Agriculture and Science 2024/02/03

Seedling Box Leave-As-Allowed" as a method of fertilizer application that is not affected by high temperatures.

Farm Frontier Inc.
　Hiroshi Fujii Chairman of the Board

1. 2023 Weather Assessment

(1) Comparison with past high temperature years in the Shonai area

　The past high temperature years in the Shonai area include 2010, 1999, and 1994 (Table 1), all of which had average maximum temperatures above 31.8°C, average average temperatures above 27°C, and average minimum temperatures above 23°C in August. The average temperature in August was above 31.8℃, the average temperature was above 27℃, and the average minimum temperature was above 23℃. In 1999, the year with the lowest percentage of first quality rice (26.8%), the number of days with an average temperature of 30°C or higher, a maximum temperature of 35°C or higher, and a minimum temperature of 25°C or higher were 6, 10, and 10 times, respectively, the highest among the past hot years.

　In 2023, the average maximum, average mean, and average minimum temperatures in Sakata City after ear emergence were 35.2°C, 30.1°C, and 26.2°C, respectively, far exceeding the temperatures at which white immature grains are most likely to occur. The average number of days with an average temperature of 30°C or higher, the maximum temperature of 35°C or higher, and the minimum temperature of 25°C or higher were 17, 18, and 24 times, respectively, far exceeding those of 1999, the year with the lowest percentage of first-class rice ever recorded. In addition, the minimum humidity, which indicates the degree of dryness, was extremely low at 42.8% (the number of days with a minimum humidity of 40% or less was also extremely high at 14 times), and there was little rainfall, resulting in extremely high wear and tear of the rice plants.

　Figure 1 shows the relationship between the high temperature index (the accumulated average temperature of 26°C or higher for the first 20 days after ear emergence) and the percentage of first quality rice of Haenuki in the Shonai area. Applying the regression equation in Fig. 1, the high temperature index in 2023 is 78 (first quality ratio is 13.71 TP3T), while the first quality ratio becomes zero at the high temperature index of 71. The rice ratio was 13.71 TP3T).

(2) Comparison of Sakata City with other regions

　Compared to the four Hokuriku prefectures (Niigata, Toyama, Ishikawa, and Fukui), which had the hottest temperatures in Japan, Sakata City had the hottest weather conditions (30 days after ear emergence) in Japan, with less rainfall, the lowest mean minimum humidity, and the highest mean minimum humidity below 40%. The weather conditions in Sakata City (30 days after ear emergence) were considered to be the harshest in Japan for rice seedling maturation (Table 2).

2. response of each variety to "high temperature" in 2023 (yield, quality, etc.)

(1) Evaluation of rice production

　Comparing the quality (percentage of grains in uniformity) in 2023 (the 5th year of production) with 2022 (the 4th year of production), the degree of quality decline was "Haenuki" > "Setsuwakamaru," with a smaller decline in quality for "Setsuwakamaru," which is considered more resistant to high temperatures. A significant increase in the number of immature grains at the base was one of the reasons for the quality decline. The same variety, "Haenuki," whose leaf color decreased after 20 days after ear emergence, showed a significant decrease in the grain yield. The shape of the grains (length, width, and thickness) showed that the width of both varieties became narrower, suggesting that the rice body was damaged by high temperatures around 7 to 15 days after flowering (Table 3).

　In 2023, the percentage of polished brown rice, which indicates the maturity rate, was higher than that in 2022, and the thousand-grain weight was lower (Table 4). The reason for the higher percentage of milled brown rice grains is thought to be that the amount of accumulated carbohydrates before ear emergence was high due to the extremely large amount of sunshine hours (160 to 200 compared to the normal year) during the 10 days before ear emergence for all varieties, and that the high temperature after ear emergence but the large amount of sunshine hours did not stop the maturation of the growing rice (the occurrence of stunted rice was extremely small), and that the maturation of the growing rice progressed without any stoppage of the maturation of the growing rice. The amount of carbohydrates accumulated prior to ear emergence was high. The high preemergence accumulation of carbohydrates and the enhanced formation of rice hulls (high silicon accumulation in the hulls) may have contributed to the reduction of damage caused by the excessively high temperature conditions.

