How Biochar Addition Affects Denitrification and the Microbial Electron Transport System (ETSA): A Meta-Analysis Based on a Global Scale

Abstract

Biochar has a significant effect on denitrification, especially in agriculture. The effects of biochar and soil properties on denitrification and ETSA have been invested in individual studies but have not yet been summarized on a global scale. We conducted a meta-analysis of the data from 37 studies to examine the effects of biochar properties, soil physicochemical properties, and ecosystem types on denitrification. Biochar can decrease soil NO₃⁻ and N₂O emissions by 14.16% and 76.69%, respectively, while denitrification function genes nirK, nirS, and nosZ increased by 10.98%, 34.62%, and 13.19%, respectively. Biochar enhanced ETSA by 8.65%. The results indicate that the effects of biochar on nitrogen cycling and greenhouse gas emissions vary significantly with specific properties, such as feedstock source and pyrolysis temperature, as well as soil characteristics like pH, organic matter, and cation exchange capacity (CEC). In summary, soil-specific biochar applications are necessary to realize optimized agricultural and environmental advantages of biochar, but several limitations have been recognized in this study, including variability across different types of biochar and a lack of longer-term experimental data. Future research should focus on long-term studies that can give a comprehensive understanding of how biochar interacts with microbial communities to create an accurate understanding.

1. Introduction

Nitrogen cycling in soil is a multifaceted process encompassing nitrification, denitrification, nitrate reduction to ammonium, and various abiotic transformations [1]. Among these, denitrification has drawn significant attention as it represents a primary pathway for nitrous oxide (N₂O) production and reduction—an important consideration given N₂O’s potency as a greenhouse gas [2]. In biological denitrification, microorganisms sequentially reduce NO₃⁻ and NO₂⁻ to NO, N₂O, and ultimately N₂ under anoxic conditions, using nitrate as an electron acceptor and organic carbon as a donor [3]. However, many denitrifying microorganisms lack the full set of enzymes required for complete denitrification, making it a collaborative process among multiple microbial groups [4].

Denitrification is generally divided into autotrophic and heterotrophic pathways. In heterotrophic denitrification, the addition of external organic carbon as an electron donor can significantly influence the denitrification rate [5,6,7]. The rate of denitrification is also modulated by various soil physicochemical properties, climatic factors, and anthropogenic influences. Soil pH, moisture (SM), organic carbon content (SOC), and fertility have been identified as primary determinants [8,9]. Denitrification generally occurs optimally under neutral to slightly alkaline conditions, with high SOC promoting microbial activity and temperatures between 15 °C and 30 °C, significantly enhancing denitrification rates [10,11,12].

In recent years, biochar has gained traction as a soil amendment due to its potential to improve soil structure, fertility, and microbial activity, all of which can influence nitrogen cycling [13]. It has been shown that reducing nitrogen losses through processes like leaching and denitrification while enhancing the abundance of functional genes (nirK, nirS, nosZ) involved in nitrogen transformations, thereby mitigating greenhouse gas emissions and improving soil fertility [14,15]. Meanwhile, numerous studies indicate that biochar modifies the denitrification process by mechanisms such as (1) enhancing nitrogen fixation and organic nitrogen mineralization through the provision of a carbon source for microbial activity [16,17], (2) altering soil pH to levels more conducive to denitrification [18,19,20], and (3) improving soil moisture and aeration by increasing soil porosity and surface area, which benefits denitrifying microorganisms [21,22].

Recent studies have increasingly focused on the electrochemical properties of biochar and their implications for soil denitrification [23]. Biochar contains a variety of functional groups, such as phenols (electron donors) and quinones or aromatic compounds (electron acceptors) that contribute to its redox capacity [24]. Yuan et al. demonstrated that reducing carboxyl groups on biochar surfaces could decrease soil N₂O emissions [25]. Additionally, the graphite structure and high carbon content of biochar facilitate electron transfer, enhancing its functionality in redox reactions [26]. Biochar’s electron-donating capacity (EDC) and electron-accepting capacity (EAC) are key metrics for quantifying its redox properties [27]. Biochar also optimizes electron distribution among denitrifying reductases, thus promoting denitrification [28,29].

