Rice yield is generally low (2.7 t ha-1 in 2008) in Cambodia. Increase rice production remains the central focus of Cambodia's agricultural policy for potential export for economic development and for livelihood improvement for rural population. Paddy rice is grown over diverse water environments in Cambodia; irrigation rehabilitation area of those constructed during Pol Pot time, deepwater rice ecosystem in flood plains of Tonle Sap Lake, rainfed lowland rice ecosystem sharing over 80% of Riceland in Cambodia . Rice yield should be increased in every rice ecosystem for food security and better livelihood for farmers. This study focused on (1) heterogeneous water environments within a village or within a landscape of human sight (i.e. defined as "micro-scale" in this thesis) within each of the rice ecosystems, (2) transfer and adoption of technologies available for farmers, and (3) positive externalities (i.e. multifunctional roles) of rice farming such as biodiversity conservation, landscape or cultural values, in order to improve rice yield in sustainable manner.

Analysis on the process of irrigation rehabilitation in Kamping Puoy in Battambang province, Northwest Cambodia, in the consecutive 4 cropping seasons from 2008 wet season rice (WSR) to 2010 dry season rice (DSR) revealed standing water depth (e.g. from September to November) much deeper in downstream fields than in upstream fields along the transect of the secondary drainage canals in WSR; farmers adapted to plant medium and late maturing varieties (maturity time in December and January) in the former while early and early medium maturing varieties (maturity time in November) in the latter. Water conditions were less different between upstream and downstream fields in DSR and with more uniform planting and harvesting time. As the area percentage of fields where DSR was introduced increased from 2008 (54%) to 2010 (100%), planting time in WSR shifted later (e.g., from May to July) with declining proportion of dry seeding method and mid-season tillage. On-farm grain yields in DSR were low (287 and 247 g m-2 in 2009 and 2010 on average, respectively), partly due to insufficient weed control and small amounts of fertilizers. Yields were lowest in fields which practiced DSR for the first time, and some improper management practices (such as variety mis-choice, wrong use of insecticides instead of fungicide) were observed, indicating insufficient agriculture extension support to farmers. Grain yield in WSR (286 and 291 g m-2 in 2008 and 2009 respectively) could be increased by transplanting, use of high yielding Raing Chey variety, and application of higher amount of N inorganic fertilizer.

Characterization of DWR area in the flood plain of Tonle Sap Lake in Northwest Cambodia along a transect of water depth gradient (from the shallower rainfed lowlands side to the deeper floating rice side) during wet season rice (WSR) production in 2008, 2009 and 2010 revealed (A) very gentle and almost flat slope (only 40 cm elevation differences in 1 km distance) and 3 groups of rice zones with (1) upper fields located closer to the National Road Number 5 where water depth was shallower and only lowland rice (LR) was grown; (2) middle fields where both LR and floating rice (FR) were grown and where lowest grain yield was recorded in 2009 due to the flood; and (3) lower fields located near to the Lake where water depth was deeper (average maximum depth more than 150 cm) and only FR was grown. (B) Secondly it was also revealed large yearly differences in flood from Tonle Sap Lake; 2008 and 2009 when water came to the paddy fields from both the inundation from Tonle Sap Lake and rainfall and when the presence of continuous standing water started in September, reached maximum in October (> 1 m) and became non-flooded conditions in early December vs. 2010 when flood did not come from the Lake and all the 3 rice zones had less than 30 cm of maximum water depth and when rainfed lowland rice attained higher yield due to higher N fertilizer application rate. The overall average grain yield for both years of 2009 and 2010 was low with only 1.1 t ha-1 for FR and 1.8 t ha-1 for LR. Late sowing and/or lack of basal N fertilizer application resulted in smaller plant stands when flood occurred, resulting in greater flood damage and more crop failure. Limited forecasting ability for water availability (e.g., flood occurrence) for the subsequent cropping season leave large risks for deep water rice production in flood plains of Tonle Sap Lake.

The study showed that grain yield was very low (1.5 t ha-1) in the studied area in RFLR in Kompong Chhnang province because farmers planted only local varieties on poor soil fertility with the low rate of inorganic fertilizer (14 kg ha-1). Results of on-station experiments in WSR 2009 and 2010 showed that yield was improved by planting improved variety (e.g. Phka Rumdoul) and applying the recommended amount of fertilizer. Yield of Phka Rumduol with fertilizer application could be 60% higher than that of Thmar-Ror-meal (local variety) with fertilizer and more than triple that of Thmar-Ror-meal without fertilizer in WSR 2009. However the efficient level of improved variety and fertilizer was less in WSR 2010 because of water deficit. Adoption level of improved variety and fertilizer depends on the availability and popularity of these resources among the farmers. Farmers preferred to grow Phka Rumduol in WSR 2010 but the adopted area was not high due to the seed shortage. On the other hand, fertilizer was not highly adopted by farmers in WSR 2010 due to unavailability of fertilizer (lack of money to purchase) and farmers' concerns on low benefit return in the drought situation.

Multifunctionalities of the 3 rice ecosystems (IR, DWR and RFLR) in Cambodia were recognized, such as those categorized as (1) livelihood and economic, (2) environment, and (3) social and cultural, at least in local scale among villagers, although the value of each function has not been quantitatively estimated. Bio-resources from paddy fields are important to farmers' livelihood, particularly poor people.

