A Critical Review of Biochar Application for the Remediation of Greenhouse Gas Emissions and Nutrient Loss in Rice Paddies: Characteristics, Mechanisms, and Future Recommendations

Abstract

Greenhouse gas emissions (GHG) and nutrient loss are considered to be critical environmental issues facing rice field ecosystems. Biochars (BCs), as carbon-rich materials with porous structures, large specific surface areas, and enriched surface functional groups, have shown great potential for improving environmental problems in rice fields and increasing rice yields. However, thus far, we still lack an adequate summary and review of the performance characteristics of BCs and their environmental behavior in paddy soils. In this paper, we critically review the positive effects of BC application on the growth and yield of rice, nutrient loss reduction, and GHG reduction. Biomass type, pyrolysis temperature, and modification method are considered to be the key factors that determine the performance characteristics of BCs. The application of BCs could promote rice yield and mitigate CO₂, N₂O, and CH₄ emissions by improving soil physicochemical properties and microbial communities, providing nutrient sources, and reducing nutrient losses. Finally, this paper illustrates the potential ecological risks of BC application on paddy fields, including the risks of inconclusive research results and secondary pollution. These shortcomings need to be addressed in future research to ensure the sustainability of BC application.

1. Introduction

With rapid industrial development and the extensive use of fertilizers and pesticides in agricultural production, global agricultural lands (especially paddies) are exposed to significant environmental threats, such as greenhouse gas (GHG) emissions and nutrient loss [1,2,3,4,5,6,7]. It is estimated that annual emissions of CH₄ and CO₂ from global rice fields exceeded 24 million tons and 680 million tons in the past decade, respectively [8]. According to Food and Agriculture Organization (FAO) estimates, the global use of chemical fertilizers (nitrogen, potash (K₂O), and phosphorus (P₂O₅)) exceeds 170 million tons per year. Among these chemicals, more than 2.1 million tons of N₂O are released globally each year due to the overapplication of chemical fertilizers (especially nitrogen) to rice fields [8]. These environmental problems have seriously threatened the security production and sustainability of paddy fields.

Biochars (BCs) are stable, carbon-rich materials with a high degree of aromatization and an anti-decomposition ability that are obtained by the pyrolysis of plant or animal wastes (e.g., agroforestry waste [9], sawdust [10], aquatic plant waste [11], animal manure [12], etc.) at 300–700 °C under limited oxygen/anaerobic conditions [13,14]. Currently, BCs have been widely used to improve the water, soil, and air environment due to its low cost and high efficiency. It is worth noting that BCs play an important role in increasing soil carbon sequestration, improving soil physicochemical properties, promoting crop yield, and reducing nutrient loss and GHG emissions [15,16,17,18,19], which is attributed to their large porosity and specific surface area (SSA), enriched minerals, high cation exchange capacity (CEC), enriched surface functional groups and negative surface charge [20,21]. The production and application of BCs provide a reference for the resource utilization of organic solid waste and, therefore, have certain advantages in terms of economic benefit and feasibility [22].

The trends of various application directions of BCs as a cost-effective soil amendment were presented in Figure 1, suggesting that BCs were an excellent strategy to improve existing environmental problems (i.e., greenhouse gas emissions and nutrient loss) in paddy fields and increase rice yield. However, thus far, we still lack an adequate summary and review of the performance characteristics of BCs and their environmental behavior in paddy soils. There is a huge difference between different ways of preparing biochar and its application in solving different rice paddy problems. In addition, there is insufficient information on the potential secondary risks of biochar application to rice fields. Therefore, it is necessary to comprehensively and systematically summarize the effects of applying BCs with different performance characteristics on improving environmental problems in paddy fields and increasing rice yield and the potential ecological risks that may result in subsequent studies. In this manuscript, a mainstream academic database (Web of Science) was used to search for relevant papers on the application of BCs to improve and remediate paddy soils. A total of 1729 papers were obtained using “BCs and paddy” as the keywords. Based on the titles and abstracts, we judged their relevance to the subject of this work and obtained more than 200 papers closely related to the subject.

In this study, the following aspects of BCs were reviewed comprehensively: (1) the effects of biomass type, pyrolysis temperature, and modification method on the performance characteristics of BCs; (2) the effects of applying BCs with different performance characteristics on rice yield, nutrient leaching, and GHG emissions; and (3) the potential ecological risks of BC application on paddy fields, including the risks of inconclusive research results and secondary pollution.

2. Factors Affecting the BC Performance Characteristics

The performance characteristics (e.g., C stability, functional group abundance, mineral content, cation exchange capacity (CEC), specific surface area (SSA), porosity, pH, and surface charge) vary significantly between different BCs, and those differences are mainly determined by multiple factors, such as feedstock type, pyrolysis temperature, and modification method (

Table 1. Effects of feedstock type, pyrolysis temperature, and modification method on the performance characteristics of BCs.

