The System of Rice Intensification (SRI) involves the adoption of certain changes in management practices for rice cultivation that create a better growing environment for the rice crop. Use of the intermittent irrigation with alternate wet and dry intervals (AWD) and single transplanting of the younger seedlings at wider spacing are regarded as the key factors in SRI for better crop growth and productivity.

Field experiments were conducted in Chiba, Japan during the two consecutive rice growing seasons (May-September) of 2008-09 to observe the effects of SRI components on rice crop performance, field environment, water savings, and water-wise rice production. The effects of irrigation method, age of seedling and spacing were evaluated during 2008 rice-season with 8 treatment combinations in a Split-split plot design (S-SPD). AWDI at 10 days intervals and continuous flooding throughout the cropping season were the two main plot factors while the effects of age of seedlings (14 and 21 days) and plant spacing (30x30cm2 and 30x18cm2) were evaluated as sub and sub-sub plot factors, respectively. The experiment results revealed that the SRI management with the proposed AWDI can save a significant amount of irrigation water (29%) without reduced grain yield (7.41t/h compared with 7.37t/ha from normal planting with ordinary water management).

Water productivity also observed to be significantly higher in all combinations of practices in AWDI plots: 1.74 g/liter with SRI management and AWDI as compared to 1.23 g/liter in normal planting with ordinary water management. In addition, the research outcomes showed a role of AWDI in minimizing pest and disease incidence, shortening the rice crop cycle, and also improving plant stand until harvest. Synergistic effects of younger seedlings and wider spacing were seen in tillering ability, panicle length, and number of filled grains that ultimately led to higher productivity with better grain quality.

Field experiment with the complete sets of SRI practices was carried out in Randomized Complete Block Design (RCBD) during 2009 rice growing season at the same field. SRI (with 8 days old seedlings) and Conventional (with 22 days old seedlings) practices were the first factor (cultivation method), while Organic and Inorganic managements were evaluated as second factor (management method) in the field experiments. The highest yield was observed with the conventional method with inorganic management (6.84t/h) that was at par with the Organic SRI (6.59t/h) followed by Organic Conventional (6.48t/h). It was recorded as 5.92t/h in Inorganic SRI management. Overall, the effects of SRI components were positive and significant on per plant basis however, it was not differed significantly in terms of grain yield per unit area.

Development of healthy and vigorous roots, increased stem diameter, greater productive leaf area, longer panicles, greater number of filled grains, development of plants tolerant to insect-pest and disease, and reduced plant lodging percentage were some notable achievements with SRI management. Water savings and water-wise rice production are other important issues that are likely to draw the attention of rice researcher and farm communities to adopt SRI under water scarce condition. However, still, comparatively better grain yields when using conventional management method underscore a need for further investigations in defining an appropriate combination of practices for SRI management considering local soil properties, prevailing climate, and critical watering stages in rice crop management.

Above all, the followings are the key lessons during the course of two years' experimentation.

・Soil aeration is very important operation in the intermittent plot. As the soil surface becomes hard as a result of alternate wet and dry intervals, it is imperative to make them loose through weeding or hoeing practices. It has been appeared as an utmost practice even if herbicides were applied to the intermittent plot.

・Panicle length and number of grains/filled grains were largely attributed to the use of single seedling at wider spacing. Despite of having fewer effective tillers or panicles per unit area, satisfactory yield in SRI plots were merely due to better panicle and grain characteristics. It has been possible due to the development of a large number of white and functional roots, greater leaf area with dark green foliage (productive leaf surface), and healthy and vigorous plant with thicker stem and longer roots. Normal seedlings that were transplanted singly at wider spacing also possessed longer panicles as well. So potential of single transplanting with the different seedlings' age should be further explored.

・SRI components appeared to be highly responsive to agri-input especially with the application of organic fertilizers. The application of as much as organic manures, and soil aeration together can provide the better results than the inorganic fertilizers alone.

Weeding requirements can be reduced by using just a single dose of pre-emergence herbicides after transplanting or by maintaining a shallow standing water depth for the first 15-20 days or more after crop establishment when crop canopies completely shade the land, and subsequently maintaining the alternate wetting and drying period or by creating a saturated but un-flooded soil regime until maturity.

・Intermittent irrigation with alternate wetting and drying intervals appeared to be a good practice for pest and disease management, water savings and water productivity.

・Due to its lodging tolerance effects, SRI practices with AWDI appeared to be a better practice in wind affected areas (especially during maturation).

・Organic rice farming under SRI and Conventional management was profitable than the conventional rice farming. Though the grain yield was comparatively lower in Organic SRI along with the increased labor inputs for weed management, but provides higher net return per unit area and was associated with the price premium for organically produced rice.

Overall, based on the field experiment in Chiba, Japan, it can be concluded that in certain areas and under the right conditions, use of intermittent irrigation with AWD schedule is a promising method in irrigated rice cultivation with dual benefits of resource saving and productive. The increased productivity of water and its resource-saving aspects are likely to be the critical factors that will make farmers and other stakeholders adopt AWDI in water-scarce environments, becoming increasingly common. In the meantime, use of baby seedlings (8 to 14 days after sowing) and single transplanting at wider spacing (30x30cm2) was proved to be significant with respect to the panicle and grain characteristics, among others.

