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Review

Effects of Biochar on Gaseous Carbon and Nitrogen Emissions in Paddy Fields: A Review

by
Yidi Sun
1,
Xuetao Wang
1,
Chenxia Yang
2,
Xiaoping Xin
3,
Junlin Zheng
4,
Tao Zong
1 and
Chaoyin Dou
1,*
1
College of Hydraulic Science and Engineering, Yangzhou University, Yangzhou 225009, China
2
Jiangsu Water Conservancy Survey and Design Institute Co., Ltd., Yangzhou 225009, China
3
Department of Soil and Water Science, Indian River Research and Education Center, University of Florida, Fort Pierce, FL 34945, USA
4
College of Water Conservancy, Shenyang Agricultural University, Shenyang 110866, China
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(7), 1461; https://doi.org/10.3390/agronomy14071461
Submission received: 31 May 2024 / Revised: 1 July 2024 / Accepted: 1 July 2024 / Published: 5 July 2024
(This article belongs to the Special Issue Organic Amendments to Low-Fertility Soils: Current Status and Future Prospects)
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Abstract

:
The paddy field is a major source of gaseous carbon and nitrogen emissions, and reducing these emissions is of great significance for mitigating greenhouse effects and non-point source pollution in farmland. Biochar, derived from agricultural waste, possesses a stable structure, large specific surface area, abundant pore structures, and surface functional groups. These characteristics could enhance soil physicochemical properties and microbial activity, thereby facilitating the dual goals of increasing crop yield and reducing emissions. Based on numerous studies, this review summarizes the effects of biochar on the emissions of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ammonia volatilization (NH3), as well as on global warming potential (GWP) and greenhouse gas emission intensity (GHGI). It elucidates the mechanism of emission reduction by biochar amendment from the perspective of carbon and nitrogen conversion processes and soil physicochemical and biological properties. Numerous studies showed the application of 5~40 t ha−1 biochar reduced CO2, CH4, N2O, and NH3 emissions by 1.64~89.6%, 8.6~89.6%, 10~90%, and 12.27~53%, respectively. A small number of studies found that the application of 5~48 t ha−1 biochar increased CO2, CH4, N2O, and NH3 emissions by 12~37%, 19.85~176%, 13~84.23%, and 5.47~70.9%, respectively. Most scholars have found that biochar has varying degrees of emission reduction capabilities in different parts of the world. Therefore, future research directions have been suggested for utilizing biochar to reduce carbon and nitrogen emissions in paddy fields.

