Introduction

Biochar (BC) is known to effectively mitigate methane (CH₄) emission from rice (Oryza sativa L.) paddies (Liu et al., 2011; Han et al., 2016; Nan et al., 2022). In a recent data analysis, Pia et al. (2024) reported that application of BC from various feedstocks decreased the area- and yield-scaled CH₄ emission by 14.4% and 22.2%, respectively. The decreased CH₄ emission by BC application is mediated by the changes in soil properties such as improved aeration in the rhizosphere (Dong et al., 2013), decreased organic substrate for methanogens (Han et al., 2016), increased soil pH favoring methanotrophs (Wang et al., 2019; Yi et al., 2019), and increased electron accepting powers of the soils (Zhang et al., 2019a, 2019b). These changes in soil properties eventually lead to increased abundances of methanotrophs over methanogens (Nan et al., 2021).

For precise utilization of BC for CH₄ mitigation, several studies compared the efficiency of BC in reducing CH₄ emission in terms of feedstock type, pyrolysis temperature, and BC application rates as summarized in Pia et al. (2024). It is understood that BC produced from lignocellulosic feedstock (rather than herbaceous feedstock) at higher temperature (rather than lower temperature) effectively mitigate CH₄ emission due to stronger stability and more functional groups (Wang et al., 2015; Lee et al., 2023b). In terms of BC application rate, it has been suggested that annual BC application at a low rate (e.g. 2 t ha^-1) is better than heavy application at once due to potential toxic compounds contained in BC and the aging of BC (Nan et al., 2020; Baek et al., 2024; Pia et al., 2024). However, the BC effects on CH₄ emission are not consistent under different experimental conditions including fertilization rates, organic inputs, and water managements (Awad et al., 2018; Zhao et al., 2019; Yagi et al., 2020; Nan et al., 2021; Novair et al., 2023). Therefore, it is necessary to investigate the variations in the BC-induced changes in CH₄ emission under different rice cultivation conditions for more precise utilization of BC for CH₄ mitigation.

In this review, we investigated the changes in the BC performance on CH₄ emission under different rice cultivation conditions such as nitrogen (N) fertilization rates, with or without organic inputs, and traditional vs. water-saving irrigation. We hypothesized that 1) BC performance on CH₄ mitigation may be greater with increasing N fertilization rates as N is an essential element for methanotrophs, 2) co-application of BC with organic inputs may efficiently decrease CH₄ emission by reducing substrate for methanogens via sorption onto BC compared to organic inputs without BC, and 3) application of BC may further decrease CH₄ emission under water-saving irrigation by promoting CH₄ oxidation under aerobic conditions.

Literature Search and Data Analysis

We collected as much as literature from the Web of Science and Google Scholar databases using the following keywords: TS = (“biochar”) and TS = (“paddy”) and TS = (“methane”) and TS = (“fertilization” or “organic” or “irrigation”). The number of publications was 17, 87, and 12 for fertilization, organic, and irrigation, respectively. Among those, 6, 3, and 3 publications that investigated BC effects on CH₄ emission with N fertilization (e.g., without vs. with N fertilization at varying rates), organic inputs (e.g., without vs. with organic inputs), and irrigation methods (e.g., traditional flooding vs. water-saving irrigation), respectively were selected finally (Tables 1,2,3). We are aware of that the number of the publications is very limited due to lacking of relevant studies that investigated the effect of BC on CH₄ emission with paired treatments. However, we believe the review may provide an insight into the rice cultivation practices to mitigate CH₄ emission using BC in combination with other agricultural measures such as N fertilization, organic inputs, and water-saving irrigation.

The average CH₄ emission data for each treatment were directly obtained from the tables and from the figures using GetData Graph Digitizer (version 2.26). The changes in the CH₄ emission by BC were computed as a percentage difference from the control without BC application.

N fertilization effects on the changes in CH₄ emission by biochar

The effects of N fertilization on BC-induced changes in CH₄ emission were not consistent, but there were evidences on the enhanced decreases in CH₄ emission by BC under N fertilization (Fig. 1) because N is an essential element for methanotrophs particularly in N-poor soils (Bodelier and Laanbroek, 2004). For example, Iboko et al. (2023) analyzed the effects of different rates of BC on CH₄ mitigation under varying N fertilization rates through literature analysis. In the study of Iboko et al. (2023), application rates of BC were grouped into low (≤ 9 t ha^-1), medium (> 9 and ≤ 20 t ha^-1), and high (> 20 t ha^-1) while N fertilization rates were classified into low (≤ 140 kg N ha^-1), medium (> 140 and ≤ 240 kg N ha^-1), and high (> 240 kg N ha^-1). They found that compared to N fertilization alone, co-application of BC at a high rate with medium N rates decreased CH₄ emission by 82%. In comparison, co-application of BC at a low rate with low N rates increased CH₄ emission by 114%. Increased CH₄ emission after low N application has been reported regardless of BC co-application because of N limitation for methanotrophs (Bodelier and Laanbroek, 2004; Banger et al., 2012; Shaukat et al., 2019). Sun et al. (2016) also reported that N application at < 100 kg N ha^-1 stimulated CH₄ emission while N application at > 200 kg N ha^-1 inhibited CH₄ emission. In a study to investigate the effects of BC from rice straw (1.78, 14.8, and 29.6 t ha^-1) on CH₄ emissions from paddy without or with N fertilization (210 kg N ha^-1), Sui et al. (2016) also found that BC increased CH₄ emission under no N fertilization by 10.2 - 55.8% but decreased CH₄ emission by 13.5 - 138.7% with N fertilization, suggesting that CH₄ mitigation by BC is dependent on N fertilization rates.

