Rice Genomics and Genetics 2025, Vol.16, No.3, 116-131 http://cropscipublisher.com/index.php/rgg 120 and excess nutrients should be consumed through crop rotation or planting green manure. Therefore, under the conditions of integrated farming, soil N, P, and K show the characteristics of "overall increase and dynamic balance", and it is necessary to maintain a benign cycle of nutrients between soil-rice-animals through optimized management. 3.3 Enhancement of micronutrients and soil biotic activity In addition to the main macroelements, integrated farming in rice fields can also affect the supply of trace elements in the soil and the biological activity of the soil. Rice growth requires trace nutrients such as zinc, boron, and molybdenum. Integrated farming may improve the effectiveness of trace elements by increasing organic matter and changing the physical and chemical environment of the soil. Studies have shown that rice-fish farming can increase the effective zinc content in the soil, which is attributed to the fact that fish stirring increases the release of zinc in the soil, and the chelation of organic matter keeps the zinc available. In the rice-duck model, since duck manure contains a certain amount of trace elements (such as copper and zinc), continuous return to the field can also slightly increase the accumulation of these elements in the soil. More importantly, integrated farming improves nutrient cycling by promoting soil biological activity. Soil biological activity can be measured by indicators such as microbial quantity, diversity, and soil enzyme activity. A large number of studies have confirmed that integrated rice farming can significantly increase the microbial biomass and enzyme activity in rice fields. Yan et al. (2023) found that the soil urease activity in this model increased by 17% to 72% compared with monoculture, and the acid phosphatase activity increased by 23% to 66%. These enzymes are involved in the transformation of soil nitrogen and phosphorus, respectively, and their enhanced activity means an improvement in the nutrient mineralization supply capacity. Similarly, Sun et al. (2025) reported that the total soil microbial index (PLFA content) increased by about 18% when the rice-fish farming system was operated for 5 years compared with conventional rice fields, and increased by nearly 49% when it was operated for 30 years. The number of various groups such as bacteria, fungi, and actinomycetes in the soil increased significantly. The growth and diversification of microbial communities make the soil nutrient cycle more active and efficient. In addition, rice-duck farming can also enrich soil macrobial communities, such as the number of earthworms. Li et al. (2025) observed that the number of earthworms per square meter in rice-fish farming fields was significantly higher than that in control fields, which helped to improve soil structure and nutrient status. In general, integrated farming in rice fields stimulates the biological activity of the soil through organic matter input and ecological disturbance, forming a pattern of "animal-microorganism-soil" linkage and synergy. This enhancement of biological activity not only promotes the release and circulation of trace elements (for example, microorganisms secrete organic acids to dissolve insoluble mineral nutrients), but also improves the efficiency of soil utilization of external inputs, thereby consolidating the foundation of soil fertility. 4 Mechanisms of Soil Nutrient Regulation 4.1 Nutrient recycling via animal excreta In the integrated rice field farming system, animal manure is one of the key links in the soil nutrient cycle. Farmed animals obtain nutrients from the rice field ecosystem and convert a considerable proportion of their intake into manure and discharge it back into the field, realizing the nutrient feedback of "raising to promote rice". This is particularly evident in the nitrogen cycle. Taking rice-duck farming as an example, each duck excretes dozens of grams of nitrogen-containing manure every day, which is equivalent to continuous "topdressing" for the soil. Studies have shown that the increase in total nitrogen and available nitrogen in rice-duck fields comes from the contribution of duck manure. Since duck manure is organic nitrogen, its decomposition and release are relatively slow, and it can continuously supply nitrogen during the rice growth period, reducing the loss caused by excessive one-time application of chemical fertilizer nitrogen. The experiment of Xiao et al. (2024) further revealed that the rice-red claw crayfish farming system did not significantly increase ammonia volatilization losses under high nitrogen input conditions, and there was a
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