Methane and nitrous oxide emissions from agricultural fields in Japan and mitigation options: a review

ABSTRACT

Agriculture is a significant anthropogenic source of methane (CH₄) and nitrous oxide (N₂O). This review provides an overview of the emissions of the two greenhouse gases (GHGs) from agricultural fields and mitigation options based on previously published field studies mainly conducted in Japan. Rice paddy fields in Japan emitted 11,982 kt CO₂eq of CH₄, accounting for 43.6% of the total CH₄ anthropogenic emissions in 2021. Global meta-analysis shows that mid-season drainage (MD) is more effective in reducing CH₄ emissions from rice paddy fields than continuous flooding. In the regions where MD is already practiced, prolonged MD (a week longer than usual MD) has been proposed to further reduce CH₄ emissions compared to conventional MD. A field survey conducted at nine sites across Japan showed that prolonging MD reduced CH₄ emissions and global warming potential (CH₄ + N₂O) by 30% and 28%, respectively. Moreover, in Japan, the autumn incorporation of rice straw was shown to reduce annual CH₄ emissions by 54% on average compared with spring incorporation. Subsurface drainage and the application of iron-containing materials have been shown to reduce CH₄ emissions from paddy fields, although further field studies testing various conditions are needed. In addition to CH₄ emissions from rice paddy fields, agricultural soil in Japan in 2021 emitted 5962 kt CO₂eq of N₂O, accounting for 28.9% of the total anthropogenic N₂O emissions. Meta-analyses indicated that nitrification inhibitors and polymer-coated fertilizers were effective in reducing N₂O emissions after the application of synthetic N fertilizers. Moreover, biochar application was shown to be effective in sequestering soil carbon and has also been investigated as a mitigation option to reduce CH₄ and N₂O emissions; however, the results were inconclusive. The Ministry of Agriculture, Forestry, and Fisheries is now promoting prolonged MD and autumn incorporation of rice straw through a direct payment program to reduce CH₄ emissions from paddy fields in Japan. In addition, prolonged MD and biochar application have been approved as J-Credit methodologies to reduce CH₄ emissions from paddy fields and sequester soil carbon, respectively. Further studies are warranted to develop practical mitigation strategies to reduce GHG emissions while maintaining productivity.

KEY WORDS:

• CH₄
• N₂O
• soil
• greenhouse gas
• mitigation

1. Introduction

Agriculture is a significant source of two important greenhouse gases (GHGs): methane (CH₄) and nitrous oxide (N₂O). Specifically, agriculture account for 39% of CH₄ and 78% of N₂O emissions, equivalent to 12% (6.2 ± 3.4 GtCO₂ equivalent yr⁻¹) of global anthropogenic GHG emissions (Jia et al., 2019). CH₄ is a short-lived GHG with a lifetime of 11.8 years, and a global warming potential (GWP) of over 100 years of CH₄ is 27 times that of carbon dioxide (CO₂) (Forster et al. 2021). The atmospheric concentrations of CH₄ have increased from 729 ppb in 1750 to 1866 ppb in 2019 (Forster et al. 2021). The Global Methane Pledge signed by over 100 countries at the COP26 climate summit in 2021 agreed to reduce anthropogenic CH₄ emissions by 30% in 2030, stating that this is the most effective strategy to achieve the Paris Agreement’s 1.5°C temperature goal due to the short lifetime of CH₄ in the atmosphere (Global Methane Pledge, 2023). Paddy rice fields are a significant source of CH₄, accounting for 9% of the global anthropogenic CH₄ emissions (Canadell et al. 2021). In contrast to CH₄, N₂O is a long-lived GHG with a lifetime of 109 years; it has a GWP of 273 times that of CO₂ and also contributes to the depletion of the ozone layer in the stratosphere (Forster et al. 2021). The concentrations of N₂O have increased from 270 ppb in 1750 to 332 ppb in 2019 (Forster et al. 2021). Agriculture is the largest source of anthropogenic N₂O, accounting for 51% of the global anthropogenic N₂O emissions (Canadell et al. 2021). This review provides an overview of CH₄ and N₂O emissions from agricultural fields and potential mitigation options based on field studies mainly conducted in Japan.

