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

Abstract

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.