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

Ultra-fine straw can present strong hydrophilic properties for the nutrient supply, due to the less than micrometer size. This study aims to explore the effects of ultra-fine straw return to the field on soil carbon and nitrogen retention, water productivity, yield, and quality. Its adaptability was also evaluated across different irrigation regimes. A lysimeter experiment was employed with Dongyan No. 18 (japonica rice) as the test material. A split-plot experiment was also designed. Two irrigation modes included conventional flooding (I_CF) and alternate wetting and drying (I_AWD). The subplots were 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 raw material in this experiment was taken as rice straw that was preserved by natural air drying at the irrigation experimental station from the previous season. The ultra-fine straw was prepared as follows. The original straw was firstly crushed and then sieved using a pulverizer and sieve with a mesh size of 125 μm. The obtained straw was then added into a 500 mL beaker filled with pure water and stirred for 1 min. Subsequently, the rice straw was broken down by ultrasonic cavitation (power 1800 W, time 3 h). Finally, a series of measurements were carried out to accurately quantify the physicochemical properties of the straw. The storage properties were then facilitated to prevent natural decay. The straw after ultrasonic cavitation was freeze-dried using a freeze dryer. As such, the ultra-fine straw was obtained after drying. The results indicated that the straw shared the more significant water-saving property, as the straw size decreased. Among them, the Sμm treatment reduced the water consumption by an average of 13.63%, 19.42%, and 8.87% over two years, respectively, compared with the S₀, S_cm, and S_mm treatments. There was a decrease in the content of lignin and cellulose hydrophobic substances after straw crushing. While the increase was found in the content of hydrophilic groups, such as hydroxyl groups. The water-holding capacity enhanced the efficiency of irrigation water use, indicating the more pronounced performance under water-saving irrigation patterns. Furthermore, the water productivity in the Sμm treatment increased by an average of 32.91%, 37.25%, and 9.67%, respectively, over two years, compared with the S0, Scm, and Smm treatments. The contents of lignin, cellulose, and hemicellulose in the straw decreased by 26.91%, 63.48%, and 16.28%, respectively, after straw ultra-fine crushing, compared with S_cm treatment. The carbon-to-nitrogen ratio (C/N) decreased after straw return. The decomposition of the S_μm promoted the growth and development of rice, thereby increasing the rice yield. The I_AWDS_mm, I_AWDS_μm, I_CFS_mm, and I_CFS_μm treatments all significantly increased the rice yield. Moreover, the I_AWD treatment improved the taste value of rice by 8.16%, compared with the I_CF treatment; The Sμm treatment exhibited significantly higher taste values, which were 13.03% and 8.23% higher than the S0 treatment, respectively; The protein content in rice from the Sμm treatment was significantly higher by 5.17% and 9.84%, respectively, compared with the S0 treatment. The entropy weight TOPSIS results indicate that the IAWDSμm treatment significantly increased the yield and water productivity, as well as the rice quality. Therefore, the IAWDSμm treatment was the most ideal straw and irrigation management model. The findings can provide a meaningful scientific basis for sustainable agriculture.