2. The GHG inventory of Japan

Agriculture is a significant source of anthropogenic GHGs globally. However, according to the National Greenhouse Gas Inventory Report (NIR) of Japan, the agricultural sector accounted for only 2.7% (excluding fuel combustion in agriculture, forestry, and fisheries) and 4.1% (including fuel combustion in agriculture, forestry, and fisheries) of the total GHG emissions in 2021 (GIO and MOE, 2023; Figure 1) because CO₂ emissions from industries and transport represent the majority of GHG sources in Japan. In addition to the GHG emissions reported in the NIR, Japan imported 62% of food and feed in 2021 on a calorie basis (Ministry of Agriculture, Forestry, and Fisheries (MAFF), 2023a); however, the GHG emissions emitted during their production in the countries of origin were not included in the NIR. While being a relatively minor source of total GHG including CO₂, agriculture is the largest source of CH₄ and N₂O in Japan. The agricultural sector accounted for 81.1% of the total CH₄ emissions in 2021 (GIO and MOE, 2023; Figure 2), and the sources of CH₄ are enteric fermentation in ruminant animals, manure management, rice cultivation, and field burning of agricultural residues. The agricultural sector accounted for 49.1% of the total N₂O emissions in 2021 (GIO and MOE, 2023; Figure 2), and the sources of N₂O include agricultural soil and manure management. In 2021, rice paddy fields emitted 11,982 kt CO₂eq of CH₄, accounting for 43.6% of the total CH₄ emissions, and agricultural soil emitted 5,962 kt CO₂eq of N₂O, accounting for 28.9% of the total N₂O emissions in Japan.

Figure 1. Anthropogenic greenhouse gas emissions (kt CO₂eq) from the agricultural sector in Japan in 2021.

The data used were obtained from the National Inventory Report (NIR) of Japan (GIO and MOE 2023) and included CO₂ emissions from fuel combustion associated with the agriculture, forestry, and fishery sectors (15710 kt CO₂) as well as from ammonia production used for synthetic N fertilizers (650 kt CO₂). It should be noted that CO₂ emissions from fuel combustion included not only the agricultural sector but also forestry and fishery sectors because no data for only the agriculture sector were provided in the NIR. For ammonia production, a total of 42% was estimated to be used as synthetic N fertilizer, based on the total synthetic N fertilizer (374,879 tN; GIO and MOE 2023) and total ammonia (906,647 tN; Japan Fertilizer & Ammonia Producers Association, 2023) demands, assuming the ratio of synthetic N fertilizer to ammonia supply was the same for imported ammonia (22% of the total ammonia supply), ammonia produced in Japan (77% of the total ammonia supply), and the carryover stock (2% of the total ammonia supply). Note that emissions from fuel combustion associated with agriculture were included in the energy sector instead of the agriculture sector in IPCC guidelines to avoid double counting of GHG emissions, although these emissions were included in this figure.

Figure 2. Anthropogenic CH₄ (a) and N₂O (b) emissions in Japan in 2021.

Data from the National Inventory Report (NIR) of Japan (GIO and MOE 2023).

The methods used to compile GHG inventories for the NIR must follow the Intergovernmental Panel on Climate Change Guidelines for National Greenhouse Gas Inventories (IPCC GLs). In the guidelines, Tier 1 is the basic method to estimate GHG emissions at the national scale using default emission factors (EFs) and a common set of activity data. Countries can also adopt Tier 2 (country-specific EFs) or Tier 3 (model-based approach) methods to improve accuracy and reduce the overall uncertainty when adequate data are available. In addition, continuous efforts are needed to update the national inventory system to maintain a record of unbiased estimates with the highest possible accuracy for the creation of government policies aimed at reducing GHG emissions (Ogle et al., 2020).

The 2019 IPCC GLs (Ogle et al. 2019) provided a default method for calculating CH₄ emissions from paddy rice fields based on global statistical analysis (Wang et al. 2018). In these guidelines, the default EFs to estimate direct N₂O emissions from synthetic and organic nitrogen (N) fertilizer applications, return of crop residues to soils, and decomposition of soil organic matter of mineral soils (EF₁) were revised as an aggregated EF of 1% and disaggregated EFs of 1.6% for synthetic and mixed fertilizer inputs (synthetic and organic) in wet climates, 0.6% for other N inputs (i.e., organic amendments, animal manures, N in crop residues, and mineralized N from soil organic matter decomposition) in wet climates, and 0.5% for all N inputs in dry climates (Hergoualc’h et al. 2021; Hergoualc’h et al. 2019).