In spite of this considerable research, our understanding of its effect on denitrification in different ecosystems and soil types is still lacking, particularly at large scales. The research regarding the electron transfer capacity and electrochemical mechanisms of biochar in relation to denitrification has been scant and requires comprehensive studies in an integrative approach [30,31]. Therefore, the present study attempts to take up some of the following research lacunae: to explain the drivers behind the denitrification rate of various biochar-amended soils, to agree on how biochar affects the electron transport system responsible for denitrification and what its most influential driver is, and to identify the physicochemical properties most associated with the changes in denitrification in soil mediated by biochar. In this study, three hypotheses have been raised: first, the response of denitrification to biochar would vary within soil type; second, the type and pyrolysis temperature of biochar are crucial factors affecting its redox and electron transfer capacity, thus influencing the denitrification-related indices; lastly, biochar affects denitrification through a change in soil pH and aeration, and soil pH and moisture content are two main factors that decide the shifting patterns of denitrification rate.

This meta-analysis investigated the data from 37 studies to evaluate the global effects of biochar on soil denitrification and ETSA. It integrates the impacts of biochar properties and soil physicochemical properties to provide a comprehensive understanding of its role in nitrogen cycling. Introducing ETSA as a performance indicator adds a novel dimension to soil denitrification. This study aims to (i) characterize the effects of biochar, soil physicochemical properties, and ecosystem types on denitrification and (ii) identify and elucidate factors affecting denitrification with biochar addition. We hypothesize that globally: (1) the effect of biochar on denitrification is affected by feedstock source and pyrolysis temperature; (2) soil pH and organic matter are key factors influencing denitrification; (3) denitrification is significantly differentiated between ecosystem types due to differences in soil properties.

2. Material and Methods

2.1. Date Collection

Data were collected from PubMed (https://pubmed.ncbi.nlm.nih.gov (1 November 2023)) and Web of Science (https://webofscience.clarivate.cn/wos/alldb/basic-search (1 November 2023)) by searching for the keywords Biochar, Nitrogen Cycle, and Electron (Figure S1). The collected reports were collated using Endnote 20, and in order to ensure the accuracy of studies, which were screened to exclude those that were not relevant to the study as well as those with missing data, 37 publications were included in this study. Literature data were extracted using webPlotDigitizer and summarized in Excel. Data were collected from the experimental and control groups in terms of mean, sample size (n), and standard deviation (SD), and if only standard error (SE) was available in the text, which was calculated based on $SE\ast \sqrt{n}$.

2.2. Meta-Analysis

The effect of biochar addition on NO₃⁻, NH₄⁺, and N₂O emissions, key genes (nirK, nirS, and nosZ), and the electron transport system (ETS) associated with the denitrification process was analyzed using a random-effects model. The natural log of the response ratio (RR) was calculated as the effect size, representing the degree of influence on the relevant metrics:

$$RR=lnR=ln({\displaystyle \frac{ Y_e}{Y_c}})=ln(Y_e)-ln(Y_c)$$

(1)

where Y_e and Y_c represent the means of the treatment and control groups, respectively. The variance (v_i) of RR was calculated as:

$$v_i={\displaystyle \frac{{S_e}^2}{{{N_{e}Y}_e}^2}}+{\displaystyle \frac{{S_c}^2}{{{N_{c}Y}_c}^2}}$$

(2)

where N_e and Y_e represent the sample size of the treatment and control, respectively, and S_e and S_c represent the standard deviations of the treatment and control, respectively. Then, for the precision of our analysis, the weighted effect size was calculated as:

$${RR}_{+}={\displaystyle \frac{\sum \left(w_i\times {RR}_i\right)}{\sum w_i}}$$

(3)

where w_i is the weighting factor of the ith experiment in the group and was calculated as follows:

$$w_i={\displaystyle \frac{1}{v_i}}$$

(4)

And we calculated the percent change depending on biochar-adding practices using the equation:

$$RR\left(\%\right)=\left(e^{{RR}_{+}}-1\right)\times 100$$

(5)

2.3. Statistical Analysis

Data were analyzed in R software (version 4.2.1) using the “meta” and “metafor” packages. The I² was used to determine if there was a large heterogeneity in the results; if I² > 65% and p < 0.05, it indicated that there was a strong heterogeneity and needed the subgroup analysis to look for the source of the heterogeneity. If Q_m and p < 0.05 of the result in subgroup analysis, indicating the source of heterogeneity. Egger’s regression test and fail-safe analysis with the Rosenberg method were used to test publication bias in the studies [32,33]. If p-values were >0.05 in Egger’s regression test or coefficients > 5N + 10 in the fail-safe analysis (N is the sampling size in this study), then the effect sizes of variables are considered statistically significant, and the observed pattern indicated no sign of publication bias (

Table 1. Test for publication bias for the response of biochar to denitrification using Egger’s regression test and fail-safe analysis with the Rosenberg method.

