Rhizosphere development under alternate wetting and drying in puddled paddy rice

Alternate wetting and drying (AWD) irrigation can save large amounts of water in rice cultivation. By repeatedly wetting and drying the soil under AWD, accentuated pore structure of the rhizosphere compared to flooded rice may occur. This could affect root growth and resource capture, but to date the physical structure and behaviour of the rhizosphere of rice under AWD has not been explored. In a controlled glasshouse experiment, two different textured soils were used in split rhizotrunks to separate a root-zone from bulk soil using mesh. To mimic a paddy field, the top of the rhizotrunk was filled with puddled soil and below the puddled layer there was a sieved soil layer. Root-zone physical properties were measured using a combination of high resolution X-ray CT imaging (pore structure), a miniaturised infiltrometer (hydrological) and a small indenter (mechanical). Soil under AWD irrigation had 46% greater macroporosity and 20% more pore connectivity compared to continuous flooding (CF). Compared to the bulk soil, root-zone soil under AWD or CF had greater macroporosity, water sorptivity and mechanical hardness. In the root-zone, AWD compared to CF increased the rate of water absorption by around 36%, but did not affect mechanical hardness. Our results suggest AWD interacting with rice roots could promote more effective water transmission through a more stable, larger and better-connected pore system. The results of this study also suggest that soil physical changes by AWD could improve the utilization of resources in a rice production system.