In the NIR of Japan, CH₄ emissions from rice paddy fields were estimated using EFs calculated using the Tier 3 method, i.e., through the DeNitrification-DeComposition-Rice model (DNDC-Rice model; Fumoto et al. 2010; Katayanagi et al. 2017). In addition, the report also accounted for a CH₄ reduction rate of 30% associated with prolonged mid-season drainage (MD) (Itoh et al. 2011), which is described in section 3.2.2.

In the NIR of Japan (GIO and MOE 2023), N₂O emissions from synthetic N fertilizers were estimated using the Tier 2 method, i.e., country-specific EFs for synthetic N fertilizers (0.31% for rice paddy, 2.9% for tea, and 0.62% for other crops) were used based on mean N₂O EFs for synthetic N fertilizers in Japanese agricultural fields (Akiyama, Yan, and Yagi 2006). In addition, the NIR included a 38% reduction rate in N₂O for nitrification inhibitors (Akiyama, Yan, and Yagi 2010).

The EF for other crops (0.62%) were lower than the IPCC default value of 1.6% for wet climate, although Japan is included in wet climate. This would be mainly because N₂O emissions from Andosols are generally lower than those from non-Andosols (Akiyama, Yan, and Yagi 2006). Japan is part of the Ring of Fire and, consequently, volcanic tephra significantly affects its soils. Fluvisols occupy 47%, followed by Andosols (29%) of cultivated lands, whereas the former are mainly used as paddy rice fields while the latter are used as upland fields; consequently, andosols occupy 47% of upland fields (NARO, 2023).

The total amount of N in organic amendments (444 ktN) used in Japan in 2021 was larger than that of synthetic N fertilizers (375 ktN) (GIO and MOE 2023). However, the current NIR (GIO and MOE 2023) assumed that the EF for organic amendments was the same as that for synthetic N fertilizers because country-specific EFs for organic amendments were not available. Therefore, a field survey across Japan and a systematic review were conducted to estimate the country-specific N₂O EFs for organic amendments, and the results showed that the mean EFs for poultry manure compost, swine manure compost, cattle manure (compost and slurry), and organic fertilizers derived from non-animal manure were 0.83%, 0.70%, 0.39%, and 1.16%, respectively. The mean EF for all organic amendments was 0.84% (Akiyama et al. 2023).

3. CH₄ emissions from paddy rice fields and mitigation options

Rice is an important staple food in Japan as well as in other Asian countries. In 2021, the rice cultivation area in Japan was 1549 kha, accounting for 36% of the total cultivated area (3922 kha) (GIO and MOE 2023). Rice paddy cultivation is a significant source of CH₄ (Canadell et al. 2021), while N₂O emissions from paddy fields are generally lower than those from upland fields (Akiyama, Yan, and Yagi 2005). Due to CH₄ emissions, the GWP of rice cultivation is the highest among cereals; for example, it is 467% and 169% higher than that of wheat and maize, respectively (Linquist et al. 2012). Because rice paddy fields are the largest source of CH₄ emissions in Japan (Figures 1 and 2), they are the primary target for reducing GHG emissions derived from the agricultural sector.

3.1. CH₄ production and oxidation in soil and transportation to the atmosphere

In paddy rice fields, CH₄ is generated by CH₄-producing archaea (methanogens) during the anaerobic decomposition of organic matter (such as crop residue) and rhizodeposition (Conrad 2020a, 2020b; Kimura, Murase, and Lu 2004). Methanogenesis is the final step in the anaerobic degradation of organic matter, and occurs only after inorganic electron acceptors, such as nitrate, ferric iron, or sulfate, have been depleted (Conrad 2020a, 2020b). Part of the produced CH₄ is oxidized by CH₄-oxidizing bacteria (methanotrophs) (Conrad 2020b; Hayashi et al. 2015). The accumulated CH₄ in paddy soils is transported to the atmosphere mainly through the aerenchyma tissues of rice plants and the ebullition of gas bubbles, while the contribution of diffusion is considered marginal (Kajiura and Tokida 2021). More than 90% of CH₄ is emitted through rice plants in paddy fields (Schutz, Seiler, and Conrad 1989). However, a recent field study showed that ebullition contributed to only 4% of the total CH₄ emissions at the panicle formation stage, whereas it accounted for 32% and 60% of the total emissions at the booting and heading stages, respectively (Kajiura and Tokida 2021).

