Morio Iijima

Ultrafine bubbles (UFBs) are defined as gas bubbles with diameters smaller than 1 μm (ISO 20480–1:2017). When generated in water, UFBs exhibit physical properties that differ markedly from those of...

Irrigation with water containing ultrafine bubbles (UFB) promotes crop growth under environmental stress conditions. This study evaluated the changes in soil physical properties when using low-cost UFB for paddy field irrigation. An experimental paddy field was divided into eight sections, with regular agricultural (control) and UFB irrigation, and rice cultivation was conducted over three seasons. The findings indicate that the sand content, linked to the fragmentation of gravel, increased 2.6% due to UFB irrigation over two years. A one-week stirring experiment was conducted in the laboratory to observe the accelerated fragmentation of the soil particles. A UFB containing distilled water significantly decreased the sand content by 3.5–7.1% and increased the clay content by 5.3–5.6% compared to the control distilled water. The thickness of the viscous colloidal layer on the soil surface increased by a maximum of 29 mm, and the penetrometer resistance decreased with UFB irrigation under submerged paddy conditions. These results indicated that continuous irrigation with low-cost UFB water from air and agricultural water promotes soil particle fragmentation and reduces soil penetrometer resistance. This study provides valuable insights into the potential benefits and limitations of UFB irrigation in sustainable agricultural practices.

In semiarid regions, soil salinity, like drought, restricts crop productivity, causing food shortages among most inhabitants—the smallholder subsistence farmers. Seasonal wetlands formed in these regions during the rainy seasons could be utilised for rice (Oryza spp.) cultivation to increase food security, but rice is sensitive to salinity. Field and greenhouse-pot experiments were performed at the University of Namibia-Ogongo Campus, North-Central Namibia, to evaluate rice genotypes’ responses to seasonal wetland salinity. The field experiments assessed 16 rice genotypes for growth during the dry season and grain production during the rainy season, in saline and non-saline (control) seasonal wetlands. The saline-wetland salinity was predominantly NaCl; electrical conductivity levels increased from 2.8 dS m⁻¹ (rainy season) to 34.3 dS m⁻¹ (dry season), resulting in a 0–14% dry-season plant survival rate. The rainy-season wetland salinity decreased paddy yields in all rice genotypes; however, Pokkali produced the highest paddy and relative yields. The pot experiment assessed CG14 (salt-sensitive) and Pokkali (salt-tolerant) genotypes for growth using soils collected bi-monthly from the saline wetland. The rainy-season soil salinity reduced shoot growth in CG14 but did not affect growth in Pokkali, while the dry-season salinity killed both genotypes. The results of this study suggest the possibility of cultivating the salt-tolerant rice genotype, Pokkali, in the saline wetland during the rainy season due to salt dilution; however, the dry-season salinity levels would be detrimental to rice. More salinity-tolerance screening studies are warranted, in order to increase rice production and food security in Namibia and other flood-prone semiarid regions worldwide.

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.

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.

Coffea arabica, an economically important crop, accounts for most of the coffee consumed globally. Increasing temperature due to climate change can cause a decrease in productivity in many crops, including coffee plants. The maximum temperature at which damage is induced has been reported for many crops, but it remains unclear in coffee plants. Here, we investigated the effect of different temperatures and the physiological damage induced by heat stress using both leaf disks and intact plants of Coffea arabica ‘Typica’. In the experiment using intact plants, we observed leaf damage by a decrease in soil plant analysis development value, and an increase in electrolyte leakage after exposure to 45°C for 96 h, whereas no leaf damage was observed for 72 h. The leaf surface temperatures after exposure to 45°C for 72 and 96 h were 44.0 and 46.3°C, respectively. Thus, a tolerance threshold in leaves of C. arabica ‘Typica’ under heat stress are likely between 44.0 and 46.3°C. The activities of catalase (CAT) and superoxide dismutase (SOD) decreased at 45°C in both leaf disks and intact plants. The decrease in the activities of SOD and CAT under heat stress may be responsible for the increased levels of reactive oxygen species, such as O₂⁻ and H₂O₂, and the resulting cellular damage. Our findings provide valuable insights into the physiological responses of Coffea arabica ‘Typica’ to heat stress, which may contribute to the breeding and screening of tolerant cultivars in the future. Graphical abstract

Close mixed-planting is a new concept of mixed cropping that allows greater complementary effects under various stress conditions; in this system, oxygen and/or water transfer occurs through the tightly entangled root system of the two species. We aimed to assess whether rice can alleviate flood stress in soybeans via close mixed-planting. A three-year field experiment and one water culture experiment were conducted to compare the crop performance between single and close mixed-planting under short-term flood (or anoxia) stresses. The survival rates, photosynthetic rates, and biomass production were higher in the mixed soybean than in the single soybean plants. Moreover, as the modified land equivalent ratio under flood stress was always >1, rice exhibited greater complementary effect on soybean against short-term flood stress in an upland field converted from paddy. Thus, rice planted as the partner crop of soybean can alleviate short-term anaerobic stress to enhance soybean growth. Graphical Abstract