Variables	Estimate	N	Egger’s Regression		Fail-Safe Coefficient
			z	p
NH4+	−0.0882	132	−2.3103	0.0209	>5N + 10
NO3−	0.1325	124	−0.7091	0.4782	>5N + 10
N2O emission	−1.1554	34	0.8104	0.4177	>5N + 10
nirK	0.1042	28	1.5337	0.1251	>5N + 10
nirS	0.2973	31	−0.1285	0.8978	>5N + 10
nosZ	0.1239	30	+0.1243	0.9010	>5N + 10
ETS	0.0830	11	−0.0159	0.1251	>5N + 10

Note: N is the sample size. For Egger’s regression test, p-values > 0.05 indicate the absence of publication bias. For fail-safe analysis, coefficients > 5N + 10 indicate the effect sizes of variables are statistically significant.

). Correlation analyses and significance tests were conducted using R software for soil physicochemical properties and target variable effects.

3. Results

The results of the meta-analysis showed that biochar resulted in different effects on variants of denitrification. Biochar increased the soil NH₄⁺ by 8.44% but decreased N₂O emission by 76.67% (Table 1, Figure 1). In addition, biochar can decrease soil NO₃⁻ by 14.16%. Meanwhile, biochar increased nirK, nirS, and nosZ by 10.98%, 34.62%, and 13.19%, respectively (p < 0.05). ETS was also increased by 8.65% (p < 0.05). Figure 1 indicates that the result of the random-effects model is considered statistically significant, and the observed pattern indicated no sign of publication bias (Egger’s regression p < 0.05 or fail-safe coefficient > 5N+10).

3.1. Effects of Biochar Characteristics on Denitrification

3.1.1. Biochar Physicochemical Properties

The application of rice husk-derived biochar led to a substantial increase in both NH₄⁺ and NO₃⁻ concentrations, with increases of 98.86% and 91.08%, respectively (Figure S3). Notably, when agricultural waste was used as a feedstock, there was a significant 1071% reduction in NO₃⁻, accompanied by an increase in the abundance of nirK and nirS genes by 71.46% and 94.35%, respectively (Figure S3). The pyrolysis temperature, particularly within the range of 550–750 °C, exerted a pronounced influence on NH₄⁺ and NO₃⁻. Additionally, biochar pH played a crucial role in regulating N₂O emission (Figure 2c). Specifically, N₂O emission was significantly elevated at a biochar pH of 7–10, while a pH exceeding 10 led to a marked reduction in emissions. The biochar C was also found to affect NO₃⁻ reduction. Biochar with 55–56% carbon significantly reduced NO₃⁻ by 74.03%. However, a contrasting effect was observed when biochar C exceeded 65%. Similarly, biochar N demonstrated opposing trends: NO₃⁻ reduction capacity and N₂O emission were significantly enhanced when biochar N was greater than 0.5%, while reductions occurred at N below 0.5%.

Furthermore, biochar ash greater than 40% led to a significant reduction in the nirK gene (Figure 2e). Conversely, when ash content was below 30%, nirS gene abundance increased markedly (Figure 2f). The electrical conductivity of biochar (EC) in the range of 10–50 S·m⁻¹ also promoted a reduction in NO₃⁻ (Figure S2). Lastly, the specific surface area (SSA) of biochar showed a negative correlation with ETS, decreasing by 84.40% when SSA exceeded 100 m²·g⁻¹ (Figure S2).

Figure 3a illustrates the correlations between biochar characteristics and denitrification variables. Biochar pyrolysis temperature was significantly negatively correlated with nirS expression (p < 0.05) but showed no significant correlations with other denitrification indicators (p > 0.05). The N₂O emission was negatively correlated with biochar pH, SSA, EC, and ash content (p > 0.05).

A significant positive correlation (p < 0.05) was observed between ETS and biochar N (Figure 3). Additionally, the pyrolysis temperature was positively correlated with biochar pH, SSA, EC, EAC, ash content, and biochar C. Furthermore, the EDC displayed a positive correlation with pH and EAC but a negative correlation with SSA (Figure 3). Similarly, EAC was positively correlated with EC and C% but negatively correlated with pH.