3.2. Mitigation options for CH₄ emissions from rice paddy fields

Water and straw management are basic and low-cost strategies to mitigate CH₄ emissions derived from paddy rice fields. Wang et al. (2023) estimated that improved water and straw management could reduce global CH₄ emissions from rice paddy fields (27 Tg CH₄ year⁻¹) by one-third (8.8 Tg CH₄ year⁻¹).

3.2.1. Water management: MD and multiple drainages

Water management largely influences CH₄ emissions from paddy rice fields because CH₄ production requires anaerobic conditions. MD, which is the traditional practice of draining water for about a week during the growing season to enhance grain yield, is common in Japan, China, and other Asian countries. Intermittent irrigation is also a traditional practice in which drainage and irrigation are repeated within a cycle of a few days during the growing season and is commonly practiced in Japan, China, India, and other Asian countries. According to a worldwide statistical analysis by Wang et al. (2018), CH₄ emissions from single MD and multiple drainages, such as intermittent irrigation and alternate wetting and drying (AWD), were lower by 29% and 46%, respectively, compared to those of continuous flooding. Intermittent irrigation consisting, for example, of 3-day flooding and 4-day drainage cycles after MD is widely practiced in Japan, while AWD is uncommon.

3.2.2. Water management: prolonged MD

According to a worldwide statistical analysis by Wang et al. (2018), CH₄ emissions from MD were lower by 29% compared to those from continuous flooding. In Japan, all paddy fields are irrigated, and MD is already practiced in about 90% of them, except for those in the Hokkaido region. Thus, introducing MD or intermittent irrigation is the applicable mitigation measure only in Hokkaido. In this region, continuous flooding is practiced in 52% of paddy fields (GIO and MOE 2023) to prevent potential reductions in rice grain yield due to cold weather. However, a recent study suggested that, even in this cold region, intermittent irrigation reduced CH₄ emissions by 21–91% compared to continuous flooding, without any increase in N₂O emissions or decrease in rice grain yields (Nishimura et al. 2020).

To further reduce CH₄ emissions in the regions where MD is already adopted, prolonged MD (1 week longer than the local MD practice) was proposed instead. Based on the results of a field survey conducted in nine sites across Japan for 2 years, prolonging MD reduced CH₄ emissions and GWPs (CH₄ + N₂O) by 30% and 28%, respectively (Itoh et al. 2011). Since 2015, the MAFF has been promoting prolonged MD as part of a direct payment program ‘the Direct Payments for Environmentally Friendly Agriculture’ to reduce GHG emissions (MAFF, 2023b). As of 2021, prolonged MD is practiced in 15,000 ha as part of the above-mentioned direct payment program (MAFF, 2023b). However, according to a survey by MAFF, 25.9% of rice farmers had already been adopting this strategy (MAFF, 2022), suggesting that the overall area subjected to prolonged MD would be much larger than 15,000 ha. In addition, prolonged MD was approved as a J-Credit methodology in 2022 (METI 2023). J-Credit is a carbon credit scheme certified by the Japanese government.

3.2.3. Water management: improvement of drainage by installing subsurface drainage

Katayanagi et al. (2017) reported that field drainage is an important factor along with organic C application rate and water management regime to estimate CH₄ emissions from paddy fields using the DNDC-Rice model. Accordingly, these factors have been used to run the model, whose results have been included in the NIR of Japan (GIO and MOE 2023)).

