table 1. Summary of research cases, primary mechanisms, and underlying causes of biochar’s effects on nitrate mitigation and control under specific conditions.
Soil TypeCropping SystemBiochar Application (Mode and Rate)Positive Observed EffectsNegative Observed
Effects		Main MechanismsReferences
Acidic red soilField crop (maize, paddy rice)/greenhouse (vegetables)Sole/split application;
8–12 t/ha;
woody/straw biochar		NO3− leaching reduced by 33–40% ↓; pH raised from 5.5 to 7.0 ↑; microbial activity increased by 20–40% ↑Nutrient utilization of crops requiring acidic conditions reduced by 10–25% ↓Positive: Increased pH, enhanced nitrogen-fixing microbial activity, improved soil porosity.
Negative: Impaired nutrient uptake due to increased pH.		[56,57,58]
Sandy soilGreenhouse (tomato)/field crop (wheat)Co-applied with organic fertilizer;
5–6 t/ha;
bamboo/agroforestry residue biochar		NO3− leaching reduced by 35–42% ↓; water-holding capacity increased by 30–45% ↑NO3− leaching increased by 10–20% ↑; microbial biomass reduced by 15–25% ↓Positive: Improved nitrate retention, water-holding capacity, and nitrogen use efficiency.
Negative: Reduced nutrient retention and microbial activity due to pH alteration and ionic imbalance.		[25,40]
LoamField crop (maize)/greenhouse (cucumber)/pasture simulationSole/split/surface application;
8–15 t/ha;
corn stalk/peanut shell biochar		NO3− leaching reduced by 36–45% ↓; root activity increased by 20–35% ↑Nutrient availability reduced by 15–25% ↓; microbial biomass decreased by 10–20% ↓Positive: Enhanced soil structure, microbial diversity, and nitrate adsorption, promoting nitrogen retention and uptake.
Negative: Weak nitrate retention due to high water flux, mismatched nitrogen release, and competition for sorption sites.		[10,40,42,55,64]
Loamy sandWheat monoculture/maize–wheat rotationSurface broadcast/split application;
5–10 t/ha;
wheat straw/corn stalk biochar		Water-holding capacity increased by 20–40% ↑; nutrient retention increased by 10–25% ↑Microbial biomass decreased by 18–22% ↓; denitrification decreased by 20–25% ↓ Positive: Improved water retention and nutrient availability due to enhanced soil porosity and ion exchange capacity.
Negative: Weak nitrate retention due to high water flux, mismatched nitrogen release, and competition for sorption sites.		[41,45,63]
Sandy loamGreenhouse (tomato)/field crop (wheat)Co-applied with compost/sole application;
5–8 t/ha;
bamboo/peanut shell biochar		Water-holding capacity increased by 30–40% ↑; soil aeration increased by 15–30% ↑NO3− leaching increased by 10–15% ↑; soil EC increased by 25–45% ↑Positive: Improved soil aeration, root activity, and nutrient retention due to enhanced porosity and ionic exchange capacity.
Negative: Ionic stress from soluble salt-elevated EC.		[25,45,62]
Silty clayRice–wheat rotation/wheat monocultureCo-applied with DCD/sole application;
8–10 t/ha;
wheat straw/rice husk biochar		Nitrogen cycling efficiency increased by 15–25% ↑; root activity increased by 20–30% ↑No reduction in NO3− leaching under high groundwater level; Nitrate mobility increased by 10–15% ↑Positive: Enhanced microbial activity and root growth due to improved soil structure and aeration.
Negative: Limited nitrate retention due to anaerobic conditions and high groundwater levels, reducing biochar’s effectiveness.		[60,67,68]
Saline–alkaline soilField crop (sorghum/wheat)Deep placement with compost/single high dose;
7–10 t/ha;
wheat straw/corn stalk biochar		Root zone salt accumulation reduced by 15–20% ↓; microbial stability improved ↑ and ionic stress reduced ↓pH raised from 8.2 to 9.0 ↑; NO3− leaching increased by 10–18% ↑Positive: Reduced root zone salt accumulation, improved nitrate retention, and maintained microbial stability.
Negative: Excessive biochar application leads to salt accumulation and reduced nitrate retention due to ion competition.		[19,59]
Notes: ↑ indicates an increase; ↓ indicates a decrease.

5. Multifunctional Role of Engineered Optimization and Material Design of Biochar in Controlling Nitrate Leaching

Building upon the context-specific limitations identified in the preceding section, the following discussion focuses on targeted optimization strategies to enhance the performance of biochar in mitigating nitrate leaching (Table 2). Specifically, it explores advances in material engineering, fertilizer integration, and site-specific deployment methods. The goal is to align biochar properties with agronomic demands and environmental constraints. These approaches seek to maximize its nitrate retention potential across diverse agroecosystems.