3.1.2. Biochar Application Rate

The biochar rate did not significantly affect the soil NH₄⁺ and NO₃⁻ consumption capacity overall. However, at rates between 3% and 5%, there was a significant decrease in NH₄⁺, while a rate below 3% significantly increased NH₄⁺ consumption (Figure 4a). Additionally, the reduction in soil NO₃⁻ increased progressively with a higher biochar rate (Figure 3).

The N₂O emission rose when the biochar rate was below 5% but notably decreased by 99.18% when the biochar rate exceeded 5% (Figure 4c). Meanwhile, nirK increased by 28.99% to 76.61% at a biochar rate below 5%, whereas nirS decreased with an increasing biochar rate (Figure 4e). The nosZ content significantly increased by 55.62% at a biochar rate between 3% and 5% (Figure 4g).

The ETS also rose as the biochar rate increased (Figure 3). Correlation analysis further revealed that biochar rate was significantly negatively correlated with both N₂O emission and nirK (p < 0.05). In contrast, a positive correlation was observed between biochar rate and ETS (p < 0.05).

3.1.3. Experiment Duration

The experimental duration significantly influenced all denitrification indicators under biochar addition. Over time, NH₄⁺ and NO₃⁻ levels gradually decreased. At durations shorter than 7 days, N₂O emission was reduced by 72.24% compared to the control (Figure 4c). Throughout the incubation period, nirK and nirS contents remained lower than the control, while nosZ content was significantly higher from 7 to 90 days, increasing by 41.28% to 66.06%.

Although experimental duration had no significant effect on ETS, biochar was observed to enhance ETS throughout the entire experiment (Figure 4d).

3.2. Effect of Soil Physicochemical Properties Under Biochar Addition on Denitrification

Soil pH had a significant impact on NO₃^- reduction, with a 23.15% increase observed under alkaline conditions and decreased when soil pH ranged from 5 to 7 (Figure 5b). However, soil pH did not significantly affect NH₄⁺ concentration, N₂O emission, or the abundance of nirK, nirS, nosZ, and ETS.

Correlation analysis between soil physicochemical properties and denitrification indicators revealed that NH₄⁺ effect size was significantly negatively correlated with soil EC, cation exchange capacity (CEC), and total carbon (TC) but positively correlated with total nitrogen (TN) (Figure 3). Similarly, NO₃⁻ effect size showed a significantly negative correlation with soil CEC and a positive correlation with TN.

Additional correlations between soil properties were observed: soil pH was significantly positively correlated with EC, CEC, and TC (Figure 3). In contrast, EC exhibited a significantly negative effect on TN.

3.3. Effect of Different Ecosystem Types Under Biochar Addition on Denitrification

The meta-analysis of soils reveals a heterogeneous impact of biochar addition on denitrification indicators across various soil types. Biochar exhibited a soil-specific influence on ammonium and nitrate concentrations, with notable reductions in nitrate levels, particularly in agricultural soils like vegetable fields and grasslands, potentially indicating enhanced denitrification or nitrate immobilization (Figure 6b). Ammonium dynamics showed variability, with some soils exhibiting positive effects, possibly reflecting reduced nitrification or increased ammonification (Figure 6a). Furthermore, biochar consistently reduced N₂O emissions in several soil types, most prominently in farmland and forest soils, suggesting its role in mitigating greenhouse gas emissions (Figure 6c). However, its effects were less pronounced in grasslands, highlighting the importance of soil-specific characteristics in determining biochar’s efficacy.

In terms of microbial genes, biochar differentially influenced key denitrification genes (nirK, nirS, and nosZ). The reduction in nirK and nirS genes in certain soils suggests a suppression of nitrate reduction (Figure 6c,f), while the positive effect on the nosZ gene indicates an enhancement of the final step in denitrification, reducing N₂O to N₂ (Figure 6d). This points to biochar’s potential to promote complete denitrification, thereby minimizing N₂O emission.

3.4. Correlation of Soil Properties, Biochar Characteristics, and Denitrification Indicators

The results of correlation reveal complex relationships among biochar properties, soil physico-chemical characteristics, and denitrification indicators across various treatments (Figure 3). Key findings indicate that biochar properties, such as pH, SSA, and carbon-to-nitrogen ratio, exhibit strong correlations with nitrogen species and denitrification-related gene abundances (nirK, nirS, and nosZ). High biochar pH, for instance, positively correlates with nirK and nosZ gene abundances, suggesting enhanced microbial-driven denitrification, likely due to biochar’s effect on increasing soil pH and improving microbial habitats. This correlation aligns with previous findings, where higher pH biochar promoted microbial activity and nitrification in acidic soils.