　On the other hand, the factors that caused the decrease in thousand-grain weight were excessively high temperature during the ripening period, low rainfall frequency and amount, and significantly many days with low minimum humidity, resulting in a decrease in leaf color of the rice plants (decrease in leaf color of lower leaves that supply photosynthetic products to the roots), decrease in water absorption capacity of the roots, and decrease in cytokinin (hormone that inhibits chlorophyll degradation in the leaf blade) produced by the roots, which reduced the photosynthetic capacity of the rice plants and decreased thousand-grain weight. The decrease in the amount of cytokinin (a hormone that inhibits the decomposition of chlorophyll in the leaf blade) produced by the roots reduced the photosynthetic capacity of the rice plants, resulting in lower thousand-grain weight and increased occurrence of "basal immature grains," which are determined in the latter half of the ripening stage in terms of quality. It is considered that the more severe the decrease in leaf color was, the more "basal immature grains" increased, resulting in a decrease in quality.

(2) Yield and quality by leaf color (between plots)

　For each cultivar, the larger the decrease in leaf color (lower leaves and mortality rate) in the second half of the ripening period, the lower the yield and the greater the quality loss (Table 5). Setsuwakamaru, which is considered to be more tolerant to high temperatures, tended to have a lower percentage of lower leaves dying and less quality loss.

3. the importance of a continuous nitrogen supply (continuous nitrogen absorbed by the upper roots)

　In Niigata Prefecture (2019), aggressive application of ear fertilizer and three times application of ear fertilizer (15 days before ear emergence, 7 days before ear emergence, and 1 day before ear emergence) were effective in reducing quality loss under high temperature conditions. Morita (doctoral dissertation: Physiological and ecological analysis of high temperature ripening injury in rice) reported that 15 small amounts of continuous fertilizer application between 16 days before and 12 days after ear emergence reduced the yield of immature grains (variety: Hinohikari) compared to the conventional two applications of fertilizer 16 days before and 6 days before ear emergence. On the other hand, in cases where quality declined under high temperature conditions, it was reported that (1) late-season nutrition was insufficient because leaf color was light in August (after ear emergence), (2) high temperatures after ear emergence resulted in insufficient nitrogen nutrition, (3) fertilizer shortage (late-season nutrition), and (4) nitrogen fertilization as usual resulted in insufficient nitrogen nutrition in high temperature years. The following is a summary of the results of the study.

　The above results suggest that under high temperature ripening conditions, a decrease in leaf color after the middle of the ripening period causes rice body senescence, which contributes to quality and yield loss, and that a small-quantity, frequent nitrogen fertilizer application is extremely effective in reducing quality loss under high temperature conditions by using a surface fertilizer that is absorbed by the "upper roots. However, it is difficult to implement the above-mentioned small-quantity, multiple-fertilizer application under the current circumstances where the production system is expanding in scale and the number of farmers is decreasing and aging.

　The most practical fertilizer application methods are "seedling-box-makase," which allows nitrogen to be absorbed by the newest root, the "upper root," and total basal fertilization with side-row fertilization. In order to maintain endurance, it is necessary to maintain leaf color color for a long period during the ripening period and to minimize the decline in leaf color of lower leaves. From this perspective, nitrogen from the "leave it to the seedling box" method is gradually absorbed by the rice plants after the ear setting stage, resulting in low senescence pressure in the lower leaves. When nitrogen supply is insufficient, the nitrogen required by rice is transferred from lower leaves to upper leaves through accelerated translocation, resulting in high senescence pressure in the lower leaves, wilting of lower leaves, late wilting, and yield and quality decline, which is especially true in high temperature years. In rice paddies with high soil fertility, nitrogen is supplied from soil fertility and the senescence pressure of lower leaves is considered to be low. However, in recent years, the senescence pressure of lower leaves is assumed to be high due to insufficient root mass and progressive reduction of nitrogen.