5.1. Material Engineering Optimization to Enhance Nitrate Retention

The functional performance of biochar in nitrate-leaching control is fundamentally determined by its feedstock characteristics, pyrolysis conditions, and surface chemistry (Figure 5). Woody biomass-derived biochars typically exhibit greater aromaticity and specific surface areas, favoring physical adsorption, while straw-derived biochars are richer in oxygen-containing functional groups, which enhance nitrate sorption through chemical interactions [60,61,66]. In contrast, manure-based biochars often exhibit high ash and salt contents, elevating the soil electrical conductivity and potentially inhibiting nitrate adsorption [62]. Aghoghovwia et al. [71] reported that straw biochar exhibited a 45% higher nitrate adsorption capacity than that of wood biochar under identical conditions, primarily due to its higher surface polarity and abundance of –COOH and –OH groups.
The pyrolysis temperature is another key determinant, as it governs pore development and surface functional group retention [11,71]. Biochars produced at moderate temperatures (around 500 °C) often strike a balance between porosity and chemical reactivity, whereas high-temperature biochars (>700 °C) may show reduced surface functionality [71,72]. Liu et al. [11] observed that 500 °C biochar achieved a 35% greater nitrate sorption capacity and a 20% longer breakthrough time in column leaching experiments compared to biochar produced at 700 °C. Thus, medium-temperature biochars tend to achieve higher nitrate sorption capacities and prolonged breakthrough times, reflecting their improved nitrate retention efficiency.
Surface modifications can further enhance nitrate affinity. Metal doping (e.g., Fe3+, Mg2+) can increase the pHPZC, thereby enhancing the biochar’s positive surface potential and electrostatic attraction to NO3− [58,60]. Oxidative pretreatments with H2O2 or HNO3 introduce additional oxygenated functional groups, improving both nitrate sorption and structural integrity [55,72]. For instance, Zhang et al. [58] demonstrated that Fe-modified biochar increased the pHPZC from 4.2 to 7.1 and doubled the nitrate removal efficiency in batch sorption tests. Moreover, Mg-doped biochars exhibited up to 60% higher nitrate retention compared to unmodified controls under simulated rainfall conditions [34]. However, these surface modifications come with potential trade-offs. Metal doping, particularly with Fe3+ and Mg2+, could lead to metal leaching. For example, Zhao et al. [72] reported that Fe-doped biochar leached up to 15% of its metal content under acidic conditions. Additionally, while metal-doped biochars are more effective at nitrate retention, their cost is considerably higher, with Fe-modified biochars priced at approximately 1.5 times that of unmodified biochars [73]. Long-term risks include the degradation of modified biochars over extended use, as shown by Wang et al. [74], where biochar’s nitrate retention capacity decreased by 25% after six months due to the breakdown of surface functional groups.
In summary, the deliberate manipulation of the feedstock selection, thermal processing, and post-treatment modifications enables the development of biochars with tailored properties—high pHPZC values, elevated stabilities, optimized charge dynamics, and superior nitrate-binding capacities. These advances provide a mechanistic basis for optimizing the biochar performance in nitrate-leaching mitigation in diverse soil and environmental contexts.
Water 17 02590 g005
Figure 5. Strategies to enhance biochar’s nitrate adsorption: feedstock, pyrolysis, and surface modifications [58,60,61,72,73,74]. Notes: ↑ indicates an increase; ↓ indicates a decrease.
Figure 5. Strategies to enhance biochar’s nitrate adsorption: feedstock, pyrolysis, and surface modifications [58,60,61,72,73,74]. Notes: ↑ indicates an increase; ↓ indicates a decrease.
Water 17 02590 g005