Feedstocks	Modified Methods	Temperature (°C)	pH	SSA (m2 g−1)	Ash (%)	Recalcitrant Index (R50)	Atomic Ratio		Content of Mineral Elements				CEC (cmol kg−1)	Reference
							H/C	O/C	P	K	Ca	Mg
Pine sawdust		350	5.75	3.39	12.30		1.19	0.44				0.07 a	56.13	[29]
		450	6.31	179.77	15.60		0.87	0.32				0.08 a	52.43
		550	6.66	431.91	11.90		0.80	0.26				0.10 a	47.43
		650	6.84	443.79	21.70		0.66	0.14				0.10 a	39.22
Vine pruning		250	7.35		5.00		1.22	0.55		0.06 b	0.07 b	0.03 b	60.95	[30]
		350	10.26		8.30		0.75	0.25		0.07 b	0.04 b	0.02 b	47.38
		600	11.31		11.50		0.41	0.13		0.11 b	0.01 b	0.01 b	32.23
Orange pomace		250	7.29		6.70		1.29	0.44		0.03 b	0.11 b	0.03 b	52.57	[30]
		350	9.88		11.30		0.84	0.19		0.06 b	0.02 b	0.01 b	35.23
		600	10.45		16.30		0.42	0.11		0.10 b	0.01 b	0b	25.59
Conocarpus waste		200	7.37		4.53		0.06	0.41	0.84 a	0.38 a	43.4 a	3.43 a		[31]
		400	9.67		5.27		0.04	0.18	0.88 a	0.54 a	51.8 a	3.98 a
		600	12.21		8.56		0.02	0.08	1.11 a	0.90 a	64.7 a	4.79 a
		800	12.38		8.64		0.01	0.06	1.34 a	1.15 a	67.5 a	7.81 a
Algal		250	8.72		22.90		1.21	0.71		3.24 b	0.75 b	0.07 b	81.23	[30]
		350	12.98		33.40		0.86	0.33		4.12 b	0.22 b	0.08 b	62.80
		600	13.66		42.70		0.38	0.15		5.49 b	0.16 b	0.08 b	49.80
Tire		300	6.95		13.1				0.01 b	0.13 b	0.63 b	0.04 b	5.53	[32]
		500	8.94		10.3				0.03 b	0.49 b	2.72 b	0.10 b	51.9
		700	10.2		10.9				0.01 b	0.58 b	3.15 b	0.17 b	10.90
Sewage sludge		500	8.81	25.42	74.21		0.48	0.45	18.19 a	8.52 a	59.29 a	14.74 a	76.76	[33]
		700	11.11	32.17	81.53		0.15	0.30	20.35 a	9.94 a	64.37 a	16.37 a	50.34
		900	12.15	67.60	100.09		0.09	0.12	20.34 a	9.68 a	69.56 a	17.52 a	247.51
Palm tree rachis (leaves)		600	10.23	164.73	38.68	0.62	0.53	0.2					39.86	[34]
	silica impregnated	600	9.02	140.37	71.39	0.75	1.48	0.11					33.20
	zeolite impregnated	600	9.09	153.28	68.37	0.57	1.94	0.46					76.27
Rice straw		350		166.9	18.0	0.51	0.94	0.22						[35]
		600		391.0	27.1	0.60	0.42	0.04
	FeCl3	350		206.2	21.1	0.42	0.92	0.27
	FeCl3	600		363.0	28.9	0.56	0.43	0.07
Swine manure		350		123.5	30.8	0.50	1.03	0.24						[35]
		600		325.8	45.0	0.61	0.45	0.06
	FeCl3	350		164.4	28.6	0.45	1.02	0.30
	FeCl3	600		267.6	43.3	0.57	0.56	0.17
Rice straw		450	7.1			0.55	0.40	0.10						[36]
	FeCl3	450	3.4			0.61	0.30	0.10
	AlCl3	450	3.1			0.53	0.90	0.2
Poultry litter		450	10.4			0.60	0.30	0.10						[36]
	FeCl3	450	1.8			0.61	0.50	0.10
	AlCl3	450	4.7			0.45	1.10	0.30

a represents total concentration (g/kg); b represents water-soluble (g/kg).

) [23,24]. Hence, when targeting specific objectives or objects, the physicochemical properties and functional modification treatments of BCs should be fully considered to maximize their treatment capacity and practical application capabilities. Among these functional modification treatments, materials such as clay minerals, rare earth elements (e.g., La and Ce), metal compounds, and phosphates are often considered for BCs in improving and remediating paddy soils [25,26,27,28].

2.1. Carbon Stability

The carbon stability of BCs is an important factor affecting the use of BCs for soil nutrient retention and GHG reduction because the unstable structure of BC will promote the release of nutrient ions by itself and provide C and N sources for GHG production [37,38]. Generally, low H/C and O/C ratios and a high thermal recalcitrance index (R₅₀) reflect BCs with higher aromaticity and stability [39]. Due to differences in crystallinity and polymerization degree, lignin is the most stable component in biomass, followed by cellulose and hemicellulose [40]. Therefore, BCs produced from feedstocks with higher lignin contents have more stable aromatic structures and slower mineralization rates than BCs produced from other components [41,42]. For example, BCs made from palm shell (which typically has a high lignin content) were the most stable, while wheat straw-derived BCs with low lignin content were the least stable [43]. In addition, pyrolysis temperature is another key factor affecting the stability of BCs [44,45]. The aromatic C content and stability in BCs increased with increasing pyrolysis temperature (most significantly at ≥400 °C), which was ascribed to the breakage or conversion of aliphatic C to more stable aromatic and heteroaromatic C [44,46]. The aromaticity of most BCs could reach 80% when the pyrolysis temperature was increased to more than 400 °C [46], and the content of aromatic C in BCs increased sharply [47]. In addition, inorganic or mineral modifications may increase or decrease the stability of BCs by changing their H/C and O/C ratios and the formation of metallic mineral phases on the surface or by promoting catalytic degradation [39,48,49]. FeCl₃-modified biochar increased their stability and R₅₀ by decreasing their H/C and O/C ratios, while AlCl₃ modification decreased the stability and R₅₀ of both BCs, which was ascribed to the formation of Al₄(SO₄)(OH)₁₀ on the surface of BCs increasing its O/C ratios [36]. The silica modification increased the R₅₀ of BC, which can be explained by the possible prevention of thermal degradation by encapsulating biochar particles with amorphous silica via Si-C bonding [34].

