The nutrient utilization rate of salinized farmland soils is low, the nitrogen loss is high, and soil salinity inhibits the hydrolysis of urea and the release of nutrients. In this work, the effect of microbial fertilizer on the nitrogen transformation characteristics and nitrogen morphology of salinized soils was studied using indoor constant temperature incubation tests with different nitrogen application rates—without (A0) and with microbial fertilizer application (A1 (15 t/ha)) or nitrogen application (N) of 0 kg/ha (N0), 97.5 kg/ha (N1), or 195 kg/ha (N2). The results show the following: (i) When no microbial fertilizer was applied, an increased nitrogen application promoted nitrogen fertilizer’s ammonification and nitrification reactions. Furthermore, the maximum net nitrification rate with the high nitrogen fertilizer application decreased; the apparent ammonification rate and net ammonification rate A0N2 increased by 26.1% and 24.6%, respectively, compared with A0N1 on the first day of incubation; the maximum net nitrification rate of A0N1 was more than that of A0N2; and A0N1 > A0N2 on day 3, while A0N2 > A0N1 on days 3 to 15. At 3 d, the nitrogen conversion process of A0N1 was dominated by the nitrification reaction, while the ammonification reaction dominated in A0N2. (ii) Microbial fertilizers significantly increased the ammonification and nitrification rates under the low N fertilizer application. The intensity of ammonification and nitrification under the low N fertilizer application was greater than that under the high N fertilizer application. The apparent ammonification rate and net ammonification rate of A1N1 increased by 60.9% and 52.6% compared with A0N1 and 21.9% and 21.7% compared with A1N2 on the first day of incubation, and the peak net nitrification rates of A1N1 and A1N2 (28.19 mg/kg d and 11.02 mg/kg d, respectively) and net nitrification rates of A1N1 and A1N2 were 113.7% higher than those of A0N1. The net nitrification rates of A1N1 and A1N2 were 82.3% and 58.6% lower than the maximum net nitrification rates on the 15th day of incubation, respectively. (iii) In saline soils, low-nitrogen microbial fertilizers led to more ammonium nitrogen in the soil, and the high-nitrogen fertilizer application resulted in higher nitrate nitrogen in the soil, leading to nitrogen leaching. Therefore, when applying microbial fertilizer, choosing the most suitable period for reduced chasing is important for the efficient use of fertilizers, the alternative role of biofertilizers, and the study of environmental pollution.

In order to discuss the problems of the transformation of soil nitrogen and nitrogen leaching in saline farmland, this study carried out a split-plot experiment with returning straw and various nitrogen application rates. The main treatment of the experiment was returning corn straw, at quantities of 0.64 g (C1) and 0 g (C0), and the secondary treatment was nitrogen fertilizer (urea) at the quantities of 0 g (N0), 0.015 g (N1) and 0.03 g (N2). The results showed that, firstly, with the extension of the incubation time, the nitrogen nitrification rate of saline soil in each treatment decreased gradually until it stabilized without straw. For Days 0–7 of incubation, the nitrogen mineralization rate of saline soil decreased rapidly, and the mineralization rates of C0N0, C0N1 and C0N2 decreased by 86.91%, 89.26% and 83.64%, respectively. The nitrification rate of nitrogen in saline soil was C0N0 > C0N1 > C0N2, which decreased by 68.01%, 67.42% and 60.52%, respectively. Secondly, under the condition of returning straw to the field, the nitrogen mineralization rate of saline soil in each treatment decreased gradually and became stable with the extension of the incubation time. The nitrogen mineralization rate of saline soil in each nitrogen application treatment was C1N2 > C1N1 > C1N0 within 0–3 days of incubation, which decreased by 87.46%, 87.20% and 81.83%, respectively. The nitrification rate of saline soil under different nitrogen treatments was C1N2 > C1N0 > C1N1, and the nitrification rates of C1N0, C1N1 and C1N2 decreased by 66.62%, 62.54% and 47.21%, respectively. Thirdly, during the incubation period, returning straw slowed down nitrogen mineralization and nitrification in saline soil under the reduced nitrogen and no-nitrogen fertilizer treatments, but it enhanced nitrogen mineralization and nitrification in saline soil under the high nitrogen treatment. In conclusion, returning straw and reducing the application of nitrogen fertilizer to saline soil can retain more ammonium nitrogen, thus inhibiting nitrification of the soil nitrogen, reducing the environmental pollution risk of nitrate leaching and reducing nitrogen losses, all of which are of great significance for environmental pollution.