5.2. Fertilizer Co-Application Strategies and Synchronization with Nitrogen Release Dynamics

Integrating biochar with advanced nutrient management strategies offers significant potential to enhance the nitrate retention and nitrogen use efficiency across diverse agroecosystems [61,62,63].
Co-application with organic or controlled-release fertilizers improves both microbial immobilization and the sorptive stabilization of nitrate (Table 2). For instance, Liu et al. [61] reported a 32% reduction in cumulative nitrate leaching over 60 days when biochar was applied with compost, compared to compost alone, due to enhanced sorption and microbial immobilization. Biochar-coated urea formulations further extend the nitrogen release duration by up to 40% and decrease leachate nitrate concentrations by 29% [63], aligning the nutrient availability more closely with the crop demand and minimizing early-stage nitrate surpluses that commonly lead to leaching. In addition, combining biochar with nitrification inhibitors such as DCD or DMPP significantly improves ammonium retention and restricts the conversion of ammonium to nitrate, thereby reducing leachable nitrogen pools [67,68]. Li et al. [75] observed that biochar + DMPP treatments reduced soil nitrate levels by 25–35% relative to DMPP alone, indicating the synergistic inhibition of nitrification and increased nitrate adsorption.
A key mechanism underpinning these improvements is the synchronization of nitrogen release with crop uptake patterns. Unlike conventional fertilization, which often leads to early nitrogen surpluses and late-season deficits, biochar-integrated systems provide a more stable and extended nitrogen supply [70,71]. Chen et al. [40] found that such synchronization improved the nitrogen use efficiency by up to 20% and reduced nitrate leaching by 30% in maize systems. Beyond nutrient dynamics, biochar can serve as a carrier for beneficial microorganisms or be incorporated into biopolymer matrices to form multifunctional amendments [76,77]. For example, Yu et al. [76] demonstrated that biochar embedded with Azospirillum brasilense in biopolymer hydrogels enhanced the nitrogen uptake by 18% and reduced nitrate leaching by 22% compared to uninoculated controls. These integrated approaches underscore the potential of biochar as a platform for precision nitrogen management, enabling environmentally sustainable and agronomically effective nitrate control.

5.3. Land-Type-Specific Adaptation of Biochar Application Methods

The effectiveness of biochar in nitrate mitigation is highly dependent on the soil type and field conditions, necessitating site-specific deployment strategies. In acidic soils, the application of neutral or alkaline biochars is particularly beneficial, as they buffer soil acidity and enhance nitrate retention via pH-driven mechanisms [57,58]. Li et al. [9] demonstrated that the application of alkaline wood-derived biochar (pH: 9.2) to red soil (initial pH: 4.8) raised the soil pH to 6.3 and reduced the cumulative nitrate leaching by 38% over a 90-day leaching period. This effect was attributed to reduced nitrate solubility and a shift in microbial nitrogen pathways, favoring DNRA over denitrification [52,64]. In saline–alkaline soils, however, high-EC biochars can exacerbate osmotic stress and promote nitrate mobility due to elevated background EC and ionic stress [19,59].
Therefore, the use of low-EC biochars combined with desalination pretreatment and subsurface application is recommended. Yuan et al. [63] reported that the deep placement (15 cm) of washed straw-derived biochar in saline–alkaline soil reduced nitrate leaching by 29% and increased the maize yield by 11%, compared to surface-applied untreated biochar. This approach limited salt accumulation in the rhizosphere and localized nitrate adsorption in the root zone, thereby mitigating both chemical and hydraulic stresses.
In high-input systems characterized by frequent irrigation and intensive fertilization, coordinating water and nutrient delivery becomes crucial [16,17,25]. Integrating biochar with superabsorbent polymers (SAPs) enables co-regulation of the soil moisture and nitrate availability [5,6,62]. Spitalniak et al. [77] found that a biochar–SAP composite improved the water retention by 25%, delayed peak nitrate leaching by 5 days, and increased the nitrogen use efficiency by 17% in greenhouse tomato systems under high-frequency irrigation. These benefits were linked to reduced percolation and the improved synchronization of nitrogen release with crop uptake patterns.
In summary, soil-specific biochar deployment strategies, including pH buffering in acid soils, reducing the electrical conductivity, and applying biochar deeply in saline–alkaline soils, as well as coordinating water and nutrient dynamics in intensive agriculture, underscore the importance of context-aware designs.
Table 2. Engineered biochar optimization strategies for enhancing nitrate retention and mitigating nitrate leaching.
Table 2. Engineered biochar optimization strategies for enhancing nitrate retention and mitigating nitrate leaching.
Optimization StrategyKey ParametersEffect on Surface AreaEffect on pHPZCEffect on Nitrate Retention EfficiencyReferences
Feedstock selectionWood/straw/manureWoody biochar: 250–350 m2/g; straw biochar: 180–230 m2/g; manure biochar: 50–120 m2/gWoody biochar: 6.0–7.0; straw biochar: 4.5–5.5; manure biochar: 7.0–7.5Straw biochar: 45% higher NO3− adsorption than woody biochar, primarily due to higher −COOH and −OH groups.[60,61,66]
Pyrolysis temperature400 °C/500 °C/700 °C400 °C biochar: 200–300 m2/g; 500 °C biochar: 300–400 m2/g; 700 °C biochar: 200–250 m2/g400 °C biochar: 5.5–6.0; 500 °C biochar: 6.0–7.0; 700 °C biochar: 5.0–5.5500 °C biochar: 35% higher NO3− sorption and 20% longer breakthrough time compared to 700 °C biochar. 400 °C biochar: moderate NO3− retention efficiency.[11,18,30]
Metal dopingFe3+/Mg2+/Cu2/Zn2+/dopingFe-doped biochar: 250–350 m2/g; Mg-doped biochar: 200–300 m2/g; Cu-doped biochar: 220–300 m2/g; Zn-doped biochar: 210–280 m2/gFe-doped biochar: increased from 4.2 to 7.1; Mg-doped biochar: increased from 5.5 to 7.0; Cu-doped biochar: increased from 5.0 to 7.0; Zn-doped biochar: increased from 5.2 to 6.8Mg-doped biochar showed up to 60% higher NO3− retention compared to unmodified biochar. Fe-doped biochar demonstrated 50% increase in NO3− adsorption. Cu- and Zn-doped biochars also enhanced NO3− retention by 40–55% compared to unmodified biochar.[55,58,60]
Oxidative modificationOxidation with H2O2/HNO3 Oxidation increased surface area: 50–100 m2/g increased ↑pHPZC increased by 0.5 to 1 unit after oxidation Oxidation improved NO3− retention efficiency by 30–40% compared to unmodified biochar, primarily through the increase in −OH and C=O groups.[39,40,55]
Co-application with fertilizersBiochar + organic fertilizer/controlled-release fertilizer/Nitrification inhibitorsSurface area increased ↑ due to functional group interactionspH adjustment for optimized nutrient release, slow-release nitrogen, and ammonium retentionNO3− leaching reduced by 25–32%; nitrogen release extended by up to 40%; nitrate adsorption enhanced through −COOH and −OH groups.[55,56,67,68]
Notes: ↑ indicates an increase; ↓ indicates a decrease.