Currently, subsurface drainage systems (tile drainage systems) are installed in more than 60% of the paddy fields in Japan to improve farming efficiency and increase rice productivity (Shiratori et al. 2007). In a Gray lowland soil at Niigata, it was shown that CH₄ emissions from a paddy field with subsurface drainage were 70% lower than those from an adjacent paddy field with no drainage system, although with large interannual variations (Shiratori et al. 2007). Although the improvement of drainage has a great potential to reduce CH₄ emissions and increase productivity, it has not been widely tested so far, and further studies are needed for a wide variety of soil types and climates.

3.2.4. Organic matter management

According to a worldwide statistical analysis by Wang et al. (2018), at a typical rate of 6 t ha⁻¹, the application of rice straw off-season, known as autumn incorporation of rice straw in temperate regions such as Japan (rice straw from a previous season is incorporated into the soil long before cultivation so that it decomposes under aerobic conditions), reduced CH₄ emissions by 50% compared to the application of rice straw on-season, known as spring incorporation in temperate regions (rice straw application just before cultivation).

Similarly, a statistical analysis by Kajiura et al. (2018) showed that the autumn incorporation of rice straw reduced CH₄ emission by 54% on average compared with spring incorporation. Takakai et al. (2021) also reported that autumn incorporation reduced CH₄ emissions and GWP by 21–37% compared to spring incorporation in the Tohoku region. The autumn incorporation of rice straw has been approved as a component of ‘the Direct Payments for Environmentally Friendly Agriculture’ in 2020 and, as of 2021, it is practiced over an area of 884 ha (MAFF, 2023b). However, this area is estimated to be larger because a MAFF survey revealed that 59.5% of rice farmers had already been practicing autumn incorporation in Japan (MAFF, 2022).

Especially in cold regions, such as Hokkaido, Tohoku, and Hokuriku, the slow rate of rice straw decomposition during winter leads to an increase in CH₄ emissions. Therefore, accelerating straw decomposition during winter could mitigate CH₄ emissions. To explore this strategy, Takakai et al. (2021) tested the application of lime nitrogen in a field in the Tohoku region, but the effects on straw decomposition and CH₄ emissions were not observed. Additionally, in the same region, Nakajima et al. (Nakajima et al. 2017) tested shallow autumn tillage to incorporate straw and accelerate decomposition, but CH₄ emissions were not reduced, probably due to a small difference in soil water content between shallow autumn tillage treatment and conventional autumn tillage treatment. The study concluded that, in winter, moderate soil water content is more important for rice straw decomposition than tillage depth.

3.2.5. Other mitigation options to reduce CH₄ emissions

Studies have shown that the application of iron-containing materials, such as steel slag, was effective in reducing CH₄ emissions in pot experiments (Furukawa and Inubushi 2004; Singla and Inubushi 2015). Field experiments investigating the effect of iron-containing materials on CH₄ emissions have been conducted in China and Indonesia (Susilawati et al. 2015; Zhang et al. 2023), and further field studies are needed under various conditions.

It is known that CH₄ emissions differ depending on rice cultivars (Gupta et al. 2021; Hayashi et al. 2015). Identifying or breeding low-CH₄ rice varieties can be an effective mitigation strategy in the future (Chirinda et al. 2018; Hayashi et al. 2015; Jiménez and Pedersen 2023; Wang et al. 2023).

4. N₂O emissions from agricultural fields and mitigation options

4.1. N₂O production in agricultural soils

In agricultural soils, N₂O is emitted as a result to soil management practices, such as the application of synthetic N fertilizers and organic N, crop residue N inputs, and the enhanced mineralization of N of soil organic matter caused by continuous land cultivation or land use change. The application of synthetic or organic N stimulates N₂O production predominantly via the microbial processes of nitrification and denitrification (Hayatsu, Katsuyama, and Tago 2021; Hayatsu, Tago, and Saito 2008). The nonbiochemical process of chemodenitrification also produces N₂O; however, its contribution would be minor in cropland (Hayashi et al. 2015). The N application rate and climate type (wet or dry, affecting soil moisture) are major factors influencing N₂O emissions, followed by land cover, soil texture, C content, and pH (Hergoualc’h et al. 2021; Hergoualc’h et al, 2019). The water-filled pore space (WFPS) of soil is a key factor controlling N₂O fluxes and N₂O production, i.e., the relative contributions of nitrification and denitrification (Davidson 1991). Congreves et al. (2019) showed that at WFPS values < 53%, nitrification was the dominant N₂O production process; however, it decreased, and denitrification increased as WFPS rose from 53% to 78%, becoming the dominant N₂O production process at WFPS values > 78%.