Fig. 1.

Summary of the directions and mechanisms of the changed CH₄ emission by biochar with N fertilization, organic inputs, and water-saving irrigation.

In a pot experiment using nine treatment combinations of BC (0, 2, and 4%) and N fertilization (0, 70, and 140 kg N ha^-1), however, Shaukat et al. (2019) found that BC application increased CH₄ emission relative to no BC treatment at a given N fertilization rate. Specifically, BC application at 2 and 4% increased CH₄ emission by 136% and 156%, 51% and 63%, and 38% and 32% for 0, 70, and 140 kg N ha^-1, respectively. The increased CH₄ emissions by BC application were attributed to supply of C substrate for methanogens (Gaihre et al., 2011) and releases of inhibitory chemicals for methanotrophs from BC (Spokas et al., 2010; Pia et al., 2024). Despite the stimulative effects of BC on CH₄ emission in the study of Shaukat et al. (2019), however, the increment of CH₄ emission by BC decreased with increasing N fertilization rates, suggesting that co-application of BC and N fertilizer reduce CH₄ emission from soils amended with BC alone through supply of N, which is an essential element for methanotrophs (Bodelier and Laanbroek, 2004). Therefore, the study of Shaukat et al. (2019) implies that the possible BC-induced increases in CH₄ emission can be alleviated by N fertilization. In contrast, Zhang et al. (2010) reported that CH₄ emission marginally changed by N fertilization, and BC application (10 and 40 t ha^-1) increased CH₄ emission in soils without N fertilization by 65 - 68%. However, with N fertilization (300 kg N ha^-1), co-application of BC at 10 t ha^-1 did not change CH₄ emission, but BC application at 40 t ha^-1 increased CH₄ emission by 54%.

The decreased CH₄ emission from N-fertilized soils by BC is often related to the beneficial effects of N on methanotrophs (Bodelier and Laanbroek, 2004). However, high N application may also reduce CH₄ emission by decreasing root biomass and activity releasing C substrate for CH₄ production within rhizospheres as roots do not have to grow extensively to explore N under high N availability (Chen et al., 2018). In contrast, it has also been reported that a high N application may increase CH₄ emission by inhibiting CH₄ oxidation under a high NH₄⁺ concentration as methanotrophs uses NH₄⁺ instead of CH₄ for energy sources while using O₂ as an electron acceptor (Adviento-Borbe and Linquist, 2016). It has also been suggested that unless N availability for methanotrophs is not limited, a high N fertilization increase CH₄ emission via decreased CH₄ oxidation through competitive exclusion of CH₄ at binding sites of methanotrophs by NH₄⁺ (Hu and Lu, 2015; Yang et al., 2020).

Despite the inconsistency of N fertilization effects on BC-induced mitigation of CH₄, in a meta-analysis study of Jeffery et al. (2016), it was reported that co-application of BC with N fertilizer at < 120 kg N ha^-1 decreased CH₄ emission while it did not affect CH₄ emission with N fertilizer at > 120 kg N ha^-1. The threshold N fertilization for switch between stimulation and inhibition of CH₄ oxidation has been reported to be 100 - 140 kg N ha^-1 (Aronson and Hellilker, 2010; Banger et al., 2012). In this context, the standard N fertilization rate for rice of Korea (90 kg N ha^-1) may be suitable for CH₄ mitigation by BC application. In Korea, a study also indicated that the effects of BC on CH₄ mitigation did not differ between the standard N fertilization and reduced N fertilization using a slow-release fertilizer (Kim et al., 2017). In the study of Kim et al. (2017), it was found that co-application of rice husk BC at 2 t ha^-1 decreased CH₄ emission from paddy fertilized with urea (90 kg N ha^-1) by 34% and with a slow-release fertilizer (45 kg N ha^-1) by 36%.