This study shows that (1) water environments are different not only between the rice ecosystems but also within each of the rice ecosystems at micro-scale i.e. between upstream and downstream fields along secondary canals in irrigated rice ecosystem, transect from rainfed lowland side towards the lake in deep water rice ecosystem in flood plains of Tonle Sap Lake, toposequential differences within a village in rainfed lowland ecosystem. The micro-scale variation in water conditions has large influences on farming practices and yield, so the characterization of field water environments is important for technology development and dissemination; (2) insufficient and ineffective usage of agricultural resources (e.g., inorganic N fertilizer, herbicides, fungicides, photoperiod sensitive variety) caused lower farm level yield than possible attainable level in all the 3 rice ecosystems in Cambodia. In rainfed lowland ecosystem while an improved variety of Phka Rumduol was rapidly and popularly adopted by farmers but in short of seeds multiplication, adoption of sufficient amount of inorganic fertilizer was hindered by water deficit and influenced by the cost-benefit balance; and (3) multifunctionlaities of rice production were recognized at village or landscape levels such as other biological resources for livelihood, should be up-scaled in order to draw attention of policy makers to attain sustainable rice farming in Cambodia.

1.論文の概要

カンボジアは1970年からの20年間に及ぶ戦争により、農業分野においても研究・開発・普及システムが破壊され発展が遅れたが、治安が回復し国際社会にも復帰した現在、食料安全保障、貧困解消、農村基盤の安定、経済発展のために、基幹作物であるイネの生産強化が必要である。しかし、カンボジアのイネの収量は近隣のアジア諸国に比べて依然として低い。カンボジアの稲作は、水環境の異なる多様な水田稲作生態系で行われているが、それぞれの生態系内の水環境の変異、村レベルや農家圃場群レベルでの栽培実態、生産の制約要因と改良の可能性を調べた研究は見られなかった。

そこで、本研究では、灌漑修復水田、深水水田、天水田という3つの稲作生態系ごとに、農家の生活範囲、あるいは見渡せる程度の空間的な広がりの範囲内(=景観レベル)での、水環境の変異・不均一性を調査し、農家圃場の収量の低さの原因を、既存の栽培技術と新規の技術の受容という観点から解析し、それぞれの稲作生態系における、持続可能な収量改善方法を明らかにすることを目的とした。以下の7章で構成されている。

第1章は序論で、研究の背景、問題の所在、研究目的と仮説が述べられている。

第2章では、カンボジアの国レベルの米生産政策や、生産統計、水田分布について、既存資料をもとにまとめられている。

第3章は、内戦時に造成されたが十分には機能していなかった灌漑システムの修復地域での2年4作期の事例研究である。灌漑水路に沿ってトランセクトを設定し、雨季の水環境は水路の上流と下流の間に大きな水深差があること、乾季の灌漑栽培が拡大するにつれて品種・作期・雑草管理法も変化すること、乾季作の低収が施肥量と利用効率の低さ、新規二期作への未習熟に起因することを明らかにしている。

第4章は、トンレサップ湖の広大で平らに近い氾濫原での事例研究である。浅い天水田側から深い浮稲側までの10kmにおよぶトランセクトを設定し、水環境、品種、栽培、収量を3年間モニタリングし、湖水氾濫の年次間差が著しいこと、天水田品種と深水稲品種の双方の選択が可能な境界的な圃場群では洪水被害のリスクと多収化の可能性が混在していることを明らかにし、農家のエンパワーメントが重要であるとしている。

第5章は、在来品種を無施肥に近い状態で栽培している天水田地域での3年間の事例研究である。モニタリングやインタビュー調査により、地形連鎖的に高位と低位の水田の水環境の違いを明らかにし、参加型比較試験やコスト・ベネフィット分析も活用して、改良品種と無機質肥料の農家による受容について解析・解釈し、天水田地域での改良品種の種子生産の重要性を提案している。

第6章では、質問票とグループディスカッションによる迅速農村調査法(RRA)により、3章、4章、5章の地域にある村をサンプルとして、水田稲作の持つ米生産以外の有用性(=多面的機能)を明らかにし、水田生態系の多様な生物資源の利用や、農家のイネの品種の多様性、農家による水田景観評価について、知見を得ている。

第7章では、カンボジアの国レベルでの稲作の課題や政策を踏まえ、3章から5章までの稲作改良のための事例研究の知見と、6章の米の生産以外に及ぼす稲作や水田生態系の機能を、総合的に考察し、第1章で立てた仮説を検証すると共に、3つの稲作生態系での持続可能な生産の指針をまとめている。

2.論文の審査の結果

本論文は、以下の5点で評価できる。

(1)カンボジアの3つの多様な水田生態系を総括的に研究対象とし、量的に多い課題を精力的に行ったこと。

(2)大きな水環境の違いと、各生態系の中の小さな水環境の違いを対比させて、農家や村で生産する者の視点での、技術改良のニーズを明らかにしたこと。

(3)農家現場での実態解明という事例研究が論文の中心的なデータではあるが、国の政策や統計、地図に照らし合わせながら、解釈を試みていること。

(4)「緑の革命が成功しなかった地域=天水田」とするステレオタイプ的な説明に対して、無機質肥料と改良天水田品種という非常に基本的な技術要素に注目して、天水田農家の収量や収益の改善という観点から、それらの技術要素の有用性、農家による受容の可能性・限界を丁寧に調査・報告していること。

(5)RRAを効果的に用いて、稲作や水田生態系が持つ外部経済性を、カンボジアという途上国の場において、半定量的に明らかにしていること。

一方、本論文では、低収の要因について生理・形態的な角度からの掘り下げや、それを踏まえての斬新な技術革新の提案、ということはなされていない。それらは本研究の手法と目的の範囲外ではあるが、農業分野の革新的技術研究の成果情報を追跡しながら、農家現場を考えてゆくことは、重要な課題であろう。また、第6章の知見をもっと活用すれば、生産力の劣る稲作生態系での大胆な農村開発の提案も可能であったかもしれない。それらはともかく、上記5点において十分な成果を上げたと判断できる。

以上より、審査委員一同は、本論文に対し博士(農学)の学位を授与できると認める。