4.2. Mitigation options for N₂O emissions from agricultural fields

4.2.1. Optimizing the N application rate

N₂O emissions generally increase with the increase of N fertilizer application rates (Gerber et al. 2016; Shcherbak, Millar, and Robertson 2014; Van Groenigen et al. 2010). Thus, optimizing N use efficiency is a basic strategy for mitigating N₂O emissions, although this does not always result in lower N₂O emissions due to the complexity of factors affecting them (Venterea et al. 2012). Yield-scaled N₂O emissions (N₂O emissions per aboveground N uptake or crop yield) are useful to assess the strategy quantitatively. A meta-analysis showed that, for non-leguminous arable crops, optimizing the use efficiency of N fertilizers led to the lowest yield-scaled N₂O emissions (N₂O emissions per aboveground N uptake) and was thus preferable to minimizing N application rates (Van Groenigen et al. 2010). Specifically, the yield-scaled N₂O emissions were the lowest (8.4 g N₂O-N kg⁻¹ N uptake) at N rates ranging from 180 to 190 kg N ha⁻¹ and increased sharply above this level. Another meta-analysis also reported that fertilizer optimization was effective in reducing N₂O emissions (mean: −31%) (Grados et al. 2022). This strategy is not only effective to mitigate N₂O emissions but also to reduce other types of environmental pollution, e.g., water pollution and ammonia volatilization.

4.2.2. Enhanced-efficiency fertilizers

Enhanced-efficiency fertilizers (i.e., nitrification and urease inhibitors and polymer-coated fertilizers) have been used to increase the N use efficiency of crops. Nitrification inhibitors delay ammonia oxidation by inhibiting the activities of nitrifiers, and urease inhibitors delay urea hydrolysis. Polymer-coated fertilizers (slow or controlled-release fertilizers) release N at a slower rate compared to conventional fertilizers. Nitrification inhibitors and polymer-coated fertilizers have been shown to reduce N₂O emissions in the Andosol and Fluvisol fields (Akiyama and Tsuruta 2002; Akiyama et al. 2013, 2015; Akiyama, Tsuruta, and Watanabe 2000). The effect of enhanced-efficiency fertilizers on N₂O emissions has been reported worldwide; however, the observed effectiveness differed among studies probably due to environmental factors and different agricultural practices adopted in different regions. Therefore, statistical models and meta-analyses are useful tools to estimate the overall effectiveness of mitigation options. A meta-analysis by Akiyama et al. (2010) reported that the mean N₂O reduction rates of nitrification inhibitors and polymer-coated fertilizers were −38% and −35%, respectively, compared with those of conventional fertilizers, while urease inhibitors did not reduce N₂O emissions. Moreover, nitrification inhibitors and polymer-coated fertilizers were effective in reducing nitric oxide emissions (−46% and −40%, respectively). Other meta-analyses have also reported significant N₂O reductions by nitrification inhibitors (Chen et al. 2023; Grados et al. 2022; Ruser and Schulz 2015; Tufail et al. 2023) and polymer-coated fertilizers (Grados et al. 2022; Pan et al. 2023). Another meta-analysis suggested that dicyandiamide (DCD), a nitrification inhibitor, resulted in lower emissions of both CH₄ (−18%) and N₂O (−29%) from paddy rice fields (Linquist et al. 2012). Some studies suggested that nitrification inhibitors would increase NH₃ volatilization (Adu-Poku et al. 2022; Kim, Saggar, and Roudier 2012); however, such an increase would be minimal in Japan, where NH₃ volatilization is generally low because of the relatively low pH and high cation exchange capacity of volcanic ash soils (Hayashi and Yan 2010).

