§高温に負けない施肥法としての「苗箱まかせ」
Farm Frontier Inc.
取締役会長　藤井　弘志
§土のはなし－第29回
農業と環境問題－その４
農地由来の窒素による大気汚染－一酸化二窒素排出
前 ジェイカムアグリ株式会社
北海道支店　技術顧問
松中　照夫

　

　

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

Farm Frontier Inc.
取締役会長　藤井　弘志

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).

　各品種とも，2023年は2022年に比べて登熟歩合を示す精玄米粒数歩合は高く（精玄米率も高く)，千粒重は低くなった（表４)。

　精玄米粒数歩合が高い要因としては，いずれの品種も出穂前10日間の日照時間が極めて多く（平年比160～200）出穂前の蓄積炭水化物量が多かったこと，出穂後も高温で経過したが日照時間が多かったことで，着生籾の登熟が停止（発育停止籾の発生が極めて少なかったこと）しないで，進行したことが考えられる。出穂前蓄積炭水化物が多かったこと，籾殻の形成が充実（籾殻のケイ酸集積量も多かった）したことは，過高温条件での被害軽減に寄与したと考えられる。　

　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)

　高温条件における品質低下を軽減した事例として，
①新潟県（2019年）では積極的な穂肥の実施や穂肥を３回（出穂前15日，出穂前7日，出穂前1日）実施が効果的であったこと，
②酒田市（2023年）では，窒素1kg/10aの４回穂肥を実施した圃場で品質や収量低下を軽減したこと，
③森田（博士論文：イネの高温登熟傷害に関する生理・生態学的解析）によれば，出穂前16日～出穂後12日の間で15回少量継続追肥により，慣行の出穂前16日と出穂前6日の２回の追肥に比べて，未熟粒歩合が低下した（品種：ヒノヒカリ）こと
が報告されている。一方，高温条件で品質低下した事例では，
①8月（出穂期以降）に葉色が淡かったので後期栄養が不十分であったこと，
②出穂後の高温によって，結果的に窒素栄養が不足したこと，
③肥料不足（後期栄養)，④例年通りの窒素施肥では，高温年は窒素栄養が不足したこと
が報告されている。

　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

　㎡当たり籾数と精玄米粒数歩合との関係によれば，㎡当たり籾数に係わらず「苗箱まかせ｣＞｢対照」であり，特に㎡籾数が高いレベルでは，｢苗箱まかせ｣＞＞｢対照」であった（図10)。

　㎡当たり籾数と千粒重との関係によれば，㎡当たり籾数に係わらず「苗箱まかせ｣＞｢対照」であり（図11)，｢苗箱まかせ」によって登熟後半の下位葉の葉色を維持し，根の吸水能を維持し，稲体の老化を抑制したことが，通常は登熟能（精玄米粒数歩合，千粒重）が低下しやすい㎡当たり籾数が多いレベルでも，登熟能を向上させ，老化しやすい高温年においても収量を確保し，品質低下を軽減できたと考えられる。

(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
農業と環境問題－その４
農地由来の窒素による大気汚染－一酸化二窒素排出

前 ジェイカムアグリ株式会社
北海道支店　技術顧問
松中　照夫

　今回も，前回のアンモニア揮散に続き，農地に由来する窒素(N)による大気汚染の話題で，温室効果ガスの主要なものの一つ，一酸化二窒素(N₂O,亜酸化窒素ともいう）を取り上げる。なお，N₂O以外の温室効果ガスについてや，地球規模からみた温室効果ガスと農業との関わりは，次回，総合的に考えることにする。

１．N₂Oの発生（直接排出）と土の水分条件

　N₂Oの温室効果は，二酸化炭素（CO₂）の298倍も強力である。また，成層圏オゾン層の破壊にも関与しており，環境への悪影響が大きい。

　農地でN₂Oが発生するのは，微生物の働きが関わる二つの経路がある。一つは，家畜排泄物などから生産される有機質肥料や化学肥料の形態で，農地に与えられたアンモニア態窒素（NH₄－N) が酸素のある条件（酸化的条件）で，硝酸態窒素 (NO₃－N）に変化する時（この変化を硝酸化成という）の副生成物として発生する経路である。もう一つは，硝酸化成でできたNO₃－Nが，酸素不足の条件（還元的条件）に置かれて，窒素ガス (N₂）へ形態変化する（この変化を脱窒という) 時の中間産物として発生する経路である。