1. Introduction

Global climate change, driven by the greenhouse effect, has become a major environmental issue receiving close attention from the international community [1]. Agricultural production activities are one of the major sources of greenhouse gas emissions, accounting for about 58.8% of the total human-induced greenhouse gas emissions [2,3]. In agricultural systems, approximately 12% of carbon dioxide (CO2) and 18% of methane (CH4) emissions come from paddy fields [4], contributing to about 30% of global anthropogenic carbon emissions [5]. The collective contribution of these two greenhouse gases to global warming is approximately 82% [6]. Consequently, the Earth’s temperature has risen by nearly 1.09 °C compared to the early 20th century [7]. This has caused many plants to advance their growth cycles, with flowering and fruiting occurring earlier than usual, leading to a mismatch with the activities of pollinators and affecting pollination efficiency. Additionally, global warming creates a more favorable environment for pathogens and pests, resulting in an increase in plant diseases and pest infestations, which in turn affects plant health and yield [8]. Therefore, achieving carbon sequestration and emission reduction in paddy fields is imperative. Additionally, rice plays a crucial role in food production, leading to a significant increase in nitrogen fertilizer usage in paddy fields to improve grain yield. The excessive application of N fertilizers resulted in substantial N loss into the atmosphere and water bodies [9]. Nitrous oxide (N2O) emissions produced by nitrification–denitrification processes constitute one of the primary pathways for nitrogen loss in paddy fields. The greenhouse gas contribution potential of N2O over 100 years is 265 times greater than that of CO2 [10]. Additionally, ammonia volatilization (NH3) is one of the most important pathways of N loss, ranging from 10~50% of N fertilizers applied annually in global rice production systems. Gaseous ammonia in the air can react with other gaseous compounds, combining to form secondary particulate matter, which is an important component of air pollution. These particles not only deteriorate air quality and increase the frequency of fog and haze weather but also pose a threat to human health by triggering respiratory diseases and cardiovascular issues [11]. When these particles settle into water bodies, they can cause eutrophication, leading to the excessive proliferation of algae and the formation of algal blooms. This process consumes a large amount of dissolved oxygen, resulting in the death of fish and other aquatic organisms [12]. Therefore, addressing the reduction of nitrogen emissions and improving nitrogen fertilizer utilization efficiency in paddy fields are also pressing concerns.
Biochar, derived from the resource utilization of agricultural waste (such as wood, straw, manure, etc.), is a high-carbon solid product obtained through the high-temperature pyrolysis of biomass in the absence of oxygen or under anaerobic conditions [11]. It possesses a stable structure, a large surface area, a well-developed pore structure, and abundant surface functional groups [13]. Later, technological advancements have led to the continuous exploration of biochar’s ecological benefits. The benefits of biochar, including carbon sequestration, nitrogen fixation, yield improvement, and reduction of carbon and nitrogen emissions, have gained widespread attention [14].
Biochar, with its inherent stability, can utilize by-products of the decomposition of organic carbon in the soil, enhancing the soil carbon sequestration capacity and indirectly reducing the emissions of CO2 and CH4. The large surface area and abundant oxygen-containing functional groups of biochar enable it to perform well in controlling nitrogen emissions. Biochar can adsorb and fix NH4+-N in the soil, thereby preserving nitrogen and reducing NH3 volatilization at the source [15]. The alkaline nature of biochar can improve the soil pH, indirectly promoting the reduction of N2O to N2 [16]. Moreover, biochar itself is a type of organic amendment. Applying biochar to paddy fields can effectively enhance soil nutrient content, promoting crop growth and resulting in increased yields and income. Moreover, biochar’s high stability in soil allows it to act as a carrier for slow-release fertilizers, which gradually release nutrient elements, reducing fertilizer loss and improving fertilizer utilization efficiency. The long-lasting effects of biochar contribute to the continuous improvement of soil fertility [17]. In comparison to other measures of carbon and nitrogen emission reduction, biochar offers significant advantages in terms of raw materials, cost, production methods, and emission reduction potential [18]. Therefore, studying the effects of biochar on gaseous carbon and nitrogen emissions in paddy fields is of great importance. This article comprehensively describes the effects and mechanisms of emission reduction by biochar amendment in paddy soil. It provides a theoretical foundation and technological basis for the efficient development and utilization of biomass resources and the reduction of carbon and nitrogen emissions from agricultural fields.