Moreover, the results illustrate that soil properties like CEC and TN are significant contributors to nitrogen cycling, as they directly influence nitrogen retention and availability for microbial processes. Meanwhile, a strong positive correlation between TN and ETS activity suggests that nitrogen-rich soils support higher electron transport activity, indicative of active microbial metabolism. In contrast, biochar attributes, such as SSA, show negative correlations with ETS activity, underscoring the role of biochar surface characteristics in supporting denitrification pathways. This finding emphasizes the importance of tailoring biochar characteristics to specific soil types to maximize benefits for soil microbial activity and nitrogen cycling.

4. Discussion

4.1. Effect of Biochar Physicochemical Properties and Experimental Conditions on Denitrification

The impact of biochar on ammonium concentrations is generally negative across biochars with low C/N ratios, high ash content, and acidic pH (<7.5). This suggests that biochars with such properties enhance nitrification, converting NH₄⁺ to NO₃⁻ more effectively. Conversely, biochar with high C/N ratios and alkaline pH tends to retain NH₄⁺ in soils, likely through enhanced adsorption due to higher surface area and cation exchange capacity, aligning with previous studies [34,35]. For nitrate, biochar’s effect is predominantly negative, particularly for biochars with high ash content, neutral pH, and low C/N ratios [36]. This suggests that biochars with such attributes may accelerate denitrification or other nitrate-reducing pathways. High ash content might provide additional nutrients that stimulate microbial activity, leading to nitrate consumption [37].

The strongest reduction in N₂O emission is observed with biochars that have high C/N ratios (>30) and pH values above 7.5, suggesting that these biochars promote complete denitrification, driving N₂O reduction to N₂. High-C biochars, by promoting organic matter decomposition, create anaerobic microsites, which favor the activity of nosZ (N₂O reductase) and reduce the formation of N₂O [38,39]. In contrast, both low C/N ratios and acidic pH show less pronounced effects, likely due to limited microbial activity or inefficient electron donors for complete denitrification.

The gene expression patterns reflect biochar’s influence on microbial denitrification pathways. Both nirK and nirS show minimal to negative responses across biochars, particularly in those with low pH and high ash content, indicating a possible inhibition of the early denitrification stages under these conditions. However, biochars with higher pH and C/N ratios show positive effects on nosZ, suggesting that these biochars preferentially enhance the final step of denitrification, reducing N₂O emission [40,41]. This is consistent with the observed reduction in N₂O emission, which correlates with enhanced nosZ activity under specific biochar conditions [42].

The results highlight the role of biochar physicochemical properties, such as SSA, in promoting microbial activity and denitrification, while high ash content appears to negatively influence it. This is consistent with findings that emphasize the importance of optimizing pyrolysis conditions to improve biochar structural properties and nutrient retention capacity [43,44]. The rate of biochar demonstrates positive correlations with reduced N₂O emissions and increased denitrification genes (nirK, nirS, nosZ) at moderate rates but diminishing effects or adverse outcomes at higher rates. This observation supports the findings by Cayuela ML et al. [30], which suggest that balanced biochar rates are essential to avoid soil imbalances or microbial inhibition.

The ETS activity is an indicator of overall microbial respiratory activity in the soil. The forest plots suggest that biochars with higher C/N ratios and produced at higher temperatures stimulate ETS activity more effectively, reflecting an overall enhancement in microbial metabolism [45,46]. Biochar feedstocks rich in lignin tend to increase ETS activity, as lignin-derived biochars support microbial colonization by providing a stable carbon source and a large surface area for microbial attachment. Moreover, biochars produced at higher temperatures create more recalcitrant carbon structures, which can serve as long-term electron donors for microbial respiration [47]. This increased ETS activity can stimulate denitrification processes by providing the necessary electron flow for microbial enzymes involved in nitrogen cycling [48]. However, biochars produced from lower-temperature pyrolysis (<400 °C) and nutrient-rich feedstocks, such as manure or crop residues, might not consistently enhance ETS activity due to their higher volatile content and more labile organic compounds, which are quickly mineralized by microbes [26,27].