　土の中が酸化的条件であるか，還元的条件であるかは，土の中のすべての隙間（全孔隙）が水分でどの程度埋め尽くされているか（これを水分飽和度（略称WFPS＝Water Filled Pore Space）という）で決まる。土の水分飽和度が60％（硝酸化成の微生物活動にはほどよい水分状態）から70％ (土がやや湿った状態）の範囲で，硝酸化成や脱窒の二つの作用が進行するため，N₂Oの発生が多くなって土から大気に排出される（図１)。水分飽和度が80％を上回る湿潤な状態では，還元的条件が強まり脱窒によって，主にN₂が生成して排出され，N₂Oの排出は少ない。逆に，水分飽和度が 50％より低い場合は，乾燥状態となり硝酸化成が中心で，主に一酸化窒素（NO）が排出される。

　N₂O排出に好適な水分飽和度60～70％とは，水分状態が作物生産に比較的良好な状態と重なる。したがって，農地へのN施与量が適正量であったとしても，N₂Oの発生を完全に抑止するのは難しい。また，地温が高まると微生物活性が高まってN₂Oの発生量も多くなる。このように土の中で発生したN₂Oがそのまま大気に排出されることを直接排出という。

２．水に溶けたN₂Oからの排出－間接排出

　農地で発生したN₂Oの一部は，土の中の水分 (土壌溶液）や地下水に溶け込み，過飽和の状態で溶存している。この過飽和でN₂Oを含む溶液が，暗渠排水や湧水，河川水として大気に開放されると，N₂Oは過飽和状態が解除されるため，大気に排出されていく。さらに，前回お話ししたアンモニア揮散によって大気へ出ていったNH₄－Nが，降雨に溶け込んで土に戻ってくると，土の中で硝酸化成を受け，その副生成物でN₂Oが発生する。

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

３．わが国のN₂O排出量と農業との関わり

　2021年のわが国で排出されたN₂Oの総量は，CO₂（毎月１日発行）令和６年２月１日  第758号に換算して19.9Mt（メガトン＝百万トン）だった (日本国温室効果ガスインベントリ報告書，2023 年)。これは，1990年に比較し40％も削減されている（図２)。このうち，農業分野から排出されたN₂O量はCO₂換算で9.6Mt，総排出量の48％を占める大きな排出源である。しかも工業プロセス及び製品の使用の分野では，1990年から2021年に排出抑制が大きく成功したのに対して，農業分野の抑制はわずかにすぎない（図２)。

　農業に由来するN₂Oは，主に家畜排泄物の管理からと農地の土から排出される。すなわち，家畜排泄物を管理している時に，排泄物中で硝酸化成や脱窒が発生し，N₂Oが直接排出される場合と，管理過程でのアンモニア揮散を起点とする間接排出である。農地の土に由来するのは，化学肥料や有機質肥料が農地に与えられた時，これら資材に含まれるNが，農地の土の中で硝酸化成や脱窒の作用を受け，N₂Oが直接排出される場合と，生成したNO₃－Nが地下水などに溶存した後，間接排出される場合である。さらに，農作物残渣の燃焼 (野焼き）でも排出される。しかし，その量は極めてわずかである（図３)。

　わが国の農業分野からのN₂O排出量（CO₂換算量）は，1990年に11.7Mtあったのに対して，2021 年は排出量が28％削減されて9.6Mtに減少した(図３)。この削減は，農地の土から直接排出するN₂O量の減少効果の影響が大きい。しかしそれは，この期間の国内の農地面積が大きく減少したため，農地へ施与される化学肥料や有機質肥料の総量も減少したという消極的な結果である（日本国温室効果ガスインベントリ報告書，2023年)。

４．農地からのN₂O排出の抑制対策

　日本国温室効果ガスインベントリ報告書(2023) によると，一部の地域では，環境保全型農業が推奨され，それが余剰Nによる地下水の水質汚濁を緩和し，その結果，N₂Oの間接排出量の削減につながったという。農地を巡るN循環で余剰Nを発生させない環境保全型農業の実践は，N₂O排出抑制効果が大きいと期待できるだろう。

　この他にN₂O排出抑制効果が期待される技術として，硝酸化成抑制剤の利用がある(Diら，2010)｡その抑制剤の一つジシアンジアミド（DCDと略) はすでに実用に供されている。DCDは硝酸化成に関わる微生物（細菌）のうち，硝酸化成の初期段階，すなわち，NH₄－Nから二酸化窒素（NO₂－N) への形態変化に関わるアンモニア酸化細菌の活性を低下させて硝酸化成を抑制し，N₂Oの生成を減少させる働きがある。

　実際の畑や草地で，硝酸化成抑制剤のN₂O排出削減効果には不確実性がある。土の水分や地温など環境条件に影響を受けるため，その効果が明らかに認められる場合と，認められない場合があるからである。