2. Effects of Biochar on Carbon and Nitrogen Content and Its Effect Mechanism in Paddy Soil

2.1. Effects of Biochar on Carbon Content and Its Carbon Sequestration Mechanism in Paddy Soil

Soil organic carbon (SOC) results from the decomposition of organic matter by soil microorganisms, accounting for about 60~80% of soil organic matter, and is an important component of the soil [19]. SOC is not only closely related to soil fertility but also plays a dominant role in the carbon cycle, functioning as both an important “source” and “sink” for CO2 and CH4 [19,20]. Studies have found that the application of biochar significantly increased soil SOC [21,22], resulting in an increase ranging from 14.3~101.6%. Xiao et al. [23] found that the application of peanut shell biochar to saline-alkali soil increased SOC by 14.3~71.5% through integrated analysis methods. Wei et al. [24] found that the most effective black soil carbon sequestration stability was achieved with the corn straw biochar addition of 50 t·ha−1 to the soil. The capacity of carbon sequestration by biochar amendment varies significantly across different studies, with variations attributed to soil types, climatic conditions, field water, and fertilizer management.
The application of biochar directly increases SOC content and initiates biochemical reactions with soil microorganisms. This endows biochar not only with the function of carbon sequestration but also with the significant potential to “stabilize and enhance carbon sinks” [25]. The primary mechanisms by which biochar increases SOC content are as follows: ① Biochar could promote plant metabolism, and the decomposition of withering plants by biochar amendment returning to the soil could increase the input of organic carbon. Dai et al. [26] found that the application of biochar increased the soil organic carbon content by 16%, which might also be due to biochar accelerating litter decomposition, thereby promoting the conversion of litter to SOC. ② Biochar has a fertilizing and water-retaining effect, which promotes plant root growth, increases root exudate secretion, and thereby enhances carbon input into the soil [27]. Sun et al. [28] found that the production of plant exudates increased by up to 564% with biochar application compared to non-biochar-treated plants.③ The enriched nature of biochar provided ample nutrients and substrates for microorganisms, creating a basis for their propagation and, in turn, promoted their conversion to SOC. Zhang et al. [29] found that high rates of biochar application (50 to 100 t ha−1) increased microbial residues by 3~5%. However, the application of biochar did not significantly change the carbon content of fungal and bacterial residues. Zhang et al. [30] discovered that corn straw biochar significantly increased the fatty acids of bacteria and fungi while decreasing glycosamine in the organic carbon, which also suggested that microbial residues were one of the effective pathways for increasing SOC.

2.2. Effects of Biochar on Nitrogen Content and Its Nitrogen Fixation Mechanism in Paddy Soil

Nitrogen fixation is an important part of nitrogen cycling in the farmland ecosystem. Biochar has been widely used for its excellent performance in nitrogen fixation. Scholars have conducted extensive studies to explore the effects of biochar on soil nitrogen fixation and related microbial activity. Wang et al. [16] found that applying biochar to soils for different regions (China, Pakistan, Germany, Japan) could all achieve nitrogen fixation effects. Meng et al. [31] found that biochar made from corn and rice straw could enhance soil nitrogen fixation capacity, with the best nitrogen fixation effect observed at 40~60 g·kg−1 biochar addition in a pot experiment. Li et al. [32] found that the addition of peanut shell biochar to tropical farmland significantly increased soil nitrogen content when applied at 40 t·ha−1 and 60 t·ha−1, with increases of 38.12% and 62.99%, respectively. Zhao et al. [33] observed that biochar prepared from spent mushroom substrate, when mixed with fertilizer and pig manure, increased soil nitrogen content by 74.05% in subtropical red soil in a pot experiment. Ghorbani et al. [34] found that, compared to the individual application of urea, legume residue, and azolla compost, the combined application of biochar with urea, legume residue, and azolla compost significantly increased the nitrogen supply to plants. This indirectly indicated that biochar could enhance soil nitrogen fixation.
On the one hand, the nitrogen fixation by biochar is attributed to its strong adsorption capacity for ammonia due to its unique porous structure, large specific surface area, and rich functional group content. On the other hand, nitrogen-fixing microorganisms in the soil absorb and convert N2 from the air to NH3; this process is known as soil biological nitrogen fixation, and these microorganisms are called nitrogen-fixing microorganisms. The activity of nitrogen-fixing microorganisms and related enzymes are easily influenced by environmental factors, mainly including oxygen, trace elements (iron and molybdenum), and nutrient availability [35]. The addition of biochar to the soil could enhance the enzyme activity of nitrogen-fixing microorganisms, thereby increasing their nitrogen-fixing capacity [36]. The enhancement of nitrogen-fixing microorganism enzyme activity by biochar can be explained in three aspects: ① The porous structure of biochar could improve soil aggregation, soil aeration, and water retention capacity, which are crucial for the growth and activity of nitrogen-fixing microorganisms [37]. ② The porous structure of biochar provided a natural protective barrier for microbial habitats, protecting them from predation pressure and environmental fluctuations [38]. ③ The rich functional groups on the surface of biochar could participate in redox reactions in the soil. For nitrogen-fixing microorganisms, these capabilities were more conducive to their nitrogen metabolism activities [35].