2.2. Functional Group Abundance

The abundance of surface functional groups affects nutrient retention in paddy fields by biochar [21]. The abundance of functional groups on the surface of BCs is determined by the pyrolysis temperature and feedstock type [50]. The atomic ratios of H/C and O/C in BCs decreased as the pyrolysis temperature increased (Table 1), which indicated that the abundance of acidic functional groups and polar groups decreased [21]. In general, the surface of low-temperature pyrolysis BCs (<400 °C) contains more acidic functional groups, while higher-temperature pyrolysis BCs contain more alkaline functional groups, which results in deoxygenation, dehydration and decarboxylation reactions [30,44]. For example, Moradi-Choghamarani et al. [51] increased the pyrolysis temperature from 300 to 600 °C and found that the concentration of total acidic functional groups, -COOH and phenolic acid functional groups on the surface of BCs decreased from 9 to 5.8 mmol g⁻¹, 6.2 to 4 mmol g⁻¹ and 2.4 to 1.5 mmol g⁻¹, respectively, while the total basic functional group increased from 0.8 to 4.1 mmol g⁻¹. Ma et al. [52] also confirmed that as the temperature increased from 350 to 750 °C, the aliphatic carbon structure of palm kernel shell BCs (including O-alkyl-C and carboxyl-C) decreased from 11.71% and 3.72% to 1.12% and 1.11%, respectively, while the aromatic carbon structure increased from 65.98% to 93.18%. In addition, BCs modified by using rare earth elements (e.g., La and Ce) and metal compounds can increase their surface functional group abundance [53,54,55]. Rare earth element modification can increase the hydroxyl groups (i.e., La-OH or Ce-OH) on the surface of BCs [56,57,58], which can facilitate the enhanced removal of anionic pollutants by BCs. Modification of iron compounds can increase the abundance of oxygen-containing groups and metal groups on the surface of BCs, which is ascribed to the formation of minerals such as ferrihydrite, goethite (α-FeOOH), lepidocrocite (γ-FeOOH), and hematite (α-Fe₂O₃/γ-Fe₂O₃) [59,60,61].

2.3. Mineral Content and Cation Exchange Capacity (CEC)

The application of BCs with high mineral content and CEC is conducive to maintaining soil nutrients and GHG reduction [62]. The total concentration of minerals (e.g., K, Ca, Mg, and P) in BCs increased with increasing pyrolysis temperature; however, the opposite trend was observed for water-soluble concentrations of mineral components (except K), which was attributed to increased crystallinity of minerals or incorporation of silica structures (Table 1) [21,63]. The CEC of BCs decreases with increasing pyrolysis temperature, which is mainly due to the decrease in mineral water solubility content, reduction in surface acidic functional groups and decomposition of volatile organic compounds [32,33]. In addition to temperature, feedstock type is also an important factor affecting the mineral content and CEC in BCs. In general, the mineral content and CEC of algal and biosolid biomass-derived BCs are higher than those of wood-derived BCs (Table 1) [33,64]. For example, the water-soluble mineral content and CEC of poultry litter- and seaweed-derived BCs were significantly higher than those of vine pruning- and orange pomace-derived BCs [30].

2.4. Specific Surface Area (SSA) and Porosity

BCs with porous structures and large specific surface areas (SSAs) have a higher capacity to maintain soil nutrients. High-temperature pyrolysis BCs have a higher SSA and porosity, which is attributed to the fact that increasing the temperature can facilitate the development of SSA and porosity by accelerating the processes of dehydration, volatilization of organic matter and carbon condensation [33,65,66]. In addition, the average pore size of BCs tended to decrease and was more conducive to microporous development as the pyrolysis temperature (400–700 °C) increased [67,68]. However, it should be noted that with increasing temperature, the porous structure of BCs may be damaged by tar, blocked by ash and collapse, and the formation of more macropores than micropores [21,69]. Fernandes et al. [70] observed that the surface area of BCs that were produced by eucalyptus wood was reduced at 850 °C compared to that at 650 °C (362.90 vs. 410.48 m² g⁻¹). Moreover, pyrolysis of lignin- and cellulose-rich feedstocks was more conducive to the development of BC pores, which is attributed to the good thermal stability of those feedstocks, enabling them to maintain the porosity structure [65,71]. For example, the SSA (5.9–594.92 m² g⁻¹) and total volume (0.006–0.415 cm² g⁻¹) of the plant-derived BCs from rice straw, Phragmites communis, and sawdust were much higher than the SSA (2.03–5.33 m² g⁻¹) and total volume (0.004–0.009 cm² g⁻¹) of the egg shell-derived BCs [72]. In addition, the SSA and porosity of BCs can be increased by physical or chemical activation [73,74,75]. The SSA of BCs after steam activation increased from 64.4 to 332 m² g⁻¹, and the pore volume increased from 0.05 to 0.29 cm² g⁻¹ [76]. After KOH modification, the SSA of BCs increased from 114.36 to 2183.80 m² g⁻¹ [77].

