| Summary: | Freeze–thaw cycles (FTCs) occur during the nongrowing season, and residual nitrogen (N) increases the risk of N loss with melting water. To study the effect of FTCs on soil N, rice fields in improved irrigated saline soil in northeast China were selected as the research subjects. Water content (10%, 20%, and 30%), different N fertilizer levels (180 and 220 kg/ha), and multiple FTCs of soil samples were used to clarify the effects of N fertilizer application and water content on N efficiency. The results indicate that, after the third FTC, the soil ammonium nitrogen (NH<sub>4</sub><sup>+</sup>-N) level increased significantly. NH<sub>4</sub><sup>+</sup>-N increased with an increase in the initial soil moisture content and decreased with fertilizer levels. Nitrate nitrogen (NO<sub>3</sub><sup>−</sup>-N) decreases with increasing initial soil moisture. The inorganic N increased significantly compared with that in the unfrozen stage, indicating that FTCs promote soil N mineralization. However, high fertilization rates inhibit mineralization. Analysis of variance showed that NO<sub>3</sub><sup>−</sup>-N is sensitive to the N application rate, water content, and salinity (<i>p</i> < 0.05). FTCs and artificial fertilization are the factors that affect N mineralization (<i>p</i> < 0.05). The research results are significant for preventing nitrate leaching and soil acidification during spring plowing and providing a scientific basis for fertilization systems and water environment pollution in improved saline soils.
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