3. Effects of Biochar on Gaseous Carbon Emissions and Its Effect Mechanism in Paddy Fields

3.1. Effects of Biochar on CO2 and CH4 Emissions in Paddy Fields

Conventional rice farming relies on the heavy application of fertilizers, which not only increases agricultural costs but also results in several environmental problems, such as greenhouse gas emissions, water eutrophication, and soil acidification [39]. The application of biochar to soil is considered a promising measure for greenhouse gas mitigation and carbon sequestration. Numerous studies showed that biochar application could significantly alter the physicochemical properties and microbial activity of soil, thereby reducing CO2 and CH4 emissions from paddy fields [22,40,41,42,43,44]. Pei et al. [45] found that adding corn stick biochar to soil reduced the temperature sensitivity of soil decomposition, leading to decreases in soil active carbon content, soil respiration rate, and enzyme activity, thereby reducing CO2 emissions. The study by Tong et al. [46] found that aged biochar can reduce CO2 emissions by 7.33–18.78%, regardless of whether the sandy loam soil is fully irrigated or deficit-irrigated. Wang et al. [47] demonstrated that the application of biochar could decrease CH4 emissions from southern double-cropping paddy fields. Specifically, the application of 20 t·ha−1 and 40 t·ha−1 biochar significantly reduced cumulative CH4 emissions by 32.43% and 41%, respectively. Li et al. [48] reported that the application of 40 t·ha−1 biochar during rice and wheat seasons significantly reduced CH4 emissions by 8.6% and 11.3%, respectively. Cayuela et al. [49] found that biochar application significantly reduced greenhouse gas emissions in 14 types of farmland soils. Sun et al. [50] discovered that applying high amounts of wheat straw biochar could increase soil carbon sequestration, thereby reducing greenhouse gases, which was consistent with the findings of Spokas [51]. However, some researchers showed that biochar could increase CH4 emissions. For example, Yuan et al. [52] found that the application of rice straw biochar increased cumulative CH4 emission by 19.85% in a pot experiment. Wang et al. [53] found that applying 20 t·ha−1 and 40 t·ha−1 coconut chaff biochar in conjunction with nitrogen fertilizer increased CH4 emissions by 99% and 176%, respectively, compared to no biochar addition. In summary, most studies indicated that biochar could decrease gaseous carbon emissions, while a minority of studies suggested that biochar might increase CH4 emissions (Table 1). This difference primarily resulted from variations in biochar types, application rates, and soil conditions [54].

3.2. The Effect Mechanism of Biochar on Gaseous Carbon Emissions in Paddy Fields

3.2.1. The Effect Mechanism of Biochar on CO2 Emissions in Paddy Fields

The response of farmland soil carbon decomposition to temperature determines the magnitude of CO2 emissions. Soil carbon pools are typically categorized into unstable and relatively stable carbon pools, with the unstable carbon pool being chemically manageable and capable of transforming within a few months [56]. Therefore, the mechanisms by which biochar affects CO2 emissions primarily encompass the following aspects (Figure 1): ① The application of biochar reduced the decomposition of soil organic matter, significantly decreased the temperature sensitivity of the soil, and thereby reduced CO2 emissions. Yang et al. [55] found that as the aromaticity of light and mineral-free carbon components in the soil increased, the temperature sensitivity of carbon also increased, but no correlation was found between the temperature sensitivity of carbon and the carbon quality index or the carbon molecular structure of the bulk soil organic matter, which may be due to the protective role of the soil mineral matrix. ② Biochar enhanced soil carbon availability, altered microbial structure, increased microbial activity, improved the fungi-to-bacteria ratio, and reduced the microbial metabolic quotient, all of which contribute to promoting plant growth. Plants utilize carbon sources, converting them into organic carbon, and thereby suppressing CO2 emissions [44]. ③ Biochar application increased moisture content, which reduces the sensitivity of soil respiration to temperature to a certain extent [57]. In addition, enzymes and microorganisms mitigate the accelerating effect of temperature on soil organic carbon decomposition by reducing the activation energy of decomposition reactions and enhancing adaptability to temperature changes, slowing down the decomposition rate of organic matter and ultimately reducing CO2 emissions from the soil.

