Deep sowing accelerates rice emergence under water deficit: field experiments and model development

Abstract Background and aims Direct-seeded rice on unpuddled soils often suffers from delayed emergence owing to soil water deficit. A potential solution is sowing deeply to utilize soil moisture farther below the surface. We aimed to explore the potential of the deep sowing technique under drought stress in rice. Methods Field experiments were conducted to examine the effect of sowing depth on emergence date under drought. We then developed and tested a process-based model to estimate the rice emergence date based on sowing depth, soil temperature, and soil moisture. We used the model to simulate the emergence response of rice sown at different depths under representative soil moisture profiles. Results In the field experiment, sowing at 4 cm or deeper advanced emergence more than sowing at 2 cm by more than 5 days. Our model calculated the cumulative thermal time, which we multiplied by soil water coefficients for the germination and shoot elongation stages. It reproduced emergence dates of moist and dry field experiments with a root-mean-square error of < 1 day. In the severe drought, moderate drought, and moist soil moisture profiles, sowing at 1 cm greatly delayed emergence under drought, whereas sowing at 4 cm consistently gave faster emergence than at other sowing depths. Conclusion Our results suggest that deep sowing will achieve more consistent emergence in drought-prone environments. Our model can be a valuable tool to predict the risk of delayed emergence under drought in direct-seeded rice and to suggest an optimal sowing depth to avoid the risk.