Irrigation with oxygen-nanobubble water can reduce methane emission and arsenic dissolution in a flooded rice paddy

A remarkable feature ofnanobubbles (<10^-6 mindiameter) istheir long lifetimeinwater. Supplying oxygen-nanobubbles (NBs) tocontinuouslyflooded paddy soil may retard the development of reductive conditions, thereby reducing the emission of methane (CH₄), apotent greenhouse gas, and dissolution of arsenic, an environmental load. We tested this hypothesis by performing a pot experiment and measuring redox-related variables. The NBs were introduced into control water (with properties similar to those of river water) using a commercially available generator. Rice (Oryza sativa L.) growth did not differ between plants irrigated with NB water and those irrigated with control water, butNBwater significantly (p < 0.05) reduced cumulative CH₄ emission during the ricegrowing seasonby21%. The amountsofiron, manganese, and arsenic that leached into the drainage water before full rice heading were also reduced by the NB water. Regardless of the water type, weeklymeasured CH₄ flux was linearly correlated with the leached iron concentration during the ricegrowing season(r= 0.74, p< 0.001).Atthe end of the experiment, the NBwater significantly lowered the soil pH in the 0-5 cm layer, probably because of the raised redox potential. The population of methanogenic Archaea (mcrA copy number) in the 0-5 cmlayer was significantly increasedbythe NB water, butwe foundno correlation between the mcrA copy number and the cumulative CH₄ emission (r = -0.08, p = 0.85). In pots without rice plants, soil reduction was not enhanced, regardless of the water type. The results indicate that NB water reduced CH₄ emission and arsenic dissolution through an oxidative shiftofthe redox conditions intheflooded soil.We propose the use of NB wateras a tool for controlling redox conditions in flooded paddy soils.