　In the case of full-layer fertilizer application, under high temperature conditions, the nitrogen utilization of fertilizer is likely to decrease because the absorption capacity of the roots is assumed to decrease due to the decrease in leaf color of lower leaves, the decrease in root vigor, and inappropriate water management. On the other hand, the continuous nitrogen supply of coated fertilizers, which leach nitrogen in small amounts even after the ear emergence stage, is absorbed by the shoot roots, which are the newest roots, so it is thought that high nitrogen utilization can be maintained even under high temperature conditions if water management is properly conducted. Therefore, it is considered that the total basal application of fertilizer is advantageous under high-temperature conditions (Figure 2).

4. usefulness of "side-row + seedling box" under high temperature conditions

(1) Yield, quality, taste and nitrogen absorption

　In all three years, the amount of nitrogen applied was "control (total basal fertilizer)" > "side-row + seedling box". In particular, in 2022 (a year of insufficient sunlight) and 2023 (a year of high temperature), which was a year of weather-related disasters, the weights of fine rice in the "seedling box" were 112 and 111 higher than those in the control. Similar to the weight of fine brown rice, nitrogen uptake was higher in the "side-row + seedling box" than in the "control (total basal fertilizer)". The weight of fine brown rice per kg of nitrogen applied tended to be higher in "side-row + seedling box" than in "control (total base fertilizer)," especially in 2022 and 2023, the years of weather-related disasters, when the weight of fine brown rice per kg of nitrogen applied in "seedling box" was 138 and 147 higher than that in the control (Table 6).

　Quality (grain yield) was "side-row + seedling box" ≒ "control (total base fertilizer)," and the grain yield of "side-row + seedling box" tended not to decrease even at high yield levels. On the other hand, the eating quality (brown rice protein) was "side-row + seedling box" > "control (total base fertilizer)" (Table 7).

(2) Start dash, culm quality and endurance evaluation

　In all three years, the number of stems per square meter at 30 days after transplanting, which indicates a dash of initial growth, was "side strips + seedling box" > "control (full basal fertilizer)," indicating that "side strips + seedling box" was more effective in ensuring initial growth. Culm leaf fullness, which indicates culm quality, was "side-row + seedling box" > "control (total basal fertilizer)," indicating that "side-row + seedling box" was effective in securing thick stems. The color of the third leaf from the top (water-absorption capacity of roots) in the second half of the seedling, which indicates endurance, was "side-row + seedling box" > "control (total base fertilizer)," indicating that "side-row + seedling box" contributed to maintaining photosynthetic capacity in the second half of seedling maturation (Table 8).

(3) High temperature resistance evaluation (2023, which was a high temperature year)

　According to the leaf color trend by integrated temperature after ear emergence (Fig. 3), the leaf color (top two leaves) of the "control" was equal to that of the "seedling box-makase" at an integrated temperature of 180°C after ear emergence, but at an integrated temperature of 591°C after ear emergence, the "control" became "seedling box-makase" > "control", and then the leaf color of "control" decreased significantly, and at an integrated temperature of 851°C after ear emergence in the latter half of ripening, the "seedling box-makase" > "control" leaf color fell below 30. At 851°C, the control was selected from the "seedling box-made" and "control", and the leaf color of the "control" dropped to below 30.

　Figure 4 shows the degree of decrease in leaf color at the integrated temperature stage I (from 180°C to 591°C) and at the integrated temperature stage II (from 591°C to 851°C) after ear emergence. The degree of decrease in leaf color was greater in Stage II than in Stage I in both years. In comparison with the control, the degree of decrease in leaf color at both stages I and II in each year was more moderate in the "leave it to the seedling box" condition than in the "control" condition. In the case of 2023, which was an abnormally hot year, the degree of decline in stages I and II was greater than that in other years, especially in stage II in the control, and the decline in leaf color suggests that wilting progressed significantly in the latter half of seedling maturation. On the other hand, the degree of decline in the "seedling box" was smaller than that in the "control" and similar to that in the other years.