3.2.2. The Effect Mechanism of Biochar on CH4 Emissions in Paddy Fields

CH4 emissions in paddy fields are mainly determined by three processes: methane production, methane oxidation, and methane transport, all of which affect CH4 emissions. The mechanism of reducing CH4 emissions in paddy fields by biochar amendment could be attributed to the following aspects (Figure 1): ① The physical adsorption of CH4 molecules by biochar, along with its large specific surface area and loose porous structure, assisted in the absorption of a substantial amount of CH4 by the soil, consequently decelerating CH4 production [58]. ② From the perspective of changes in soil physical properties, the addition of biochar reduced soil bulk density [59] and improved soil aeration conditions, water content, and soil moisture. This reduced the activity of CH4-producing bacteria while also increasing the activity and abundance of CH4-oxidizing bacteria, ultimately leading to reduced CH4 emissions. ③ From the perspective of changes in soil chemical properties, microbial structure, and related enzyme activities, the application of biochar increased soil pH [60], leading to a decrease in the activity of CH4-producing bacteria [61,62], promoting the growth of CH4-consuming bacteria, and increasing the abundance of CH4-oxidizing bacteria [63]. Additionally, biochar promoted the growth and development of rice roots, enhanced the activity of root oxidases, and facilitated the rapid oxidation of produced CH4, thereby inhibiting CH4 emissions [64]. The possible mechanism by which biochar promoted CH4 emissions was that the excessive application of biochar increased the soil carbon pool and root exudates, providing sufficient carbon sources for CH4-producing bacteria [58].

4. Effects of Biochar on Gaseous Nitrogen Emissions and Its Effect Mechanism in Paddy Fields

4.1. Effects of Biochar on N2O and Ammonia Emissions in Paddy Fields

Biochar, as an emerging functional soil amendment material, could alter the physical and chemical properties of the soil, thereby affecting the nitrogen emission process in paddy fields [54]. Most studies showed that biochar could inhibit N2O emissions from various soils [65,66,67]. Zhang et al. [54] demonstrated that the application of 10 t·ha−1 and 40 t·ha−1 of crop straw biochar significantly reduced N2O emissions in double-cropping paddy fields. Singh et al. [68] found that the application of poultry manure biochar to the soil reduced N2O emissions by 72%. Cayuela et al. [49] conducted a 15N isotope tracer experiment and found that biochar reduced soil N2O emissions by 10~90% in 14 different soils. Wu et al. [58] conducted a field experiment and found that aged biochar significantly reduced N2O emissions by 19.5~26.3% in the rice–wheat rotation system. Hua et al. [69] summarized studies on the impact of biochar on greenhouse gas emissions in farmlands from central China, indicating that straw char was superior to other types (wheat stalk, animal manure, bamboo, woody, rice husk) of biochar in inhibiting greenhouse gas emissions from farmlands. However, Shen et al. [70] studied the effect of straw biochar on greenhouse gas emissions from double-cropping rice and found that the application of 24 t·ha−1 and 48 t·ha−1 biochar increased N2O emissions by 13~80%, compared to no biochar addition. Lan et al. [71] applied slag biochar to paddy fields and found that the N2O emission flux increased by 84.23%.
Some studies found that the CEC of biochar generally ranged between 71.0 and 451.5 cmol kg−1 [72], which was much higher than that of paddy soils (7 to 30 cmol kg−1); applying biochar at a 10% mass ratio to the soil could significantly reduce ammonia volatilization [73]. Amin et al. [74] found that, compared to no biochar application, biochar significantly reduced the cumulative emissions of ammonia volatilization in the oil-contaminated soil. Mandal et al. [75] applied three different types of biochar (poultry litter biochar, green waste compost biochar, and wheat straw biochar) produced at temperatures ranging from 250 °C to 700 °C to calcareous soil and found that they all suppressed ammonia volatilization, reducing nitrogen loss by 53%, 38%, and 35%, respectively. Qi et al. [76] applied biochar in paddy fields under controlled irrigation and found that compared to no biochar, the use of biochar reduced the cumulative emission of ammonia volatilization by 12.27%. Zhang et al. [77] found that biochar reduced ammonia volatilization by 13.57~29.5%. However, when compared to bulk biochar, aged biochar showed a decrease of 9.38~14.71% in ammonia volatilization. Contrarily, Zhang et al. [78] found that the addition of straw biochar increased the cumulative amount of ammonia volatilization by 9.45%, and during the base fertilizer and top-dressing periods, it increased by 5.47% and 13.44%, respectively. Feng et al. [79] found that wheat stalk biochar application improved the ammonia volatilization in the paddy field by 24.8~70.9% compared to the no biochar addition. In summary, biochar as a soil amendment has been shown in some studies to reduce N2O emission and ammonia volatilization; however, some studies found opposite conclusions (Table 2). Therefore, the effectiveness of biochar application on N emission reduction should be evaluated based on specific soil types and environmental conditions to fully leverage its potential and contribute to sustainable agricultural development.