2.5. pH and Surface Charge

The pH and surface charge affect the adsorption of nutrient ions by BCs. The pH value of BCs increases with increasing pyrolysis temperature, which is due to the decomposition of acidic functional groups and the increase in alkaline-rich ash during pyrolysis [78,79]. For example, as the pyrolysis temperature increased from 200 to 800 °C, the surface acidic groups of conocarpus waste-derived BCs decreased from 4.17 to 0.22 mmol g⁻¹, while the basic groups increased from 0.15 to 3.55 mmol g⁻¹ [31]. Similarly, as the pyrolysis temperature increased from 300 to 700 °C, the pH value of rice straw BCs increased from 7.94 to 10.68, which was ascribed to the decomposition of acidic functional groups and the formation of alkaline minerals such as K₂O in the BCs [80]. Generally, the pH value of wood-derived BCs is lower than that of algal and biosolid biomass-derived BCs, which is closely related to the content of non-pyrolytic inorganic elements in biomass and the degradation degree of organic components [81,82]. The pH of biochar is usually alkaline, but there are some exceptions due to the differences in feedstocks. For example, the pH value of BCs prepared from pine sawdust at 350–600 °C was acidic (5.75–6.84) [29]. Furthermore, the pH value of BCs from sycamore sawdust produced at 300 and 500 °C was acidic (4.55 and 6.03), but the BCs were alkaline (7.88) at 700 °C [72]. When the pH of a solution is greater than the point of zero charge (pH_PZC), the BC surface is negatively charged; conversely, the BC surface is positively charged. The increase in the pH_PZC of BCs with increasing pyrolysis temperature can be explained by the decrease in the abundance of oxygen-containing functional groups and polar groups (e.g., -COOH, -COH, and -OH) on the surface of the BCs, resulting in a decrease in their negative surface charge [21,45]. For example, the pH_PZC of sludge BCs increased from 8.58 to 10.2 as the pyrolysis temperature was increased from 500 to 900 °C [33]. Tan et al. [83] also confirmed that the surface negative charge of BCs decreased gradually as the pyrolysis temperature was increased from 300 to 700 °C. Moreover, modification by rare earth elements (La and Ce) or metal compounds can also change the pH_PZC of BCs [56,58,84]. Wang et al. [85] found that La doping increased the pH_PZC of magnetic BCs due to the increase in the number of hydroxyl groups (i.e., La-OH) on the surface of the BCs.

Overall, BCs properties mainly depend on feedstock type and pyrolysis temperature. Medium- and low-temperature algae- and biosolid-derived BC with higher pH value, mineral content and CEC, and BC produced by high-temperature pyrolysis of lignin-rich straw or wood has higher SSA, porosity, and C stability. In addition, the modification method is also an important factor affecting the characteristics of BCs. For example, inorganic or mineral modification can increase the CEC and mineral content of BCs, while also reduce the C stability.

3. Application of BCs in Promotion of Rice Growth and Yield

3.1. Effect of BCs on the Growth and Yield of Rice

For a long time, returning straw has provided a source of nutrients and improve soil physicochemical properties, but it has also brought about environmental problems such as the rapid loss of nutrients and an increase in GHG emissions [86,87,88,89]. The application of BCs is a strategy that provides a good alternative to straw returning [87], which promotes rice growth and yield by improving soil physicochemical properties, mitigating nutrient loss, enhancing soil microbial activity and abundance, and increasing the gene expression level and resistance of rice [90,91,92].

Table 2. Effect of BC application on the improvement of rice yield.

Feedstocks	Pyrolysis Temperature	Application Rate	Times	Soil Type/Texture	Main Impact Factors	Yield	Reference
Rice straw	350–500 °C	5, 10 and 20 t ha−1	4 year	Sandy loam	Stimulated biodiversity, increased soil aeration and soil pH with different application rates	+2.82%, 3.56% and 7.47%	[92]
Rice straw	500 °C	22.5 t ha−1 (in the first year only)	3 year		Increased soil pH (0.26 units), TC (42.3%), and TN (13.8%)	+16.4%, 9.7% and 9.2%	[93]
Rice straw	550 °C	10.5 t ha−1	2 year	Gley paddy	Increased soil pH, SOC (8.5–11.1%), soil available P (33.9 and 15.3%), and available K (99.1 and 28.6%)	+8.5–10.7%	[94]
Wheat straw	500 °C	0.5–3% w/w	2 year		Both application rates increased soil pH and soil N availability	+1.8–7.3%	[95]
Bamboo chips and rice straw	600 °C	22.5 t ha−1	2 year	Clay loam	Significantly increased the NO3−-N (89.7–102.2%) content of rhizosphere soil	+19.8–21.6%	[96]
cassava straw	300–500 °C	20, 30 t ha−1		Ultisols	soil pH, soil organic carbon, total nitrogen, soil microbial carbon, soil microbial nitrogen	+10.56% and 10.46%	[97]
Wheat straw	350–450 °C	20 t ha−1		Anthrosol	Significantly increased SOC (32.5%), TN (14.2%), and nutrient availability	+28.4%	[91]
Rice husk	500 °C	4 t ha−1		Acid sulfate soil	Increased soil pH, nutrients (K, P, Ca and Mg), and total bacterial population	+41.87%	[98]
Wheat straw	550–600 °C	5, 20, 40 t ha−1		Silty loam	Decreased N and P leaching loss and increased N use efficiency with different application rates	+4.42–16.89%	[87]
Rice straw	450–500 °C	1.8, 3.6 mg ha−1		Saline–alkaline soil	Both application rates increased P availability and retention and increased CEC by 58.8% and 107.6%	+3.66 and 8.54%	[99]
Rice straw	600 °C	15, 30, 60 t ha−1		Clay and sand (2:1)	Increased soil pH, SOC (123–417%), nutrient availability, and N use efficiency with different application rates	+10.13–24.56%	[100]
Rice husk	300, 500 and 600 °C	2% w/w		Sandy loam	Increased soil nutrient availability with different pyrolysis temperature	+24.19%, 18.58% and 35.10%	[101]
Rice straw		20, 40 t ha−1		Dark-yellow	Both application rates reduced N loss and improved N use efficiency	1.67–5.54%	[102]
Wheat/rice/maize straw	550°C	2% w/w			Soil invertase, phosphatase, and urease promoted the mineralization of C, N, and P	+51.05%, 63.04% and 102.03%	[103]
Rice husk	500 °C	10, 20 t ha−1		Clay	Both application rates increased pH, CEC, and OC and the availability of N, P, and K, respectively	+52.2 and 65.4%	[90]
Rice husk	600 °C	1% w/w		Slightly acidic	Increased essential elements and water usage efficiency	+11–19%	[104]
Chicken litter		5 t ha−1		Sandy loam	Increased soil pH, TC, TP, TN, available P and exchangeable N	+86.44%	[105]
Sewage sludge	550 °C	5, 10% w/w		Sandy loam	Both application rates increased soil pH, TN, SOC and available nutrients	+148.8 and 175.1%	[106]
cassava straw		30 t ha−1		Ultisol	N uptake was associated with enhanced activities of N metabolism enzymes		[107]
wheat straw	500 °C	24, 48 t ha−1	3–4 year	granite red soil	Mortierella and Westerdykella promoted TOC degradation		[108]
wheat straw	350–550 °C	20, 40 t ha−1		sandy loam	increased the activities of dehydrogenase and alkaline phosphatases, and decreased β-glucosidase affect the soil C and N cycling		[109]
rice straw	500 °C	24 t ha−1		stagnic anthrosol	stimulated the microbial use of N-rich substances, such as amino acids		[110]
rice husk		1, 2, 5 and 10% w/w		riparian soil	soil microbial enhanced P mineralization and reduced N leaching		[111]