The global mean N₂O emissions from ammonium-based fertilizers are higher than that from nitrate-based fertilizers (Bouwman, Boumans, and Batjes 2002). In well-drained soils where nitrification is the dominant N₂O production process, the application of nitrate-based fertilizers may be more effective in reducing N₂O emissions than the application of commonly used ammonium-based fertilizers or urea. This is because, while NO₃-N produces N₂O only via denitrification, ammonium-based fertilizers produce it via both nitrification and denitrification. To avoid N loss by leaching after the application of nitrate-based fertilizers, polymer-coated calcium nitrate would be a better option for the reduction of N₂O emissions in well-drained fields (Akiyama and Tsuruta 2002; Hasukawa et al. 2021; Nishimura et al. 2021, 2022). It should be noted that nitrate-based fertilizers may increase N₂O emissions in poorly drained fields where denitrification is the dominant N₂O production process.

4.2.3. Mitigation of N₂O emissions from acidic tea soils

In 2021, the tea cultivation area in Japan was 38,000 ha, accounting for only 1% of the total cultivated area (3,922,000 ha). However, tea cultivation was responsible for 14% of the total N₂O emissions from synthetic N fertilizers and organic N amendments that year (calculated based on the NIR data; GIO and MOE 2023), due to their high average application rates, i.e., 447.6 kgN ha⁻¹ and 162.4 kgN ha⁻¹, respectively, and high N₂O EF (2.9%) (GIO and MOE 2023). Thus, tea fields should be a primary target for the mitigation of N₂O emissions from Japanese agricultural soil because of the unique agricultural practices, such as the high application rate of synthetic and organic N amendments in the acidic soils and the large amount of tea pruning litter input. Hirono and Nonaka (2014) reported that, in a tea field, N₂O emissions from lime nitrogen treatment were 50% lower than those of conventional fertilizer treatment, suggesting that the increase in soil pH caused by lime nitrogen affected N₂O emissions. Although DCD was also produced during lime nitrogen decomposition, it would not be the main cause of the N₂O reduction because DCD treatment was not effective to reduce N₂O emissions. Similarly, Yamamoto et al. (2014) reported that N₂O emissions from lime nitrogen treatment were lower by 36% than those of conventional fertilizer treatment in a tea field. It should be noted that tea plants prefer acidic soils, with optimal pH values ranging from 4.0 to 5.0 (Kosuge 1987); therefore, the lime application rate should be limited to avoid exceeding the optimal pH of tea soil. Because tea fields are a hotspot of N₂O emissions among Japanese agricultural fields, further studies on effective mitigation options for their acidic soils are needed.

4.2.4. Other mitigation options to reduce N₂O emissions

A second-order meta-analysis by Grados et al. (2022) reported that drip irrigation significantly reduced N₂O emissions (mean: −26.5%) in addition to the optimization of fertilizer rates and application of biochar, controlled-release fertilizers, and nitrification inhibitors. In contrast, frequent fertilizer applications, crop residue removal, lime amendments, organic fertilizer applications, diversified crop rotation, deep fertilization, and no-tillage or reduced tillage were ineffective in reducing N₂O emissions, with values increasing by 36.7% for cover crops. The effect of drip irrigation on GHG emissions has yet to be reported in Japan.

Microbial methods to reduce N₂O emissions may be a potential option in the future; for example, N₂O emissions during nodule decomposition from soybean fields can be reduced by the inoculation of nosZ++ strains of Bradyrhizobium diazoefficiens, a mutant with increased N₂O reductase (N₂OR) activity (Itakura et al. 2013) or by inoculation of a mixed culture of indigenous nosZ+ strains (with N₂OR) of the same species; Akiyama et al., (2016). Increasing the number of fungivorous mites through the application of coconut husks has been shown to reduce N₂O emissions after organic fertilizer application when fungal denitrification would be the dominant N₂O production process (Shen et al. 2021).