6. Challenges and Perspectives

Biochar has emerged as a multifunctional soil amendment with great promise for reducing nitrate leaching while enhancing soil health and nutrient use efficiency. However, to fully clarify its potential in sustainable agriculture and environmental management, several scientific, technological, and practical challenges must be addressed to ensure its scalability and field applicability.

6.1. Mechanistic Uncertainties in Nitrate Transformation

The current understanding of how biochar modulates soil nitrogen dynamics—particularly nitrification, denitrification, and DNRA—remains incomplete. Future research should dissect biochar’s influence on functional microbial guilds (e.g., amoA, nirS, nosZ, nrfA) under varied redox and pH conditions. Advanced meta-omics and isotope tracing techniques are essential to reveal transformation pathways across diverse soil–biochar interfaces.

6.2. Assessing Long-Term Soil Impacts and Agroecological Effects

Comprehensive field trials and mesocosm experiments are needed to evaluate the long-term impacts of biochar on key soil physicochemical attributes, including the pH, CEC, water-holding capacity, and aggregate stability. These studies should also address biochar’s capacity to enhance crop yields and quality, as well as its potential suppressive effects on soil-borne pathogens. Understanding the role of biochar in shaping soil health over multiple cropping cycles is crucial for promoting its integration into farming systems.

6.3. Advancing Biochar Design and Functionalization

Innovation in biochar engineering is essential to note its functionality to site-specific needs. Research should explore how feedstock diversity (e.g., agricultural residues, municipal biosolids) and production parameters (e.g., pyrolysis temperature, residence time, oxygen content) influence the structural, chemical, and surface properties. In parallel, efforts to functionalize biochar—through nanotechnology, metal doping, microbial inoculation, or surfactant coating—should aim to enhance its sorption selectivity, microbial compatibility, and redox activity. These modifications could open new pathways for the targeted remediation of nitrogen pollutants or co-contaminants.

6.4. Field-Level Application Strategies and Environmental Trade-Offs

To ensure ecological safety and optimize field performance, future research should prioritize life cycle assessments (LCAs) and techno-economic evaluations of biochar use. These include quantifying carbon emissions across the production–application–decomposition chain and assessing trade-offs between nitrogen retention and unintended side effects such as nutrient imbalance or greenhouse gas release. Research should also investigate the context-specific performance of biochar under diverse climatic regimes, soil textures, and cropping systems while developing standard protocols for environmental risk assessment and regulatory compliance.

