Ultrasonic-assisted separation and extraction of submicron and nano-sized straw particles: Reconstruction of physicochemical characteristics and a promising breakthrough in overcoming technical bottlenecks of straw return to the field.

Addressing the issues of slow decomposition and low nutrient release efficiency associated with traditional straw returning, this study innovatively applied ultrasound-assisted centrifugal separation technology to prepare submicron/nano-straw particles and systematically conducted a multi-scale investigation from microscopic to macroscopic levels. The core finding reveals that when the particle size reaches the 1 μm threshold, ultrasonic cavitation vigorously disrupts the straw structure, leading to efficient lignin removal (77.45 %) and a significant reduction in cellulose crystallinity, thereby fundamentally enhancing the degradation rate. Concurrently, the cavitation effect optimizes elemental ratios (e.g., C, N, and K elemental proportions increasing by 1.05 to 8.50 times) and exposes active functional groups such as C-N and N-H bonds, effectively overcoming the bottleneck in nutrient release. Furthermore, cavitation increases the abundance of hydrophilic groups on the straw surface, enhancing its water-holding capacity by 13.84-18.52 %. Soil columns experiment and pot trials confirmed that the nano-straw prepared by this technology substantially reduces nutrient loss, significantly increases soil available potassium content, ultimately synergistically increasing rice yield by 25.27 %. In summary, by simultaneously optimizing the straw's degradability, fast-acting nutrient release capacity, and water retention, ultrasonic technology solves the core challenges of traditional straw returning and provides a novel strategy for developing new fast-acting straw fertilizers.