　Even under severe climatic conditions during the ripening period, the leaf color of the "control (total basal fertilizer)" crop remained stable, indicating that the nitrogen supply to the rice body of the "control" crop was stable (Figure 5). The leaf color of the "control (total fertilizer)" began to decline after 400°C and significantly declined after 600°C, especially in the lower leaves of the "control. Therefore, it is considered to be a fertilizer with high temperature tolerance and especially useful in areas where high temperatures are the norm for rice maturation.

　In 2023, an abnormally high temperature year, the color of the first leaf and the second leaf (leaf-1) of the "control" and the "boxed" rice plants were lower than those of the "boxed" rice plants when the accumulated temperature after ear emergence was 845°C. The tendency of rice plants with lower leaf color at the first leaf to have lower leaf color at the second leaf was greater in the "control" than in the "boxed" rice plants (Fig. 6). The tendency for the next leaf color to be lower in rice plants with lower leaf color at the leaf stop was greater in the control (Fig. 6). In the comparison of leaf color of the lower leaves and the third leaf from the top in the "control" and "seedling box left to the seedling box," rice plants with the third leaf from the top dead were in the "control" > "seedling box left to the seedling box," indicating that under high temperature conditions, a large decrease in leaf color of the lower leaves in the latter half of the ripening period leads to a decrease in water absorption capacity of the rice plants, causing a decrease in quality and yield (Fig. 7). This leads to a decrease in the water-absorption capacity of the rice plants, resulting in lower quality and yield under high temperature conditions (Figure 7).

　The color of the lower leaves (third leaf from the top), which is related to root vigor, was higher in 2023, an abnormally hot year, than in 2022, a year with insufficient sunlight. In 2023, when the temperature was high, the mortality rate was 9/19 (47%) in the "control" and 4/41 (10%) in the "box-assigned". The number of rice plants with leaf color of 20 or less on the third leaf blade from the top was 0/15 (0%) in the "leave to seedling box" and 2/14 (14%) in the "control" in 2022, while it was significantly higher in the "control" with 8/41 (20%) in the "leave to seedling box" and 15/19 (79%) in the "control" in 2023, which was a high temperature year. This result was consistent with the results of the "Control" group. This result indicates that the lower leaves, which supply photosynthates to the roots, died more frequently in the "control" in the high-temperature year, indicating that the water-absorbing capacity of the roots decreased. Under these conditions, the "seedling box" maintained the water-absorbing capacity of the roots with less lower leaf mortality and leaf color (Fig. 8).

(4) Evaluation of root water absorption capacity

　Leaf color at 24 days after ear emergence (top two leaves) was higher in "seedling-box-makase" than in "control" for each cultivar. In particular, the decline in leaf color from 17 days after ear emergence was smaller in "seedling-box-makase" than in "control". The rate of efflux, which indicates the water-absorption capacity of the roots, was also higher in the control than in the 'seedling-box-makase'. This suggests that the continuous nitrogen supply by 'seedling-box-makase' reduced the decline in leaf color (lower senescence pressure) and maintained a high rate of efflux, which was one factor that reduced late wilting of rice plants under the hot ripening conditions in 2023 and reduced quality decline, thus ensuring stable yields. This is thought to be one of the factors that reduced the late wilting of rice plants and ensured stable yield even under the high temperature ripening conditions of 2023 (Table 9). Under high temperature conditions, a decrease in root water uptake capacity during the ripening period can be fatal. The continuous supply of nitrogen by "seedling box planting" maintained leaf color, supplied nutrients to the roots, and resulted in high water uptake capacity as indicated by the rate of emergence.

