Objectives To study microbial response to nitrogen fertilizer in saline soils with the goal of optimizing fertilizer application, improving environmental quality, and sustaining agricultural production.

Methods Field experiments were conducted in Baleng Demonstration Base located in Dengkou County, Inner Mongolia. The tested soil was silt loam and the tested crop was maize. The experiment had a split design with the soil salinity level as the main effect while the nitrogen fertilizer rate was the split effect. Two fields had mild (0.77–1.24 mS/cm) and moderate (1.24–1.77 mS/cm) degree of salinities, and four nitrogen levels of 0, 135, 270 and 405 kg/hm² coded as N0, N1, N2 and N3 were used. The soil salinity under different N levels was measured, and the population of soil microorganisms were investigated.

Results Soil salt concentration increased with the increase of N application rate under the same degree of salinization. Populations of bacteria, fungi and actinomycetes and overall microbial biomass carbon and nitrogen contents first increased with the increase of N application followed by decreasing. In N2 treatment (i.e., N 270 kg/hm²), microbial populations and biomass reached peaks. Microbial populations showed a significant non-linear relationship with nitrogen fertilizer rate and soil salt concentration. In the soil of mild salinity, both N fertilizer and increase of salinity led to increase of microbial population, where the opposite was observed for the soil with moderate salinity.

Conclusions The higher the degree of soil salinization, the lower the microbial populations. The microbial populations and biomass all reached the maximum with the N application rate of N 270 kg/hm². Soil microbial population had a significant binary quadratic nonlinear relationship with N fertilizer rate and soil salinity level. The increase of soil salinity and nitrogen fertilizer promoted the growth of soil microbes in mild saline soil, but the effect became negative in moderate saline soil.