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.

Struvite (MgNH₄PO₄·6H₂O) is commonly found in the recovered products after removal of nitrogen (N) and phosphorus (P) from wastewater by magnesium modified biochar, which can be used as a slow-release fertilizer. This study aims to investigate the effects of the struvite-based magnesium modified biochar (MAP-BC) combined with different fertilization rates on N and P reduction, rice yield and quality under different irrigation modes. MAP-BC was also applied in farmland under the different modes of irrigation and fertilization. Dongyan 18 (japonica rice) was used as the test material in the field experiment. Two irrigation modes were set, including the conventional flooding (I_CF) and alternate wet and dry (I_AWD) irrigation. Five types of fertilizer application were the conventional fertilization (N₁B₀), conventional fertilization + 5 t/hm² MAP-BC (N₁B₁), conventional fertilization + 10 t/hm² MAP-BC (N₁B₂), 25% reduction in N and P fertilizer + 5 t/hm² MAP-BC (N_3/4B₁), and 25% reduction in N and P fertilizer + 10 t/hm² MAP-BC (N_3/4B₂). Furthermore, MAP-BC was applied with a struvite purity of 82%. Excellent slow-release properties, N and P nutrient loads were obtained to solidify the subsequent field applications. Both MAP-BC and basal fertilizer were thoroughly mixed to simultaneously apply into the topsoil. Moreover, the MAP-BC was only applied in 2021 without 2022, in order to determine the positive effect of MAP-BC in the next growing season. The results showed that: Compared with I_CF, I_AWD significantly increased the chlorophyll content at milk-ripe stage, whereas, there was the significant decrease in the ineffective tiller number in 2021 (P<0.05). Compared with I_CF, I_AWD was significantly reduced the number of ineffective tillers by 17.39% in 2021 (P<0.05). MAP-BC was avoided the adverse effect of fertilizer reduction on chlorophyll content of rice, indicating the super compensation. Compared with N₁B₀, N_3/4B₁ and N_3/4B₂ treatments significantly increased the leaf chlorophyll content at tillering, jointing-booting, heading and flowering, and milk-ripe stage by 3.00%-6.83%, 2.99%-6.92%, 3.66%-5.69%, and 3.42%-9.02%, respectively (P<0.05). The high-purity struvite components in the MAP-BC were ensured the long-term supply of N and P nutrients to plants through the slow release. Compared with N₁B₀, N_3/4B₂ was fully met the demand for N and P nutrients at the later stage of rice growth. There was the significant increase in the N and P uptake at panicle by 4.77%-7.06% and 4.26%-12.69%, respectively (P<0.05). Compared with N₁B₀, the application of 10 t/hm² MAP-BC significantly increased the highest tiller number and the final tiller number at two years by 6.75%-9.64% and 13.16%-16.88%, respectively (P<0.05). In the two-year experiment, compared with N₁B₀ in I_AWD mode, the yield in the N₁B₁ and N₁B₂ treatments significantly increased by 7.66%-8.43% and 11.49%-12.64%, respectively (P<0.05). Meanwhile, 10 t/hm² MAP-BC was compensated for the adverse effects of 25% reduction in N and P fertilizer on yield. In I_AWD, N_3/4B₁ and N_3/4B₂ were significantly reduced the setback, chalky rice rate and chalkiness, whereas, there was the significant increase in the break down value (P<0.05). As such, the appearance and eating quality of rice were then improved significantly (P<0.05). In addition, N₁B₀ and N₁B₂ significantly increased(P<0.05) the protein content of rice by 2.66% and 5.79% compared with N₁B₀ treatment, indicating that the application of MAP-BC could improve the nutritional quality of rice. Therefore, 25% reduction of N and P fertilizer combined with 10 t/hm² MAP-BC in I_AWD was achieved in the fertilization reduction, yield increase and quality improvement under water-saving conditions. Thus, the finding can provide a theoretical basis for the green and efficient rice production. Subsequent studies should be considered on the utilization of actual eutrophic water in the MAP-BC preparation, in order to enhance the recycling efficiency of agricultural straw and eutrophic water.