　This suggests that the continuous supply of nitrogen by "leave it to the seedling box" maintains the leaf color of the lower leaves, which supply photosynthates to the roots, and that cytokinin (a hormone that inhibits chlorophyll degradation in the leaf blade) produced by the roots is also supplied to the aboveground area, maintaining a high rate of root emergence (water absorption), increasing nitrogen absorption, and improving photosynthesis rate in the leaf blade, resulting in higher yield and quality (Figure 9). This is thought to increase the rate of photosynthesis of the leaf blade, resulting in improved yield and quality (Fig. 9).

(5) Improvement of ripening capacity

　m²According to the relationship between the number of paddy grains per unit of rice and the percentage of milled brown rice grains, m²The results showed that the "seedlings in the box" > "control" regardless of the number of paddy per box, especially in the case of the m²At high paddy number levels, the "seedling box left in place" > > "control" (Fig. 10). m²According to the relationship between the number of rice grains per unit area and the weight of 1,000 grains, the number of rice grains per m²The results showed that "leaving the seedling box on the seedling box" maintained leaf color of lower leaves in the latter half of rice maturity, maintained water absorption capacity of roots, and suppressed senescence of rice plants (Figure 11), which usually leads to a decline in maturity (milling grain yield and thousand-grain weight).²Even at a high paddy number per unit area, the crop improved maturity and yield was secured and quality loss was reduced even in a hot year when the crop was susceptible to senescence.

(6) Evaluation by image

　According to the results of drone-sensing images taken on July 3 (the highest stage of the highest raking season) of plots with siliceous (slag) material applied + "side strips + seedling box" and without slag application + "full basal fertilizer" (Figure 12), the "side strips + seedling box" + slag (siliceous material) had higher NDVI, less variability, and higher yields. On the other hand, the total basal fertilizer applied in the field without soil preparation using siliceous materials resulted in low NDVI growth (nitrogen uptake), indicating that there was a large variation in growth within the field. It is thought that the stable growth ensured by the "side-row + seedling box" method and the application of "slag" with silicic acid play the roles of both wheels of a car.

5. strategies for producing high temperature tolerant rice (Figure 13)

　In the same cultivar, damage was reduced in plots where leaf color (especially the third leaf from the top) was maintained until late in the ripening period, soil preparation was implemented, appropriate water management was implemented (e.g., to cope with fading), and root mass was secured. Therefore, in this era of high temperatures, the application of siliceous materials, reduction of reduction risk, rooting, and proper water management are essential technologies that need to be further promoted.

0Factors contributing to changes (e.g., weather, decline in soil fertility, omission of basic technology, and soaring fertilizer costs) are becoming more diverse and significant.

0 "Starting off" and "endurance" are important to create rice plants tolerant to high temperatures.

In order to optimize the sink capacity, the "start dash" to improve the initial growth is necessary to improve the m²Appropriate number of ears per m²It is important to induce the primary branching type of rice (a large amount of secondary branching rice is disadvantageous) by optimizing the number of rice per unit of production.

0Securing the mature roots is important, and the number of cases of late wilting caused by roots is increasing. It is necessary to secure the "nadir root" and "upper root," which are the roots that mature (securing the roots by appropriate drying out and intermittently irrigating). This is fatal under high temperature conditions.

0Under high temperature conditions, it is essential to supply small amounts of nitrogen continuously. In 2023, under abnormally high temperature conditions, the "leave it to the seedling box" method maintained high leaf color, especially in the lower leaves, which are closely related to the water absorption capacity of the roots. Especially, the leaf color of lower leaves, which is closely related to root water uptake capacity, was maintained. High leaf color indicates high water uptake by the roots, and thus it is important to maintain high leaf color in the second half of seedling maturity.

0 Reduction risk should be reduced (strong reduction => delayed vegetative growth => delayed and suppressed root elongation => inability to secure strong stems => insufficient root mass and inappropriate sink capacity). To reduce reduction risk, it is necessary to introduce technologies such as drainage measures, application of steel slag (siliceous material containing iron oxide), and application of enzyme material (Agri-Revolution spraying) that promotes decomposition of rice straw (cellulose).