5. Effect of biochar on GHG emissions

Biochar application is an effective method to sequester soil carbon (C) (Spokas 2010) and has been approved in Japanese agricultural fields as a J-Credit methodology for this purpose in 2020 (METI, MOE and MAFF 2023). In addition to C sequestration, biochar application in tropical regions was shown to increase crop yield due to the low pH and low fertility of tropical soils and low fertilizer input; however, this effect has yet to be observed in temperate climates, according to a meta-analysis (Jeffery et al. 2017). Although the effect of biochar application on GHG emissions has been intensively investigated, the results obtained are not conclusive. Kaur et al. (2023) evaluated various meta-analyses of the effects of biochar on N₂O emissions. Among them, four showed that biochar application had insignificant effects on N₂O emissions, while 14 reported average N₂O reductions of −38.8% (ranging from −10.5% to −54.8%). A second-order meta-analysis by Grados et al. (2022) reported that biochar application significantly reduced N₂O emissions (−26.6%). However, Joseph et al. (2021) pointed out that early meta-analyses including laboratory experiments with a very high biochar application rate (>100 Mg ha⁻¹) showed high mitigation of N₂O emissions, with reductions as high as 50%. In contrast, recent meta-analyses based on field studies testing realistic biochar application rates reported a considerably lower N₂O reduction of about 10%. In addition, Jeffery et al. (2016) reported that biochar reduced CH₄ emissions from paddy rice fields, particularly from acidic soils, but decreased the CH₄ sink potential of upland soils. Another meta-analysis by Wang et al. (2023) suggested that biochar application reduced CH₄ and N₂O emissions from paddy rice fields although insignificant. In contrast, another meta-analysis by Jia et al. (2023) found that biochar applications had an insignificant effect on CH₄ fluxes in both paddy and upland fields.

While the results of the above-mentioned global meta-analyses on GHG reduction by biochar application are contradictory, the results of field studies conducted in Japan suggest that biochar application does not reduce GHG emissions irrespective of biochar types, application rates, or soil types. For example, Watanabe et al. (2014) revealed that bamboo biochar application (20 Mg ha⁻¹) did not affect CO₂, N₂O, and CH₄ emissions or the yields of various vegetables and sweet potato in a Cambisol field. Similarly, the application of rice husk biochar (10–40 Mg ha⁻¹) had no effect on CH₄ and N₂O emissions from a rice paddy field (Koyama et al. 2015), and that of wood residue-derived biochar (10–40 Mg ha⁻¹) had no effect on CO₂, N₂O, and CH₄ emissions or the yield and quality of various crops in an Andosol field (Koga, Shimoda, and Iwata 2017). The application of orchard pruning residues biochar (2–10 Mg ha⁻¹) did not affect N₂O emissions in an Andosol pear field (Oo et al. 2018), and that of four different biochar types (rice husk, bamboo, hardwood, and wood briquet of mixed hardwood and softwood) at 25 Mg ha⁻¹ also did not affect N₂O and CH₄ emissions or the yield of spinach and Brassica spp. in an Andosol field (Yamamoto et al. 2019). Similarly, the application of palm shell biochar (40 Mg ha⁻¹) did not affect N₂O, CH₄, and CO₂ emissions in a Cambisol field (Basalirwa et al. 2020).

6. Conclusions

In recent years, some of the GHG mitigation techniques in the agricultural sector in Japan have been transitioning from the research stage to the implementation stage. MAFF has been promoting prolonged MD and the autumn incorporation of rice straw as parts of the direct payment program since 2015 and 2020, respectively. In addition, prolonged MD is expected to be more widely applied since its approval as a J-Credit methodology in 2022. The same approval has also been granted to biochar application for the sequestration of soil C in 2020. Mitigation options such as subsurface drainage and iron-containing material have been reported to reduce CH₄ emissions from rice paddy fields, although further studies are needed to test their effects under various conditions. Numerous meta-analyses have reported the effectiveness of nitrification inhibitors and polymer-coated fertilizers in reducing N₂O emissions from agricultural soils. Biochar application has demonstrated effectiveness in sequestering soil C and is intensively being investigated as a mitigation option to reduce CH₄ and N₂O emissions; however, the results are inconclusive. Further studies are needed to develop practical mitigation options that can be applied by local farmers to reduce GHG emissions from the agricultural sector while maintaining productivity.

Disclosure statement

No potential conflict of interest was reported by the author.

Additional information

Funding

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under Grant number JPNP18016, and the Ministry of Agriculture, Forestry and Fisheries (MAFF) under the project “Assessment and extension of technologies for mitigating greenhouse gas emissions from agricultural soil,” and the project ‘Assessment of nature-based credit’.