7. Conclusions

Biochar mitigates nitrate leaching through a combination of physical adsorption, electrochemical interactions, and biological feedback mechanisms; however, its performance is highly context-dependent. Comparative evidence shows that biochar derived from woody biomass tends to perform better in acidic soils due to its higher surface area and aromaticity, while straw-based biochars are generally more effective in alkaline or neutral soils owing to their oxygen-rich functional groups. In coarse-textured soils, biochar enhances nitrate retention mainly through improved water holding and structural stabilization. To optimize biochar’s performance across different environmental conditions, various strategies have shown promise. These include tailored feedstock selection, precise pyrolysis control, targeted surface modifications, and co-application with fertilizers or inhibitors to address nutrient imbalances. By combining these approaches, biochar can be customized for specific soil types and agroecosystems, thereby improving both its efficiency and applicability.
Future research should focus on integrating biochar into precision nutrient management systems, where its co-benefits, such as carbon sequestration and soil fertility enhancement, can be maximized. Moreover, there is a need to establish comprehensive production and field application models that consider the economic feasibility, environmental impact, and scalability of biochar use across different agricultural systems. Through such efforts, biochar has the potential to evolve from a promising soil amendment into a core component of sustainable agricultural intensification and environmental restoration, playing a key role in addressing global challenges in food security, climate resilience, and environmental sustainability.

Author Contributions

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

Funding

This study was funded by the National Key R&D Program of China (grant number 2023YFD1701803), and the National Science Foundation of China (grant number 42207048).

Data Availability Statement

No new data were created or analyzed in this study.

Acknowledgments

The authors extend their appreciation to the Chinese Academy of Agricultural Sciences Innovation Program (Agro-Environmental Protection Institute).

Conflicts of Interest

Author Tao Jing was employed by the company Zhongnong Chuangda (Beijing) Environmental Protection Technology. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
NO3−-NNitrate nitrogen
NUENitrogen use efficiency
AECAnion exchange capacity
CECCation exchange capacity
SSASpecific surface area
pHPZCPoint of zero charge
ECElectrical conductivity
SOCSoil organic carbon
WSAsWater-stable aggregates
DNRADissimilatory nitrate reduction to ammonium
SAPsSuperabsorbent polymers
LCAsLife cycle assessments

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Water 17 02590 g001
Figure 1. Bibliometric analysis of biochar and nitrate nitrogen research trends (2013–2024). (a) Data collection flowchart (3541 records, Web of Science Core Collection); (b) annual and cumulative publication trends; (c) thirty key topics identified from articles published in 2024 using Carrot2 clustering (www.carrot2.org), with parameters including keyword frequency, citation count, and relevance. Notes: * denotes a truncation wildcard, retrieving records containing any word that begins with the preceding stem; numbers denote the number of publications under the topic.
Figure 1. Bibliometric analysis of biochar and nitrate nitrogen research trends (2013–2024). (a) Data collection flowchart (3541 records, Web of Science Core Collection); (b) annual and cumulative publication trends; (c) thirty key topics identified from articles published in 2024 using Carrot2 clustering (www.carrot2.org), with parameters including keyword frequency, citation count, and relevance. Notes: * denotes a truncation wildcard, retrieving records containing any word that begins with the preceding stem; numbers denote the number of publications under the topic.
Water 17 02590 g001
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MDPI and ACS Style

Luo, L.; Li, J.; Xing, Z.; Jing, T.; Wang, X.; Zhang, G. Biochar for Mitigating Nitrate Leaching in Agricultural Soils: Mechanisms, Challenges, and Future Directions. Water 2025, 17, 2590. https://doi.org/10.3390/w17172590

AMA Style

Luo L, Li J, Xing Z, Jing T, Wang X, Zhang G. Biochar for Mitigating Nitrate Leaching in Agricultural Soils: Mechanisms, Challenges, and Future Directions. Water. 2025; 17(17):2590. https://doi.org/10.3390/w17172590

Chicago/Turabian Style

Luo, Lan, Jie Li, Zihan Xing, Tao Jing, Xinrui Wang, and Guilong Zhang. 2025. "Biochar for Mitigating Nitrate Leaching in Agricultural Soils: Mechanisms, Challenges, and Future Directions" Water 17, no. 17: 2590. https://doi.org/10.3390/w17172590

APA Style

Luo, L., Li, J., Xing, Z., Jing, T., Wang, X., & Zhang, G. (2025). Biochar for Mitigating Nitrate Leaching in Agricultural Soils: Mechanisms, Challenges, and Future Directions. Water, 17(17), 2590. https://doi.org/10.3390/w17172590

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