Rice Genomics and Genetics 2025, Vol.16, No.3, 116-131 http://cropscipublisher.com/index.php/rgg 121 downward trend. The reason is that organic nitrogen such as shrimp manure is adsorbed by soil and fixed by microorganisms, avoiding excessive nitrogen volatilization. At the same time, the activity of crayfish improved soil aeration conditions, increased nitrogen absorption by rice roots, and the total nitrogen accumulation of the whole plant increased by 10.2% compared with the control. This shows that returning animal manure to the field not only directly increases the soil nitrogen pool, but also promotes the effective use of nitrogen by rice by changing the physical and chemical environment of the soil. The global meta-analysis results of Chen et al. (2024) also support this view: the nitrogen fertilizer partial productivity (i.e., the yield per unit fertilizer) of the integrated farming system is significantly higher than that of the monoculture rice field, indicating that the same input of nitrogen is more fully converted into output under the integrated farming conditions. In the rice-fish symbiotic system, in addition to part of the nitrogen in the feed being used for fish growth, most of the rest enters the soil-water environment through fish metabolism. How to rationally utilize feed nitrogen and reduce losses has become one of the research focuses. The current experience is to achieve efficient recycling of feed nitrogen and reduce environmental pressure by optimizing the feeding amount and matching aquatic plants (such as planting floating aquatic vegetables to absorb surplus nitrogen). Overall, returning animal manure to the field changes the rice field from an open nutrient system to a partially closed cycle, and the nitrogen reuse rate is significantly improved. It is estimated that a well-functioning rice-fish symbiosis system can basically recycle the nitrogen converted from crop straw, feed, plankton, etc. into rice and fish output, and the nitrogen utilization efficiency can be increased by more than 10%. This is an important mechanism for rice field integrated farming to achieve high efficiency of weight loss. However, it is also necessary to prevent problems such as excessive soil nutrients and methane fermentation caused by excessive accumulation of manure. Therefore, in actual production, the nitrogen fertilizer application structure should be appropriately adjusted according to the number of animals and the amount of manure to ensure the dynamic balance of nutrient input and output. 4.2 Nutrient fluxes driven by water and bioturbation The disturbance caused by the activities of animals in rice field water and soil under the integrated farming model is one of the important mechanisms affecting the dynamics of soil nutrients. The trampling and paddling of ducks in the field, and the foraging and burrowing of fish and shrimp in the mud all affect the material exchange at the soil-water interface. First, animal disturbance can enhance the release of nutrients in the soil. The stirring of ducks causes the nutrients deposited on the mud surface to suspend in the water, which is absorbed and utilized by the rice roots or evenly distributed through the water layer. Benthic animals such as shrimps and crabs dig holes in the mud and release nutrients such as phosphorus and silicon in the soil into the overlying water. It has been observed that the concentration of dissolved phosphorus in the root zone of rice under crab farming conditions is higher than that in the control field, which is speculated to be related to the destruction of soil phosphorus fixation by crab digging holes (Xu et al., 2025). Secondly, animal activities can improve soil ventilation and redox conditions. Long-term flooding of rice fields can easily form an anaerobic environment with strong reduction, while the swimming of ducks in the fields and the water circulation of fish and shrimp increase the chance of oxygen diffusion into the soil. The thickening of the oxidation layer is conducive to the nitrification of ammonium nitrogen and the oxidation of ferrous iron, reducing the accumulation of toxic reducing substances, thereby protecting the root system and facilitating nutrient absorption. In addition, water disturbance can also crush large soil particles, making soil particles more finely dispersed, thereby increasing the contact surface between soil and water and accelerating the diffusion and release of nutrients. This is similar to the "micro-tillage effect" of soil, which helps to balance the nutrient distribution in various parts of the field. Thirdly, the presence of aquatic animals changes the nutrient dynamics of rice field water. Fish and snails feed on plankton, which controls the number of algae and prevents too many nutrients from being fixed by algae, thus leaving more water nutrients for rice to use. This has a stabilizing effect on seasonal changes in nitrogen and phosphorus concentrations. It is worth noting that while animal disturbance enhances
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