4.2. The Effect Mechanism of Biochar on Gaseous Nitrogen Emissions in Paddy Fields

4.2.1. The Effect Mechanism of Biochar on N2O Emissions in Paddy Fields

The mechanism by which biochar inhibits N2O emissions could be mainly attributed to the following points (Figure 2): ① The high adsorption and ion exchange properties of biochar was beneficial for nitrogen fixation, reducing the likelihood of their utilization by microorganisms, reducing the substrates for nitrification and denitrification, inhibiting the activities of urease and protease, and ultimately suppressing N2O emissions [58]. ② The application of biochar provided ample C and N sources for soil microbial activities, providing energy and nutrients for microorganisms, promoting their growth and metabolic activities, enhancing the activity of N2O reductase, accelerating the conversion of N2O to N2 and thus reducing N2O emissions [82,83]. ③ The addition of biochar increased soil pH, porosity, and aeration, and the release of ethylene in paddy fields reduced the activity of denitrifying bacteria and their related enzymes, decreasing N2O emissions [42,84]. The increase in N2O emissions could be attributed to the fact that biochar provided additional carbon substrates for denitrification, stimulated soil microbial activity, and provided more mineral nitrogen to microorganisms under high C/N ratio conditions, thereby promoting denitrification reactions and further increasing N2O emissions.

4.2.2. The Effect Mechanism of Biochar on Ammonia in Paddy Fields

The main mechanisms by which biochar reduced ammonia volatilization emissions in paddy fields could be attributed to the following three aspects (Figure 2): ① Physical adsorption mechanism. Biochar had a strong adsorption ability for NH4+ due to its large specific surface area and rich pore structure, thereby promoting nitrogen fixation and reduced ammonia volatilization. Additionally, it interacted with root exudates, increasing its surface negative charge and enhancing NH4+ adsorption [85]. ② Gas–liquid equilibrium disturbance mechanism. Biochar improved soil water retention, slowed down water dispersion, and enhanced the soil water solution’s ability to retain ammonia, thereby reducing ammonia volatilization [86]. Additionally, Liu et al. [81] found that the application of biochar increased the temperature of the surface soil, with higher temperatures resulting in slower volatilization rates. ③ Biogeochemical regulation mechanism. Biochar regulated the C/N ratio in the soil, inhibiting organic nitrogen mineralization and thereby enhancing nitrogen stability and reducing ammonia volatilization. The main factors for biochar to increase ammonia volatilization were attributed to the following points: ① Biochar was rich in basic ions such as Ca2+, K+, and Mg2+; these ions exchange with H+ and Al3+ after being applied to the soil, significantly increasing soil pH. The increase in soil pH would affect the conversion balance of NH4+ to NH3, shifting the equilibrium to the right and thereby promoting ammonia volatilization emissions [80]. ② The addition of biochar promoted the activity of soil microorganisms and accelerated the hydrolysis of urea, increasing the concentration of ammonia nitrogen on the soil surface and thus promoting ammonia volatilization [87]. ③ Biochar increased soil permeability and effectively promoted gas exchange, thereby accelerating ammonia volatilization [81].