summarizes the effects of BC application on the improvement of rice yield.

First, the application of BCs can positively affect rice yield by improving the soil pH, CEC, soil aeration, and water holding capacity, providing a source of nutrients and enhancing nutrient use efficiency. For example, Qin et al. [92] observed that BC treatment with different application rates improved soil microbial biodiversity, soil aeration and soil pH while increasing rice yield by 2.82–7.47%. Ghorbani et al. [90] found that the application of rice husk BCs increased soil pH, CEC, and OC and the availability of N, P, and K while increasing the rice yield by 52.2–65.4%. Similarly, Maikol et al. [105] and Khan et al. [106] also found that the addition of BCs increased the soil nutrient content and increased the yield of rice. In addition, some researchers have applied BCs with loaded nutrient elements or N and P fertilizer together, which is beneficial for improving the utilization efficiency of N and P fertilizer and rice yield while reducing production costs [105,112,113]. For example, Huang et al. [114] showed that BC application increased the uptake of nitrogen fertilizer by rice by 23–27%, thus increasing grain yield by 8–10%. Similarly, Zhang et al. [115] obtained similar results by applying BCs in combination with nitrogen and phosphorus fertilizers. However, there are also reports that high-temperature pyrolysis (≥600 °C) BC amendment may decrease rice productivity, which can be explained by the strong adsorption of soil nutrients by BCs, resulting in the reduction of nutrient uptake by rice [116,117]. Thus, BCs produced at pyrolysis temperatures lower than 550 °C might be more favorable for promoting rice yield [118].

In addition, BC amendment had a positive effect on improving rice yield by reducing soil nutrient loss and alleviating stress, which is relevant to the improvement of soil physicochemical properties and its slow release after adsorption [119,120]. For example, the addition of BCs improved the availability and retention of P [99], alleviated TN and TP leaching losses and increased rice yield, which was ascribed to increased soil pH and maintenance by itself [87]. Rare earth elements (La and Ce) and metal compound modifications can increase the pH_PZC and surface functional groups of BCs; thus, their application is considered a potential strategy to reduce the loss of anionic nutrients in the soil [121,122,123]. For example, the application of nano CeO₂-modified BCs can reduce the PO₄³⁻ concentration in the surface water of rice fields and increase the PO₄³⁻ content in the surface soil [25]. Wu et al. [27] also confirmed that MgO-modified BC amendments alleviated the leaching of P loss and increased the effective P content in paddy soil. In addition, soil texture (e.g., saline soil, acidic sulfate soil, clay loam and sandy loam) is also an issue worth considering before applying biochar. For example, Nguyen et al. [124] and Zhang et al. [125] found that rice straw derived BC significantly reduced the salt stress on rice in saline soil and promoted rice growth by improving soil physicochemical properties. Panhwar et al. [98] observed that the addition of rice husk BC in acidic sulfate soils alleviated low pH stress and Al³⁺ and/or Fe²⁺ toxicity, and promoted the growth of rice. Qin et al. [126] revealed that the application of aged BCs with lower mineral contents to humid acidic highly weathered soils will have limited P retention capacity. Compared with clay soil, the application of BCs may promote the leaching of nutrients in sandy soil, due to its lower retention capacity of nutrients [111,127]. Therefore, Algae and biosolid-derived BC are more suitable for improving acidic soils with low pH and poor nutrition due to its higher pH and higher mineral content. For saline and sandy soils, Wood-derived BCs with large SSA and porous structure is a good choice to reduce salt stress and leaching of nutrients.

In addition, the application of BCs can promote the growth and yield of rice by promoting the activity of microorganisms and enzymes, which is attributed to microorganisms and enzymes that promote the decomposition and mineralization of soil organic matter and nutrients [109,128]. BCs stimulated the relative abundance of acidic bacteria and fungi (e.g., Mortierella and Westerdykella) and decreased the abundance of potential phytopathogens of Athelia and Penicillium, which favored the promotion of TOC mineralization [108]. Jing et al. [103] found that different straw BC treatments favored to promote the mineralization of soil C, N and P by increasing soil invertase, phosphatase, and urease activities, and also increased the yield of rice. Tian et al. [129] reported that microorganisms start to mine N from the SOM to compensate for high C:N ratios after BC application, which consequently accelerate cycling of stable N. Similarly, Ali et al. [97] reported that BC amendment significantly promoted the relative abundance of bacteria such as Acidobacteria, Actinobacteria, Bacteroidetes, Planctomycetes, and Proteobacteria, which was strongly associated with increased rice yield.