Organic Inputs Effects on the Changes in CH₄ Emission by Biochar

The application of organic inputs such as crop residues is recommended for sustainable rice production by improving soil health (Wei et al., 2021; Lee et al., 2023a). However, the benefits of using organic inputs may be hampered by the potential increases in CH₄ emissions (Song et al., 2024). Therefore, it is highly necessary to explore agricultural management to mitigate CH₄ emission from organic-amended paddies. Three studies (Cai et al., 2018; Wang et al., 2018; Kang et al., 2021) consistently indicated that BC efficiently decreased CH₄ emissions across the experimental regimes from a lab incubation to a field experiment (Fig. 1). In an incubation experiment, Cai et al. (2018) reported that with increased application of rice straw from 1% to 2% (w/w), CH₄ emission increased by 59.6%; however, co-application of rice straw BC (1%, w/w) pyrolyzed at 300°C and 700°C decreased CH₄ emission by 23.5% and 50.8% for the treatments with rice straw at 1%, respectively. Cai et al. (2018) attributed the CH₄ mitigation by BC to decreased concentration of dissolved organic carbon (DOC), which is the substrate for methanogens, by sorption of DOC on to BC surfaces (Kasozi et al., 2010; Mukherjee and Zimmerman, 2013).

In Korea, Kang et al. (2021) reported that application of barley straw alone (20 t ha^-1) 14 days before rice transplanting did not increase CH₄ emission and co-application of BC pyrolyzed from barley straw (10 t ha^-1) with raw straw (10 t ha^-1) decreased CH₄ emission by 33% compared to the barley straw alone treatment (10 t ha^-1). Similarly, Wang et al. (2018) reported that a high application of rice straw (15 t ha^-1) increased CH₄ emission by 675% compared to that with rice straw at 7.5 t ha^-1, but this increment was reduced to 250% by the application of maize straw BC at 45 t ha^-1. Therefore, these results suggest that co-application of BC with organic inputs may be a feasible measure to reduce CH₄ emission from organic-amended paddy soils.

Water Management Effects on the Changes in CH₄ Emission by Biochar

All literature studied in this review showed that BC enhances CH₄ mitigation under water saving irrigation (Table 3 and Fig. 1). In a meta-analysis study, Jeffery et al. (2016) reported that BC decreased CH₄ emission from flooded paddy while increasing CH₄ emission from upland and unchanging CH₄ emission under flooding-drying cycles. However, Yang et al. (2019), who compared the effects of rice straw BC (40 t ha^-1) on CH₄ emissions between controlled irrigation (flooding only in regreening stage) and continuous flooding except for ripening stage, reported that BC application decreased CH₄ emission by 16% in controlled irrigation treatment but increased CH₄ emission by 60% under continuous flooding, suggesting that CH₄ oxidation under controlled irrigation was enhanced by BC application while BC promotes CH₄ emission in flooded paddies.

In Thailand, Sriphirom et al. (2019) investigated the effects of the application of Mangrove (Rhizophora apiculata) biochar at 10 t ha^-1 on CH₄ emission from chemically fertilized paddy (90 kg N ha^-1) under different water regimes, continuous flooding vs. alternate wetting and drying (AWD). They found that BC decreased CH₄ emission by 12% under both continuous flooding and AWD conditions because BC enhanced the sink capacity of CH₄ in soil by increasing the growth of methanotrophs and decreasing the abundance ratio of methanogenic archaea to methanotrophic bacteria. As AWD alone also decreased CH₄emission by 12% regardless of BC application due to shortened duration of anaerobic condition, BC application combined with AWD reduced CH₄ emission by 24% when compared to no BC application under continuous flooding. This result suggests that combined BC application and AWD effects on CH₄ emission is additive. Despite the potential increases in CH₄ emission by BC application under continuous flooding (Yang et al., 2019), middle-season drainage is common practice in Korea, and thus BC application may help CH₄ mitigation during the course of the drainage. Besides, BC may further help rice growth during AWD via soil water and nutrient retention during soil drying because of the high water and nutrient sorption capacity of BC (Haque et al., 2021).

Conclusions

Mitigation of CH₄ from rice paddy soils with the application of BC is well understood. However, the relevant studies have been conducted under fixed rice cultivation conditions; e.g., at a single dose of N fertilization, without organic inputs, and under traditional irrigation conditions. The present study shows that the direction or magnitude of changes in CH₄ emission by BC is subject to these rice cultivation conditions. Co-application of BC with N fertilizer may increase or decrease CH₄ emission compared to the N fertilization alone depending on the N fertilization rates. Although the threshold N fertilization for the turnover of increase or decrease of CH₄ emission is still unclear, N fertilization may enhance CH₄ oxidation mediated by BC via the supply of N to methanotrophs. In contrast, there is a caution that co-application of BC with N fertilizer may increase CH₄ emission through the supply of C substrate to methanogens, inhibitory effects of toxic chemicals on methanotrophs, and preferential oxidation of NH₄⁺ to CH₄ by methanotrophs. Under organic inputs such as straw addition, BC may efficiently decrease CH₄ emission by reducing C substrate availability for methanogens via sorption. The application of BC combined with water-saving irrigation may additively enhance CH₄ mitigation by promoting CH₄ oxidation under aerobic conditions. Therefore, our study suggests that application of BC may be a measure to mitigate CH₄emission from paddy soils amended with organic inputs and to enhance CH₄ oxidation under water-saving irrigation including AWD and extended or multiple middle-season drainage.