5. Effects of Biochar on Yield, GWP, and GHGI in Paddy Fields

Numerous studies have shown that biochar can improve soil porosity, enhance water retention, increase soil fertility, and consequently improve crop yields [88,89]. Zhang et al. [90] found that the addition of biochar led to a rice–wheat rotation farmland yield increase of 10~16% compared to no biochar addition. Zhang et al. [91] studied the effect of biochar produced at different pyrolysis temperatures (300, 500, 700 °C) and under different acidification levels (pH = 5, 7, 9) on rice yield, finding that high temperatures with acidic and neutral biochar had a significant yield-increasing effect, with an average increase rate of 11.71%. While most studies indicate that biochar could enhance crop yield, there are also studies suggesting that biochar has no impact on or even decreases field yield. For example, Liu et al. [92] discovered that the application of biochar significantly reduced rice yield by 20.3% compared to the untreated biochar treatment. Therefore, the increase in crop yield by biochar was closely related to soil type and the amount and types of biochar applied.
Greenhouse gas emission intensity (GHGI) stands for the comprehensive greenhouse gas intensity per unit yield. Global warming potential (GWP) is an index measuring the greenhouse effect of a substance. Compared to GWP, GHGI combines the greenhouse effect with crop yield, making it more convenient to evaluate the relationship between the agricultural ecological environment and crop yield. Wu et al. [93] found that applying biochar in a rice–wheat rotation system not only reduces GWP emissions but also increases rice yield. In addition, they found that biochar that had been aged in the soil for 3 years could still reduce emissions. The treatment with aged biochar significantly reduced the GHGI of the rice–wheat rotation system compared to the control, with a reduction of 29.3%, and compared to fresh biochar, the reduction was 14.7%. Liu et al. [94] observed that when biochar was applied in combination with humic acid in a rice–wheat rotation system, both GWP and GHGI were significantly reduced (8.2% to 43.6%). Hua found that applying 20 to 40 t·ha−1 of biochar was a good choice to achieve both increased yield and reduced emissions. Liu et al. [95] discovered that the win–win goal of increased productivity and reduced greenhouse gas emissions was achieved when applying 10~20 t·ha−1 biochar. In summary, the majority of studies indicated that the application of biochar can, to some extent, increase crop yield and effectively reduce GWP and GHGI, especially with significant effects observed with the addition of 20~40 t·ha−1 biochar.

6. Conclusions

Most studies indicate that the application of biochar could mitigate the emissions of CO2, CH4, N2O, and ammonia in paddy soil. The incorporation of biochar reduces the sensitivity of soil temperature, enhances soil carbon effectiveness, and alters microbial structure changes; microbial activity increases, fungal proportion increases, and microbial metabolic quotient decreases, thereby reducing CO2 emissions. Biochar application improves soil aeration, moisture content, soil humidity, pH, and other physical and chemical conditions. This reduces the activity of CH4-producing bacteria and increases the activity of CH4-oxidizing bacteria, thus reducing CH4 emissions. Applying biochar reduces the substrates for nitrification and denitrification by absorbing nitrogen, inhibits the activity of urease and protease, enhances the activity of N2O reductase, and suppresses denitrifying bacteria activity, thereby decreasing N2O emissions. Biochar enhances the soil’s capacity to retain NH4+, inhibiting nitrogen mineralization. It also increases soil water retention, slows down water loss, and increases the temperature of the topsoil, thus reducing ammonia volatilization. However, biochar application might increase carbon emissions by enhancing the soil carbon pool, providing additional carbon substrate for denitrification and improving the soil pH for ammonia volatilization. Furthermore, most studies find that biochar improves crop yield and reduces GWP and GHGI.
Additionally, this review has not covered the appropriate biochar application techniques, the carbon and nitrogen conversion rates in soil by biochar amendment, the long-term effectiveness of biochar, and its potential drawbacks. Therefore, the following are some suggested studies to focus on in the future:
(1)
Investigate the optimal biochar species and application rates for different soil types. Additionally, detail proper biochar application techniques, including preparatory steps and methods for blending with other materials to achieve optimal emission reduction through the rational application of biochar.
(2)
Exploring effective, environmentally friendly, and economically viable methods to modify biochar, aiming to minimize its application rate and mitigate potential adverse effects on carbon and nitrogen emissions.
(3)
Biochar, rich in organic matter, serves as an energy source for microorganisms, which participate in the fixation of carbon and nitrogen in soil. Thus, accurately quantifying the impact of biochar application on microbially mediated carbon and nitrogen conversion processes is essential.
(4)
It is essential to maximize the benefits of biochar while also recognizing its potential drawbacks. For example, biochar may impact the material and energy cycle in the soil and the soil microbial community, thereby affecting the soil ecological balance. Therefore, the correct application of biochar should take into account both the macro and micro aspects to make such research more valuable and convincing.
(5)
The current research on the long-term effects of biochar on greenhouse emissions, grain yield, and soil health in paddy fields is not yet sufficient. Key areas requiring further exploration include the stability of biochar in soil, its decomposition rate, and its long-term effects on soil chemistry and physical and biological properties.
(6)
Compared to fresh biochar, aged biochar exhibited higher stability and stronger nutrient retention capabilities. However, there has been relatively little research on the impact of aged biochar on reducing greenhouse gas and ammonia volatilization emissions, as well as on its emission mechanism. Therefore, it is necessary to further study the role of aged biochar in these aspects to assess its real effectiveness in reducing agricultural environmental pollution.