Moreover, BC amendment may improve the resistance and yield of rice by promoting gene expression levels, and inhibiting the growth and development of pests [130,131,132]. BC-inspired H₂O₂ accumulation and transcription of genes involved in the ethylene (ET) signaling pathway enhance rice plant resistance and inhibit root-knot nematode growth and development [133]. Noguera et al. [134] proved that BCs promote the growth and yield of rice by enhancing leaf protein turnover (both catabolism and anabolism). Similarly, Shen et al. [135] found that nano-BC reduced the toxic effects of root exudates (ferulic acid) from Imperata cylindrica on rice seedlings, which was attributed to the fact that nano-BC reduces the level of oxidative stress and lipid peroxidation in rice and reduces negative gene expression at the molecular level. In addition, some other researchers have found that BC treatment may also promote rice quality, which was due to the increase in related enzyme activities and gene expression levels [136,137]. BC treatment significantly increased the transcript levels of genes encoding starch synthase and starch branching enzymes, thus improving the viscosity and taste quality of rice starch [138]. Ali et al. [137] found that applying BC improved the quality and yield of rice by promoting the activities of nitrate reductase, glutamine synthetase and glutamine 2-ketoglutarate aminotransferase. However, the literature on specific enzymes produced by biochar is very limited, which may be the direction of future research.

3.2. Preparation/Modification Recommendations and Research Limitations

The effects of BCs on the growth and yield of rice were related to its C/N, mineral content, CEC, functional group abundance, and surface charge. Medium- and low-temperature algae and biosolid-derived BC with high mineral content and CEC are good choices for improving the growth and yield of rice, but biosolids (e.g., sewage sludge and poultry litter) may contain heavy metals that are still noteworthy. In addition, modification of biochar with rare earth elements (La and Ce) or metal compounds or combined application with nitrogen and phosphorus fertilizers is conducive to mitigating the loss of soil nutrients and improving N and P utilization efficiency and rice yield.

However, the shortcoming of the current stage of research is that most of the studies on the effect of BC on improving rice growth and yield have remained at the empirical or observational level, and more reasonable practical application strategies and more in-depth mechanistic studies are still lacking. First, the effects of BC addition on the growth and yield of rice vary greatly and are mainly related to soil type, biochar type, application rates and patterns, and water management practices. Practical application strategies for various BCs, different application rates and patterns, and different water management practices in different soil types are still lacking. Second, most of the studies on BC in increasing rice resistance, yield, and quality lacked analysis of its mechanism at the cellular metabolism and gene molecular expression levels. In addition, we recommended that research on the interaction of microorganisms with BC with different performance characteristics, soil nutrient cycling and rice growth, and metagenomics and metatranscriptomics could be used to further analyze the effects of BC on the soil microbiome, function, and enzyme activities.

4. Application of BCs in Mitigation of Greenhouse Gas (GHG) Emissions from Rice Fields

4.1. Effect of BCs on GHG Emissions

Paddy systems are an important source of GHGs (mainly CO₂, N₂O, and CH₄) [2,139,140]. The use of BCs, as a prospering carbon sequestration technology, is recognized as an effective strategy to mitigate GHG emissions in rice fields [15,141,142]. First, BC amendment promoted the abundance and activity of relevant genes and enzymes (e.g., nosZ, N2OR, pmoA genes and methanotrophs) during denitrification and CH₄ production and improved soil physicochemical properties [139,143]. In addition, the application of BCs with a higher SSA and porosity and a more stable aromatic structure favored the reduction of GHG emissions from paddy fields by direct adsorption, decreasing the available C and N contents in the soil and inhibiting nutrient mineralization (Figure 2a) [96,140,144].

Specifically, the application of BCs had a positive effect on the emission reduction of CO₂, N₂O and CH₄ in rice fields by promoting the abundance and activity of relevant genes and microorganisms during denitrification and CH₄ production [143,145,146]. BC amendment favors the mitigation of N₂O emissions by promoting the activity and abundance of nosZ and N2OR to convert N₂O to N₂ during denitrification [147,148,149]. In addition, the application of BC amendments can mitigate CH₄ emissions in rice fields, and the mechanism for this can be interpreted as follows: BCs promote the activity of methanotrophs and inhibit the activity of methanogens and reduce the ratio of methanogens/methanotrophs (mcrA/pmoA) [140,150,151]. For example, Han et al. [152] observed that BC treatment inhibited the activity of methanogens and increased the activity and pmoA gene abundance of methanotrophs in paddy fields. Wu et al. [151] found that BCs increased the potential methane oxidation capacity by enhancing the total, type I and type II methanotrophs in soil. Qi et al. [153] also demonstrated that the application of BCs to reduce the methanogens/methanotrophs (mcrA/pmoA) ratio in rice fields was associated with mitigated CH₄ emissions.

Second, the application of BCs increases the soil pH, CEC value and soil aeration and reduces the soil bulk density, which helps mitigate CO₂, N₂O and CH₄ emissions in rice fields [154,155,156]. The increase in soil pH promoted the abundance and activity of nosZ, N2OR, and methanotrophs [147,157,158] and increased CO₂ solubility and bicarbonate formation [159], which helped mitigate the generation of CO₂, N₂O, and CH₄ in rice fields. However, the liming effect of BCs tends to disappear over time, leading to a regression of soil pH [160,161], which limits the role of BCs in mitigating GHG emissions from rice fields. The increase in soil CEC values promoted the adsorption and retention of NH₄⁺-N in the soil [154,162]. In addition, increased soil aeration and reduced soil bulk density led to greater O₂ exposure in the soil, which promoted denitrification function and methanotrophic activity and inhibited methanogenic bacterial activity [92,152,155,163]. The application of BCs can inhibit the production of CO₂ and CH₄ by promoting rice root growth and increasing radial oxygen loss (ROL) [140]. However, more O₂ exposure inhibits the relative abundance and activity of N2OR, resulting in decreased reduction of N₂O to N₂ [164,165].

