Abstract The flotation method of ultra-fine bubbles (UFB) aims to address pollution and has been used for combating the undesirable water conditions of contaminated soils. Hence, water containing UFB is gaining increasing attention for potential agricultural applications. Although certain hypotheses have been proposed, such as the collection of ions in water through the electrical characteristic of UFB, no clear experimental data have been provided. We found that improvement in turbidity may cause the adsorption of fine soil particles in the water by the UFB, thereby improving the quality of the water. The data from the paddy field showed that a decrease in turbidity (below 2 nephelometric turbidity units) occurred over a short period of time (3 days). UFB concentration is directly related to turbidity with a coefficient of determination of 0.93. This phenomenon was also observed through the distribution of bubbles and soil particles, where the average particle size increased because of the aggregation of soil particles and the decrease in turbidity in the paddy field, indicating that UFB collect soil particles and thereby improve water quality. Therefore, UFB are highly effective in cleaning rice field water and will be a preferred method for purifying the environment in the future.

Abstract The extensive and intensive use of herbicides has resulted in the spread of herbicide-resistant weeds in many crop production systems; therefore, it is imperative to devise new organic weed control methods. Recently, the application of spent coffee grounds (SCG) in agricultural fields has been found to inhibit plant growth and germination and is thus considered a potentially effective weed control measure. This study aimed to evaluate the effects of different amounts and methods of SCG application on weed growth through field experiments. The field experiments were conducted in an upland field converted from a paddy in western Japan. The results show that the plow-in application of over 10 kg m⁻² of SCG and mulching application of 20 kg m⁻² decreased the weed dry weight compared with the control. In addition, the growth of weed species of families other than Gramineae, such as wingleaf primrose-willow and horseweed, was not significantly affected by SCG application. Weed species of families other than Gramineae are dominant in some upland fields. Hence, the inhibitory effect of SCG on weeds may be lower in original upland fields than in the upland field converted from paddy field that was investigated in the present study. Overall, this study demonstrated that the plow-in application of 10 kg m⁻² of SCG every 4 mo was effective for weed control in an upland field converted from a paddy field. Because SCG worked against grass weeds under the specific conditions in this study, it would be valuable to explore other potential applications of this novel means of weed control.

Improving agricultural research and education is highly recommended to control agricultural development and environmental sustainability in Cambodia. Agricultural research mostly focuses on interviews with farmers as a first measure in developing countries, but a lack of quantitative accuracy remains one of the major constraints. In this situation, we conducted educational activities for master’s degree students of the Royal University of Agriculture (RUA) to append agronomic information with popular equipment in interdisciplinary fieldwork in Pursat Province, Cambodia. For the popular equipment, an RGB camera, a reflectometer as well as pH and EC meters were selected. The agronomic information collected by the students supported the results obtained during the interviews. For example, the difference in fertilizer application between the irrigated and nonirrigated areas was confirmed by the soil ammonium concentration evaluated with a reflectometer; the difference in rice growth among water conditions was confirmed by the leaf area percentage evaluated with an RGB camera. Since the majority of the students lacked agricultural and statistical knowledge, the agronomic information quantified by popular equipment provided proper educational materials. The interdisciplinary fieldwork also indicated serious problems in the study area, such as low beneficial crop production and environmental sustainability. To overcome these problems, improving agricultural education is required to foster skillful agricultural professionals, and the quantification of agronomic information is an essential issue.

The production of pearl millet (Pennisetum glaucum (L.) R.Br.) is important in Namibia, in sub-Saharan Africa, owing to the prevailing low precipitation conditions. Most fields supporting crop production in northern Namibia are located in a network of seasonal wetlands. The aim of the present study was to evaluate the effects of ridging and fertilizer application on the yield and the growth of pearl millet in the seasonal wetlands under different rainfall conditions. The study was conducted for two years (2017–2018) in the experimental fields in northern Namibia, and yield, yield components, and growth parameters were evaluated in relation to the application of different fertilizers (manure and mineral) with and without ridge-furrows. Manure fertilizer application presented the highest yield in 2018, whereas mineral fertilizer application showed the highest yield in 2017. The proportion of rainfall was the highest during the mid-growth period in 2017, and the reproductive stage in 2018. Thus, pearl millet plants under manure fertilization overcame damage resulting from waterlogging stress during the seed setting stage by improving the soil and plant nutrient conditions. In contrast, the plants under mineral fertilization were more tolerant to large amounts of rain during the mid-growth period. In this study, yield was mainly determined by total dry weight, and it was closely related to panicle density in both years. Therefore, we concluded that fertilizer application, including additional fertilizer based on the growth diagnostic, could be important for improving crop production in seasonal wetlands.