0Under high temperature conditions, the application of siliceous materials is essential as fertilizer because silicic acid improves light-reception (increased leaf erectness), increases specific leaf weight (leaf blade thickness), improves root oxidation (increased water absorption), and promotes transpiration by opening pores (decreased rice body temperature). Since the risk of field reduction is increasing, steel slag-based siliceous materials containing iron oxide are useful. Rice with low silicon content ⇒ reduced transpiration ⇒ closed stomata ⇒ reduced photosynthetic capacity ⇒ reduced light reception by lower leaves ⇒ reduced photosynthetic capacity of lower leaves ⇒ insufficient supply of carbohydrates to roots ⇒ reduced root vigor ⇒ wilting ⇒ lower yield and quality.

0To reduce damage caused by high temperatures, it is necessary to improve efficiency through information-based soil preparation, introduction of countermeasures that address the issues of each field, and medical record management (drone sensing is extremely useful for such information and can be evaluated visually (with high accuracy), and is a weapon in the armory of farm management guidance. It is a weapon in the armory of farm management guidance).

　The above shows that the stable early growth of rice plants by "side-rowing + seedling box-less" and the continuous supply of small amounts of nitrogen during the seedling maturation period, as well as the improvement of photosynthesis by the application of slag (silicic acid) and the securing of roots during rice maturation by suppressing reduction (spraying Agri Revolution ⇒ promoting rice straw rotting), are essential for rice production (reducing quality decline and securing stable yields) that is not affected by weather fluctuations. This is essential for rice cultivation that is resilient to weather fluctuations (reducing quality decline and ensuring stable yields), and is considered to be a trump card for rice cultivation in the future.

No Soil - No. 29
　　Agriculture and Environmental Issues - Part 4
　　　Air pollution from nitrogen from agricultural land - Dinitrogen monoxide emissions

Former Technical Advisor, Hokkaido Branch, Jcam Agri Co.
Teruo Matsunaka

　Following the previous article on ammonia volatilization, this article also discusses air pollution by nitrogen (N) originating from agricultural lands, and will focus on dinitrogen monoxide (N2O, also called nitrous oxide), which is one of the major greenhouse gases. The relationship between greenhouse gases and agriculture on a global scale and greenhouse gases other than N2O will be discussed comprehensively in the next issue.

1. N2O generation (direct emission) and soil moisture conditions

　The greenhouse effect of N2O is 298 times stronger than that of carbon dioxide (CO2). It is also responsible for the depletion of the stratospheric ozone layer and has a significant negative impact on the environment.

　There are two pathways by which N2O is generated in agricultural lands, both of which involve the action of microorganisms. One is the pathway in which ammonia-form nitrogen (NH4-N) given to farmland in the form of organic or chemical fertilizers produced from livestock excrement is converted to nitrate-form nitrogen (NO3-N) under oxygenated conditions (oxidative conditions) (this conversion is called nitrate conversion). This pathway is generated as a byproduct when nitrate nitrogen (NH4-N) is converted to nitrate nitrogen (NO3-N) under oxygenated (oxidative) conditions. The other pathway occurs as an intermediate product of the transformation of NO3-N formed by nitric oxidation into nitrogen gas (N2) under oxygen-deficient (reductive) conditions (this transformation is called denitrification).

　Whether the soil is under oxidizing or reducing conditions is determined by the degree to which all the spaces in the soil (total pore space) are filled with water (this is called water saturation (abbreviated as WFPS = Water Filled Pore Space)). When the soil is between 60% water saturation (a moisture state that is adequate for nitrification microbial activity) and 70% (a slightly wetter soil), both nitrification and denitrification are underway, and more N2O is produced and released from the soil to the atmosphere (Fig. 1). In wetter conditions with moisture saturation greater than 80%, reducing conditions are more reductive and denitrification results in the formation and emission of mainly N2, with less N2O emissions. Conversely, when the water saturation is lower than 50%, the conditions are dry and nitrification is the main process, with nitric oxide (NO) being the main emission.