SRI components showed positive effects on crop performance and environment. For grain yield, SRI effects was positive and significant on per plant basis; but appeared to be non significant while accounting per unit area. And the associated benefits (water-wise rice production, resource savings and environment-friendly cultivation) could be much greater than those achieved with the conventional methods of rice cultivation. No extraordinary high yields and increased labor costs while using Organic SRI may limit its adoption however, the associated benefits has demonstrated the scope of SRI practices as a promising technique of sustainable rice farming over conventional method even in the temperate climatic condition. Indeed, once farmers learn that they can reduce their seed requirement, water requirement, production costs, and perhaps labor requirements, while maintaining yields, SRI methods are poised to become more widely adopted

本論文は、SRI(System of Rice Intensification)と呼ばれる、比較的新しい稲作農法を対象に、温帯地域の日本という場において、どのような可能性を有しているかについて追究したものであり、6つの章で構成されている。

第1章では、SRIという農法の概略およびその展開状況をもとに、本論文の目的が述べられている。Laulanieが提唱したSRI農法は、その後、Uphoff、佐藤、Komaらの努力により広く展開してきているが、その農法は各人の考え方および実施される場所の環境によって異なっている。灌漑施設がある場所での定義としてはStoopらの定義が最も一般的であり、出芽後10日間前後の乳苗を、広い間隔で一本植えし、栄養成長期から間断灌漑によって育てる、というものである。このように育てることによって、灌漑用水が節減されると同時に収量の増大が数多く報告されており、それゆえに、これらの報告に対する懐疑的な見方も少なくない。そこで本論文では、以下の3点の解明を目的とした。1.SRIの各要素がイネの生育と生産性にどのように影響するのか、2.SRIの各要素が環境と節水に与える影響、3.温帯日本という環境下でのSRI農法の可能性。そして、それぞれの帰無仮説として、以下を設定した。1.従来型の農法は、SRI農法と同程度の効果を持っている。2.水、種、作業時間、肥料、化学農薬の資源利用効果は、従来型農法とSRIとで同じである。3.有機栽培および化学肥料栽培下におけるSRIおよび従来型の稲の生長は、同じ収量を与える。

第2章から第4章は、上記仮説を確認するための実験について記述している。まず実験圃場として、柏市大青田の農家水田(760平米)を借用した。ここにそれぞれの実験目的に合わせた小プロットを設定し、灌漑方法、施肥方法、移植方法をそれぞれ変更させた。2008年度は、灌漑方法(連続灌漑、浅水間断灌漑)、苗の葉令(14日、21日)および栽植密度(30cm×18cm、30cm×30cm)を合計8通り設定した(第2章)。

その結果、以下が明らかになった。浅水間断灌漑によって、節水、登熟期間の短縮、倒伏率の低下、および耐病性の増大が認められた。若い苗の移植によって、穂の成長の促進、登熟率の促進、収量指数の増加が見られた。そして広い栽植密度によって、根の本数、長さ、重量の増加、有効穂数の増加が見られたが、面積あたり換算では有効穂数の有意差は見られなかった(第3章)。

この結果を受けて、第4章では2009年(2年目)の実験の目的と結果が示されている。設定としては、葉令(8日、22日)、移植本数(1本、4本、)、灌漑方法(連続灌漑、浅水間断灌漑)、栽植密度(30cm×18cm、30cm×30cm)、施肥(化学肥料、有機肥料)を合計16通り設定した。

第5章では、これらの結果を従来の研究成果と対比させ、従来の成果を検証すると同時に、本研究の独自性を鮮明に示している。温帯地域での研究はこれまでほとんど蓄積がないが、綿密な一事例を提供している。温帯地域での栽培稲は熱帯稲とは感光性が異なるが、とくに中緯度の途上国でのSRI農法の展開が期待できる。

第6章では、以上の考察のもとに、SRIという農法について、以下のようにとりまとめている。1.広い間隔で1本の苗を移植することによって、根の生育を促し、葉色を向上させ、耐病性を高め、穂の生育を促進した。2.浅水間断灌漑によって、耐病性が向上し、節水効果が認められた。3.SRI稲は強風下での倒伏が少ない。

以上のように、本研究は、温帯日本におけるSRI栽培実験を綿密に企画し、実施した最初の研究であり、その独創性が評価できる。また従来の研究は、実験圃場という狭い特殊な環境下での実験か、農家圃場を用いてはいるものの収量構成要素などが正確に取られていない実験に大別されるが、本実験は農家圃場を用い、かつ綿密な測定を行っているところに大きな特徴がある。加えて、本論文での考察は、多様で綿密な実験結果を統計的処理により信頼性を含めて議論したものであり、結果の信頼性を高め、より客観的な議論に資するデータを提供している。すなわち、本論文はSRI研究の一事例を提供することに加えて、SRI研究の学問体系に大きな寄与を与えたと言える。

よって本論文は博士(国際協力学)学位請求論文として合格と認められ、審査員一同は、本論文に対し、博士(国際協力学)を授与することを認めた。