Funding

This research was funded by China Postdoctoral Science Foundation [grant number 2023M742952], the Natural Science Foundation of Jiangsu province, China [grant number BK20220594], the Natural Science Foundation of China [grant number 52209063], and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Conflicts of Interest

Author Chenxia Yang was employed by the company Jiangsu Water Conservancy Survey and Design Institute Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Agronomy 14 01461 g001
Figure 1. Carbon dioxide and methane emission mechanism.
Figure 1. Carbon dioxide and methane emission mechanism.
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Figure 2. Nitrous oxide and ammonia volatilization emission mechanism.
Figure 2. Nitrous oxide and ammonia volatilization emission mechanism.
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Table 1. Effects of Biochar on CO2 and CH4 emissions in paddy fields.
Table 1. Effects of Biochar on CO2 and CH4 emissions in paddy fields.
Carbon EmissionBiochar
Application Rate		Decreased Range Increased RangeReferences
CO210~40 t ha−11.64~89.6% [40,43,45,55]
20~40 t ha−1 12~37%[10,44]
CH420~40 t ha−18.6~89.6% [42,47,48,49,50,51]
20~40 t ha−1 19.85~176%[44,47,52]
Table 2. Effects of Biochar on N2O and ammonia emissions in paddy fields.
Table 2. Effects of Biochar on N2O and ammonia emissions in paddy fields.
Nitrogen EmissionBiochar
Application Rate		Increased RangeDecreased RangeReferences
N2O10~40 t ha−110~90% [44,65,67,68,69]
24~48 t ha−1 13~84.23%[70,71,72]
Ammonia volatilization 5~40 t ha−112.27~53% [73,74,76]
5~40 t ha−1 5.47~70.9%[78,79,80,81]
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Sun, Y.; Wang, X.; Yang, C.; Xin, X.; Zheng, J.; Zong, T.; Dou, C. Effects of Biochar on Gaseous Carbon and Nitrogen Emissions in Paddy Fields: A Review. Agronomy 2024, 14, 1461. https://doi.org/10.3390/agronomy14071461

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Sun Y, Wang X, Yang C, Xin X, Zheng J, Zong T, Dou C. Effects of Biochar on Gaseous Carbon and Nitrogen Emissions in Paddy Fields: A Review. Agronomy. 2024; 14(7):1461. https://doi.org/10.3390/agronomy14071461

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Sun, Yidi, Xuetao Wang, Chenxia Yang, Xiaoping Xin, Junlin Zheng, Tao Zong, and Chaoyin Dou. 2024. "Effects of Biochar on Gaseous Carbon and Nitrogen Emissions in Paddy Fields: A Review" Agronomy 14, no. 7: 1461. https://doi.org/10.3390/agronomy14071461

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