　A moisture saturation of 60-70%, which is optimal for N2O emission, coincides with a moisture state that is relatively favorable for crop production. Therefore, even if the amount of N applied to farmland is appropriate, it is difficult to completely suppress N2O generation. In addition, as soil temperature increases, microbial activity increases and the amount of N2O generated also increases. This direct emission of N2O generated in the soil directly to the atmosphere is called direct emission.

2. emission from N2O dissolved in water - indirect emission

　Some of the N2O generated in agricultural land is dissolved in soil moisture (soil solution) and groundwater, dissolving in a supersaturated state. When this supersaturated N2O-containing solution is released to the atmosphere as culvert drainage, spring water, or river water, the N2O is released to the atmosphere because the supersaturated state is lifted. Furthermore, when NH4-N, which was released to the atmosphere by ammonia volatilization as discussed in the previous article, dissolves in rainfall and returns to the soil, it undergoes nitrification in the soil, producing N2O as a byproduct.

　Such emissions are known as indirect emissions and, like direct emissions, are not negligible.

3. N2O emissions in Japan and their relation to agriculture

　The total amount of N2O emitted in Japan in 2021 was 19.9Mt (megaton=million tons) in terms of CO2 (issued on the first day of every month) No. 758 on February 1, 2024 (Greenhouse Gas Inventory Report of Japan, 2023). This is a 40% reduction compared to 1990 (Figure 2). Of this amount, 9.6 Mt of CO2 equivalent of N2O was emitted from the agricultural sector, which is a major source of emissions, accounting for 48% of the total emissions. Furthermore, emission reductions in industrial processes and product use were highly successful from 1990 to 2021, while those in agriculture were only slightly less successful (Figure 2).

　N2O derived from agriculture is mainly emitted from livestock excreta management and from the soil of agricultural land. N2O is emitted directly from livestock excreta through nitrification and denitrification during the management of livestock excreta, and indirectly from ammonia volatilization during the management process. The emissions from agricultural soils are caused by direct emissions of N2O from nitrification and denitrification of N contained in chemical fertilizers and organic fertilizers when these materials are applied to agricultural soils, and indirect emissions of NO3-N after its dissolution in groundwater and other sources. In addition, N2O is also emitted from the combustion of agricultural residues (open burning). However, the amount is extremely small (Figure 3).

　N2O emissions (CO2 equivalent) from the agricultural sector in Japan decreased from 11.7Mt in 1990 to 9.6Mt in 2021, a 28% reduction in emissions (Figure 3). This reduction is largely due to the effect of a decrease in the amount of N2O emitted directly from agricultural soil. However, this is a negative result because the total amount of chemical and organic fertilizers applied to farmland also decreased due to the large reduction in the area of farmland in Japan during this period (Greenhouse Gas Inventory Report of Japan, 2023).

4. measures to control N2O emissions from agricultural land

　According to the Greenhouse Gas Inventory Report of Japan (2023), environmentally friendly agriculture has been recommended in some areas, which has mitigated groundwater pollution caused by surplus N, resulting in a reduction of indirect N2O emissions. The practice of environmentally friendly agriculture that does not generate surplus N in the N cycle around farmland can be expected to be highly effective in reducing N2O emissions.

　Another technology that is expected to be effective in reducing N2O emissions is the use of nitrate formation inhibitors (Di et al., 2010), one of which, dicyandiamide (DCD), has already been put to practical use. Nitrate formation is suppressed by reducing the activity of ammonia-oxidizing bacteria, which are involved in the initial stage of nitrate formation, i.e., the transformation from NH4-N to nitrogen dioxide (NO2-N), thereby reducing N2O production.

　There is uncertainty about the effectiveness of nitrification inhibitors in reducing N2O emissions in actual fields and grasslands. This is because the effect is influenced by environmental conditions such as soil moisture and soil temperature, which may or may not be apparent.