In addition, the application of high-temperature (>600 °C) pyrolysis BCs can mitigate CO₂, N₂O and CH₄ emissions by promoting biotic or abiotic reaction processes and inhibiting soil nutrient mineralization or reducing available C and N in the soil, which is attributed to its higher SSA and porosity and aromatic structure and oxygen-containing functional groups (C=O, -COOH and quinone groups) that can act as good electron shuttles and accelerate the denitrification process [146,166,167,168]. For example, the application of high-temperature pyrolysis BCs (700 °C) could inhibit CH₄ emissions by adsorbing some inherent organics in the soil, while low-temperature pyrolysis BCs (300 °C) increased the content of degradable fluorescent organics in the soil, which could provide more substrate for CH₄ production [169]. BC treatment significantly increased the relative abundance of iron-reducing bacteria, prompting competitive electrons between Fe(III) reduction and CH₄ generation, resulting in a reduction in CH₄ emissions [158]. Moreover, BCs with greater SSA and porosity can directly absorb CO₂, N₂O, and CH₄ and provide habitat for methanotrophs [145,158,170].

Although the application of BCs has had many positive impacts on GHG emissions reduction, the negative impacts cannot be ignored (Figure 2b). Application of BC enhanced the abundance and activity of related genes and enzymes (e.g., nirK, nirS, amoA, ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), methanogens, nitrite reductases) during nitrification, denitrification and CH₄ production [146,156,171]. BC amendment may stimulate the respiration and activity of methanogens and microbes, which are processes that contribute to increased CO₂ and CH₄ emissions [143,172]. Treatment with BCs promotes the relative abundance and activity of AOA and AOB with the amoA gene code, which stimulates ammonia-oxidation-induced N₂O production during nitrification [171,173]. Lin et al. [174] observed that 4% BC treatment increased the relative abundance of AOB amoA and the abundance and diversity of AOB in soil, which led to a 291% increase in N₂O emissions. BC amendment increased N₂O production, which was ascribed to improving the abundance and activity of nirK, nirS and nitrite reductases to convert NO₂⁻ to N₂O during denitrification [146,175,176]. In addition, the addition of less stable BCs, especially those produced through low-temperature pyrolysis (<400 °C), may decompose and release labile C themselves and promote soil nutrient mineralization and the production of rice rhizosphere secretions, which is beneficial in promoting CO₂, N₂O, and CH₄ emissions [144,177,178,179]. Intervention with NH₄⁺-N may have stimulated CH₄ emissions from rice fields, which was ascribed to the competitive oxidation of NH₄⁺-N and CH₄ by methanotrophs [161,178]. The addition of BCs promoted the growth of rice roots and increased the content of rhizosphere secretions, which can provide more available C and N for methanogens and other microorganisms [180,181,182].

4.2. Preparation/Modification Recommendations and Research Limitations

The effect of BC on GHG from rice fields was closely related to its SSA, porosity, aromaticity, and C stability. BC produced by high-temperature pyrolysis of lignin-rich straw or wood has higher SSA, porosity, and C stability and is a preferred choice for greenhouse gas reduction materials. However, current studies have mainly investigated the effects of BC on soil GHG production from the perspective of soil physicochemical properties and the abundance and activity of relevant microorganisms, and more reasonable preparation conditions for BC and more in-depth mechanistic studies are still lacking. First, more detailed studies on the effects of BC properties (C stability, porosity, SSA, oxygen functional groups, electron transferability, etc.) on GHG emission reduction are lacking, which hinders the development of new BC materials and more reasonable strategies for practical applications. Second, the effect of increased rice rhizosphere activity (e.g., rhizosphere secretion and ROL) after BC addition on GHG emissions from rice fields needs to be further explored, and high-performance liquid chromatography (HPLC) and microprobes can be used in the future to analyze the effect of rhizosphere secretion components and oxygen distribution in the micro-oxygen zone on microbial distribution and on GHG emissions. In addition, the effects of BC interactions with biotic and abiotic organisms and the related gene abundance and activity of rhizosphere microorganisms on GHG emissions from rice fields need to be further explored. In the future, techniques such as real-time quantitative PCR (qPCR) and high-throughput sequencing should be used to further analyze the effects of the abundance, activity, and community structure of related genes and microorganisms on GHG reduction in rice fields.

5. Potential Ecological Risks

Although BCs have shown great potential for alleviating environmental problems in rice fields and improving rice yields, differences in feedstock sources, pyrolysis conditions, and modification methods of BCs can lead to vastly different performance characteristics and some uncertainties, which may reduce the potential of BC application for rice field improvement and rehabilitation or even create negative outcomes, such as the release of toxic substances, increase in GHG emissions, secondary release of pollutants, block the biodegradation of pesticides, and inhibit the growth and development of rice and microorganisms. In inclusion, the potential ecological risks resulting from the application of BCs in paddy systems are still important issues of current concern (Figure 3).

5.1. Toxic Substances in BCs

BCs may also contain toxic substances such as heavy metals, volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), dioxins and furans (PCDD/Fs), and persistent free radicals (EPFRs). The production of these toxic substances is mainly related to the feedstock type and pyrolysis temperature [183,184,185]. In general, biosolid biomass such as wastewater sludge, livestock litter and food residues as feedstocks for biochar production may contain higher levels of heavy metals, dioxins and PAHs, probably due to the excessive use of additives in feedstuffs and higher chlorine content [24,186]. The formation of EPFRs is facilitated by the thermal decomposition reactions of biomass rich in lignin, cellulose and hemicellulose during pyrolysis [46,185,187]. In addition, it has been shown in many studies that BCs contain higher levels of heavy metals, VOCs, PAHs, dioxins, and EPFRs at low and medium pyrolysis temperatures (200–600 °C) and short residence times [184,188,189]; at high temperatures (>600 °C) and longer residence times, the contents of heavy metals, dioxins, PAHs and EPFRs in BCs will gradually decrease [186,187].

