Rice Genomics and Genetics 2025, Vol.16, No.5, 267-281 http://cropscipublisher.com/index.php/rgg 271 3.2 Soil health improvement and nutrient recycling Rice-duck co-cultivation has a significant optimization effect on the soil environment and nutrient cycle of rice fields, which is also one of the key mechanisms for improving its ecological service function. On the one hand, the activities of ducks improve the physical and chemical properties of rice field soil. Ducks swimming and stirring in the field are equivalent to a continuous shallow tillage, making the soil surface more loose and breathable. This is conducive to the rooting and breathing of rice roots, and also increases the activity of soil microorganisms. Lan et al. (2021) found that compared with rice fields without ducks, the soil bulk density of the 0 cm-10 cm soil layer in rice-duck symbiotic fields was reduced, the porosity was increased, and the soil ventilation was improved. At the same time, ducks trample on the field to suspend some of the mud, which helps release the nutrients fixed in the soil for absorption by rice roots. On the other hand, duck feces and leftover feed provide a continuous supply of organic nutrients to the rice fields. Duck feces are rich in elements such as nitrogen and phosphorus. After decomposing in the water environment of the rice field, they directly become a source of nutrition for rice, improving soil fertility. Yan et al. (2023) conducted an experiment in the Chaohu Basin of Anhui Province and showed that in the rice-duck-crayfish tri-aquatic co-cultivation system, soil fertility indicators such as total nitrogen and available phosphorus were significantly higher than those in the monoculture rice field, and the activities of soil urease and phosphatase increased by more than 20%. This shows that rice-duck co-cultivation strengthens the recycling and regeneration capacity of soil nutrients. The presence of ducks can also affect the water and fertilizer environment dynamics of rice field soil. Xu et al. (2024) pointed out that after organic fertilizers were used to partially replace chemical fertilizers in the Erhai Basin and combined with rice-duck co-cultivation, the nitrogen and phosphorus concentrations in the surface water of the rice fields were significantly reduced. Duck feces are more easily fixed in the soil or absorbed by rice after being diluted by water and mixed with mud and water, reducing nutrient loss and non-point source pollution. At the same time, the stirring of ducks increases the dissolved oxygen in the field water and inhibits the activity of anaerobic microorganisms, thereby reducing the production and emission of methane in rice fields to a certain extent. Wang (2018)'s research shows that the rice field farming model can reduce the greenhouse gas emission intensity per unit area by changing the redox environment. In general, rice-duck co-cultivation improves soil structure and fertility through "duck-assisted farming" and "duck fertilization": it not only builds a benign soil-plant-animal nutrient cycle, but also reduces fertilizer dependence and nutrient loss, and improves the soil quality and functional services of the rice field ecosystem. 3.3 Indirect effects on rice grain quality and nutritional value In addition to the impact on the environment and yield, the rice-duck co-cultivation model also indirectly improves the quality and nutritional value of rice through various channels. First, the use of rice-duck symbiotic farming can usually produce rice that meets pollution-free or organic standards. Since no or less chemical pesticides and fertilizers are applied during the co-cultivation process, the harmful substance residues in rice are greatly reduced, and food safety is higher. At the same time, sufficient organic nutrient supply and a healthy ecological environment contribute to the improvement of rice quality (Wang et al., 2024). Che et al. (2021) compared the quality indicators of rice under different integrated farming modes in rice fields. The results showed that the rice polishing rate and transparency of rice produced by rice-duck co-breeding were similar to those of conventional farming, while the chalkiness was significantly reduced by 8%–11%, the amylose content increased by about 10%, and the protein content decreased by 6%–10%. Reduced chalkiness and increased amylose usually mean that the rice has a crystal-clear appearance and better taste, which is consistent with consumers' demand for high-quality rice. A moderate reduction in protein content may also make the rice softer and more delicious. Although the rice yield under the integrated farming mode in the above study decreased slightly, the yield decrease in the rice-duck mode was the smallest, only about 96% of the control. The improvement in rice quality allows rice-duck rice to be sold at a higher price in the market, thus compensating for or even exceeding the loss of income caused by the slight reduction in yield. Taking Xingqing Family Farm in Deqing County as an example, the "duck rice" produced by rice-duck co-breeding won the Zhejiang Good Rice Gold Award for its fragrant and sticky taste and safe quality, and the market price is about 15% higher than ordinary rice. Secondly, rice-duck farming also provides consumers with an additional source of high-quality animal protein.
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