| Summary: | Nitrogen is an indispensable and limiting element for plant and microbial growth. To investigate the combined effects of salinity and freezing–thawing (FT) processes on soil inorganic nitrogen (SIN) transformation in seasonally freezing salinized farmland, laboratory incubation experiments were conducted under five soil salt content (SSC) treatments (0.08%, 0.25%, 0.35%, 0.50%, and 0.70%), four FT temperature treatments (C (5 °C), FT (−5 + 5 °C), FT (−10 + 5 °C), and FT (−15 + 5 °C)), and two soil water content (SWC) treatments (40% and 80% of maximum water holding capacity (WHC)). Ammonium (NH<sub>4</sub><sup>+</sup>-N) and nitrate (NO<sub>3</sub><sup>−</sup>-N) nitrogen were monitored at the first, second, fifth, and eighth incubation days. The FT processes increased relative NH<sub>4</sub><sup>+</sup>-N content by 13%, 39%, and 77% with the decreasing of freezing temperature from −5 °C to −15 °C compared with C (5 °C) treatments, respectively. FT (−5 + 5 °C) and FT (−15 + 5 °C) treatments decreased the relative NO<sub>3</sub><sup>-</sup>-N contents by 4% and 6% compared with C (5 °C) treatments, respectively. Under FT treatments, the increment of relative NH<sub>4</sub><sup>+</sup>-N content was higher in low-SSC treatments and lower in high-SSC treatments. The relationship between relative NO<sub>3</sub><sup>–</sup>-N content and SSC gradually changed from a decrease in C (5 °C) to an increase in FT (−15+5 °C) treatments. SWC decreased NH<sub>4</sub><sup>+</sup>-N content in high-SSC and low-freezing temperature treatments (SSC × freezing temperature < −2.5%· °C), while NH<sub>4</sub><sup>+</sup>-N increased in low-SSC and unfrozen treatments. The variations of SIN/<i>R<sub>min</sub></i> (nitrogen mineralization rate) were mostly affected by NO<sub>3</sub><sup>–</sup>-N/<i>R<sub>nit</sub></i> (net nitrification rate) and NH<sub>4</sub><sup>+</sup>-N/Ra (net ammonification rate) in C (5 °C) and FT treatments, respectively. Overall, the results suggested that enhanced salinity inhibited the effects of freezing temperature on NH<sub>4</sub><sup>+</sup>-N and NO<sub>3</sub><sup>−</sup>-N formation, respectively. The increase in SWC weakened the NH<sub>4</sub><sup>+</sup>-N formation induced by the decrease in freezing temperature, and this function increased with the increase in salinity.
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