Toxic substances released from BCs may have direct toxic effects on rice and soil microorganisms while increasing the bioaccumulation risk of rice and inconclusivity [13,95,190]. For example, Zhang et al. [191] reported that two nanobiochars produced under high-temperature conditions (700 °C) inhibited the germination of rice seeds, which may be related to phenolic compounds on the surface of BC. Anjum et al. [192] found that PAHs produced during pyrolysis of BC had mutagenic effects on Salmonella/microsomes. EPFRs induced the production of reactive oxygen species (ROS) that inhibited rice germination and growth and poisoned soil organisms, which was attributed to free radical-induced oxidative damage [189,193]. Bai et al. [194] found that the toxicity of BC pyrolyzed at moderate and high temperatures (>500 °C) to rice could be ascribed to free radical-induced oxidative damage, and low-temperature (200 °C) pyrolysis of BC was the direct toxicity of heavy metals and PAHs to rice. Lieke et al. [195] reported that EPFRs in BC could trigger neurotoxic effects in Caenorhabditis elegans, thereby inhibiting its living characteristics (movement and defecation) in soil.

5.2. BC Aging and Excessive Application

Due to fragmentation and decomposition by biotic and abiotic reactions, aged BCs may release DOM, nano- and micron-sized BC colloids/particles, and endogenous contaminants and promote the mineralization of native organic carbon in soils, resulting in reduced carbon sequestration potential [196,197,198]. For example, Feng et al. (2022) found that aging biochar promoted a 25% increase in CO₂ emissions and that the mitigation effect of BC on N₂O emissions was decreased by 15% compared to that of fresh biochar due to accelerated C decomposition and mineralization and facilitated digestion and weakened denitrification by aging biochar. Aging causes an increase in the specific surface area and pore volume of biochar and oxidation of π-π electron-rich aromatic C rings, which promote the rerelease of heavy metals and organic pollutants that were originally adsorbed [185,199]. In addition, the release of DOM and nano- and micron-sized BC colloids/particles may increase the mobility, bioavailability, and toxicity of heavy metals and organic pollutants in soil [198,200]. For example, Kim et al. [201] observed that DOC released from biochar increased the release and mobility of As in soils. Chen et al. [123] found that biochar nanoparticles promoted P loss in acidic or alkaline soils by mediating P retention and migration. Hameed et al. [198] reported that biochar colloids can act as a mediator to promote the transport of organic pollutants, leading to increased mobility and biotoxicity of organic pollutants.

Moreover, excessive application of BCs may inhibit soil organic matter mineralization and/or reduce nutrient availability, resulting in inhibition of rice growth and development and microbial abundance and activity [202,203,204]. BCs can alter the intercellular communication of microorganisms by adsorbing signaling molecules and promoting their hydrolysis, thus changing the microbial community structure [185,205]. In addition, the application of BC with a high C/N ratio may induce poor nutrient uptake by rice, which may inhibit its growth and yield [206,207,208].

5.3. BCs Block the Biodegradation of Pesticides

BCs may inhibit the activity and biodegradation of pesticides (e.g., herbicides and insecticides) by strong adsorption, which may lead to the overuse of pesticides and their accumulation in agricultural production and more serious negative environmental impacts [209,210]. Yu et al. [211] showed that the addition of BC (0.5% w/w) to soil significantly increased the adsorption of acetamiprid and blocked its biodegradation. Khorram et al. [212] reported that the degradation half-life of flumioxazin increased significantly from 34.6 d to 50.8, 82.7 and 160.3 d with 0.5%, 1% and 2% rice husk BC amended soil, which could be explained by the increased adsorption capacity of BC for flumioxazin. In addition, BC-catalyzed production of ROS (e.g., •OH and •O₂⁻) may convert pesticides into more toxic compounds [65,213].

6. Conclusions and Prospects

BCs have shown great potential in addressing global environmental problems in rice fields and in increasing rice yields, so their application is considered a green, economical, and environmentally feasible strategy for improving and remediating paddy fields. The type of biomass, pyrolysis temperature, and modification method are key factors that determine the performance characteristics of BCs. The application of BCs or modified BCs with different performance characteristics can promote rice yield and mitigate GHG (e.g., CO₂, N₂O, and CH₄) emissions by improving soil physicochemical properties and microbial communities, providing nutrient sources, and reducing nutrient losses. Although BCs are increasingly used as amendments in paddy remediation, as highlighted in this paper, there are still some potential ecological risks of BCs in rice fields that need to be further explored. We recommend the following future research projects.

(1)

Future research needs to sieve suitable biomass feedstocks and adjust the process parameters (e.g., increase pyrolysis temperature and residence time), and establish standardized and common evaluation mechanisms or indicators for BC toxicology to reduce the production of toxic substances.

(2)

Future research should focus on establishing standardized or recommended BC production conditions and application rates. The differences in raw material sources, pyrolysis conditions, and modification methods can lead to great differences in the performance characteristics of BCs. Various functional BCs still have problems of insufficient actual treatment capacity and high economic cost when addressing different aspects of improving rice fields. Hence, future research needs to further develop high-efficiency, economical, and green functional BCs along with applications of other substances to improve rice fields.

(3)

Future research should eliminate the gaps and uncertainties that exist in the application effects of BCs as much as possible. Currently, most of the relevant studies are short-term and at a laboratory scale. The impact of the BC digestion rate on carbon sequestration, nutrient mineralization, GHG emissions, and the adsorption–desorption dynamics of nutrients should be quantified in the field. In particular, the secondary pollution and potential ecological risks that aging BCs may pose should be fully evaluated.