イネにおいて葉群動態は乾物生産性を決定する重要な形質の一つであるにもかかわらず,その遺伝子型間変異については評価に乏しい.本研究では世界のイネ・コアコレクションおよび多収性品種群を対象に,計測が簡易なプラントキャノピーアナライザーを利用して,生育初期における葉面積指数(LAI)の相対成長速度(α),生育中期における受光率の最大増加速度(γ/4),被覆到達日数(DAT_0.8)および吸光係数(K)を求めた.さらに破壊計測による出穂期LAIを加えて,計5指標を用いて葉群動態に関する遺伝子型間変異を評価した.2007年から2009年に京都大学農学部付属農場で行った実験により,5指標にはそれぞれ大きな遺伝子型間変異が存在し,αやγ,DAT_0.8に有意な品種群間差が検出された.IRRIで育成された多収性品種は,γが大きく,葉面積展開に関してある程度の改良がおこなわれていると考えられた.一方,同じ多収性品種でもタカナリやMilyang23はそのような特徴が見られず,葉群展開に関して更なる改良が可能であると考えられた.

研究代表者は現在までに作物の生育動態に関する定量的評価手法の開発を行ってきたが，この手法では群落閉鎖後の生育状態に関する評価は難しく，また，広域評価も困難であった．そこで本研究では，生育期間を通して作物の生育情報を収集するための手法を開発し，さらに手法の広域展開可能性を検討することを目的として，以下の解析・実験を行った． ①生育期間を通した生育情報収集技術の開発：広範な栄養環境下で多様なイネ6品種を用いて行った3か年の栽培試験結果から，現在までに開発してきた生育動態を評価するパラメータのうち被覆率増加に関するパラメータがバイオマス生産をよく反映していることがわかった．さらに，これらのパラメータに加えて，群落閉鎖後の葉身窒素含量のような品種固有の生理パラメータを利用することで，収量の推定精度が改善されることを示唆した． ②ドローン計測による被覆率計測手法の検討：収量変動の大きい圃場でダイズの栽培試験を行い，異なる距離からのドローン撮影による被覆率計測を試みた．近距離（約7m）からのドローン計測と比較して，遠距離（約20m）からの計測による収量推定精度が高かった．遠距離からの撮影は雑草等の誤差要因を小さくすることが明らかとなり，より簡易的に広域のデータ収集が可能であることを示した． ③機械学習を利用したササゲ収量推定精度の検証：西アフリカでササゲ20品種を用いた栽培試験を行った。通常マメ科は水平葉であるため，被覆率を生産性評価に利用することは難しいが，本研究対象地域のような低生産性地域では被覆率が重要なパラメータであることがわかった．簡易的な3時期の非破壊的な被覆率計測によって，対象地域のササゲ収量を高精度で推定できることが示された．ここでは，4手法の機械学習を試し，手法による推定精度の違いを検出することができた．

新型コロナウイルス（Covid-19）禍により渡航が制限されたため、オンラインによりナミビア、ボツワナ、日本の3ヵ国での試験や研究成果を詳細に打ち合わせ、以下の研究と、研究成果の共同執筆・国際会議での報告などを実施した。① 接触混植試験：ナミビア北部では、ソルガムとトウジンビエの接触混植が農家の自助努力などにより徐々に広がりを見せている。そこで、ナミビア大学オゴンゴ校内の附属農場施設内で、トウジンビエとソルガムの接触混植試験を実施した。農家圃場においても栽培を希望する農家に種子を提供し、農家圃場における栽培試験を実施した。② 亀裂施肥試験：本試験のためにナミビア国で作成した新型の亀裂施肥機をベースにして日本ではコストを考慮し、亀裂処理に特化した新型アタッチメントを作成した。その走行試験を神戸大学等で実施し、今後の普及を検討中である。 ③ ボツワナ大学では、ナミビアから導入したイネ品種群の栽培試験を2019/2020作付け期に開始し、2021/2022まで3か年に渡る栽培トライアルを継続中である。2021/2022からは小規模フィ―ルドでの作付けを開始し、種子の更新と栽培試験を拡大した。 ④ ナミビアとボツワナの研究サイト周辺の農家圃場では季節湿地が形成されるためメタン発生の増大が懸念される。そこで地球温暖化の進行を阻止するためウルトラファインバブル水灌漑によるメタン削減の可能性を議論し、現地への導入を目指したフィールド試験を日本で開始した。 ⑤ 研究成果5報の原著論文がアクセプトされた。 ⑥ 国際学会（アジア作物学会議）で４報の口頭発表と、国内学会（日本作物学会）で２報の口頭発表を実施した。