Effects of ultra-fine rice straw return to the field on water productivity, rice yield, and quality in paddy fields with alternating dry and wet conditions

Ultra-fine straw has dimensions generally in the micrometer or lower range and possesses strong hydrophilic and nutrient supply capabilities. This study aims to explore the effects of ultra-fine straw on soil carbon and nitrogen retention, water productivity, yield, and quality, evaluating its adaptability across different irrigation regimes. This study employed lysimeter experiments with Dongyan No. 18 (japonica rice) as the test material, designing a split-plot experiment. The main plots included two irrigation modes: conventional flooding (I_CF) and alternate wetting and drying (I_AWD). The subplots involved: no straw treatment (S₀), traditional straw return of 3-5 cm (S_cm), millimeter-scale straw return (S_mm), and ultra-fine straw return (S_μm), with a uniform straw return rate of 6.5 t/hm². The materials used in this experiment were rice straw preserved by natural air drying at the irrigation experimental station from the previous season. For the preparation of ultra-fine straw, the straw was first crushed using a pulverizer and passed through a sieve with a mesh size of 125 μm. The sieved straw was then added to a 500 mL beaker filled with pure water and stirred for 1 minute. Subsequently, the rice straw was broken down by ultrasonic cavitation (power 1800 W, time 3 hours). Finally, to accurately quantify the physicochemical properties of the straw and to facilitate storage as well as prevent natural decay, the straw after ultrasonic cavitation was freeze-dried using a freeze dryer to obtain the ultra-fine straw. The results indicated that as straw size decreased, the straw demonstrated a more significant water-saving effect. Compared to S₀, S_cm, and S_mm treatments, the S_μm treatment reduced water consumption by an average of 13.63%, 19.42%, and 8.87% over two years, respectively. This is because after straw crushing, the content of lignin and other hydrophobic substances such as cellulose is reduced, while the content of hydrophilic groups such as hydroxyl groups increases, leading to improved water-holding capacity and enhanced efficiency of irrigation water use, with this effect being more pronounced under water-saving irrigation patterns. Compared to S₀, S_cm, and S_mm treatments, the water productivity in the S_μm treatment increased by an average of 32.91%, 37.25%, and 9.67% respectively over two years. After straw ultra-fine crushing, the content of lignin, cellulose, and hemicellulose in the straw decrease by 26.91%, 63.48%, and 16.28%, respectively compared to S_cm, and the carbon-to-nitrogen ratio (C/N) decreased after straw return, which is beneficial for the decomposition of the S_μm and subsequently affects the growth and development of rice, thereby increasing yield. The I_AWDS_mm, I_AWDS_μm, I_CFS_mm, and I_CFS_μm treatments all significantly increased rice yield. Moreover, compared to the I_CF treatment, the I_AWD treatment improved the taste value of rice by 8.16%; the S_μm treatment exhibited significantly higher taste values, being 13.03% and 8.23% higher than the S₀ treatment respectively; the protein content in rice from the S_μm treatment was significantly higher by 5.17% and 9.84% compared to the S₀ treatment. The final result using the entropy weight TOPSIS method indicates that I_AWDS_μm treatment is the most ideal straw and irrigation management model. Therefore, the I_AWDS_μm treatment significantly increases yield and water productivity, enhances rice quality, and provides a meaningful scientific basis for achieving sustainable agricultural production.