4.2. Influence of Soil Physicochemical Properties on Denitrification with Biochar

The physicochemical properties of soil, such as pH, CEC, SM, and organic matter, significantly affect how biochar influences denitrification pathways and microbial communities. Soil pH, in particular, has a profound impact on denitrification, as most denitrifying bacteria are more active in neutral to slightly alkaline conditions. Biochars produced at higher pyrolysis temperatures often have alkaline properties, and when applied to acidic soils, they can raise the pH, creating a more favorable environment for microbial denitrification [11,18,40,49]. The pH adjustment also promotes the activity of denitrifying genes such as nirK and nosZ, which are responsible for the complete reduction in N₂O to N₂ [50]. On the other hand, in already neutral or alkaline soils, high-pH biochars may have less effect on denitrification since microbial activities are already favored by such soil pH. Hence, the influence of biochar on soil pH is of special importance under acidic conditions, which alter the composition of the microbial community toward one favoring complete denitrification.

Other factors that interact with biochar in affecting nitrogen cycling include soil SM and CEC. Soils with a high CEC have potentially a high ammonium retention capacity and thus can trigger an enhancement in the ammonium retention and reduced nitrate leaching following the addition of biochars, possibly resulting in lower rates of nitrification and hence N₂O emission [51]. Biochar’s porous structure can also improve soil water-holding capacity, particularly in coarse-textured soils, providing more stable moisture conditions that support denitrifying bacteria under anaerobic or low-oxygen conditions, crucial for complete denitrification [21]. Additionally, soils with high organic matter content, such as forest or wetland soils, can leverage biochar as a habitat for microbes rather than as a primary carbon source, as these soils already contain ample organic material [39,50]. In these cases, biochar acts more as a structural amendment, enhancing microbial habitat complexity and influencing microbial dynamics by providing surfaces for biofilm formation and nutrient adsorption, further aiding denitrification processes [47]. Thus, optimizing biochar use requires an understanding of soil physico-chemical properties to target-specific mechanisms within nitrogen cycling.

4.3. Impact of Ecosystem Types on Denitrification and Biochar Interactions

The interaction of biochar properties with that of soil is highly contextual because specific types of soil may react more positively to the biochar additives due to their inherent characteristics, generally in regard to organic matter content, texture, and microbial communities. Wetland and paddy soils are often anaerobic; hence, a more dramatic reduction in N₂O emission after the application of biochar would be obtained with such a high level of organic matter [52,53]. This reduction may be attributed to biochar’s ability to create microhabitats and serve as electron donors that facilitate complete denitrification, resulting in the conversion of N₂O to N₂ [26]. Conversely, sandy or low-organic-matter soils may not derive significant advantages from biochar amendments, as these soils are deficient in microbial biomass and moisture retention capabilities essential for sustained microbial denitrification [54]. Thus, biochar amendments in sandy soils may primarily act as a source of carbon and habitat structure but might not significantly affect the reduction in nitrogen species to N₂.

Agricultural soils, such as those used for vegetable fields or vineyards, also show varying responses to biochar addition, which can be linked to management practices that influence soil microbiota and nitrogen dynamics [55,56]. In intensively managed soils, biochar may support the microbial communities responsible for denitrification by providing stable organic carbon and altering soil pH, thus enhancing microbial activity and the abundance of denitrifying genes such as nosZ [57,58]. However, in forest soils, which typically have acidic pH and low nitrogen availability, biochar’s effect on denitrification can be limited by the native soil’s low buffering capacity and mineral nutrient content [39]. Forest soils may necessitate elevated biochar application rates or biochars with certain attributes to effect a significant alteration in nitrogen cycle mechanisms. Consequently, customizing biochar treatments according to ecosystem types and land use patterns is crucial for enhancing denitrification and reducing nitrogen losses.

5. Conclusions

This meta-analysis reveals that the effects of biochar on denitrification and nitrogen cycling are highly influenced by biochar properties, soil physico-chemistry, and soil type. Biochar characteristics such as feedstock type and pyrolysis temperature determine its SSA, pH, and nutrient retention capacity, which in turn affect its performance in different soil environments. Soil physicochemical properties, including pH, organic matter content, and CEC, modulate biochar’s impact on nitrogen retention and microbial activity, enhancing nitrogen cycling processes in some soils while having minimal effects in others. Ecosystem types further influence biochar’s efficacy; for instance, sandy soils benefit from biochar’s water retention properties, while agricultural soils see increased microbial biomass and nitrogen efficiency. Future research should focus on changing biochar properties to specific soil and crop needs, improving feedstock selection, and developing functionalized biochars. Expanding long-term field trials across diverse ecosystems and regions will provide practical insights while integrating biochar into climate-smart agriculture and promoting adoption through policies and education will maximize its agricultural and environmental benefits.