Interactive Effects of Soil Texture and Salinity Levels on Nitrification-Related Nitrogen Transformations in Soils

Soil salinity substantially modulates soil nitrogen (N) transformation processes, yet how soil texture mediates the salinity-induced changes in soil nitrogen transformation remains poorly quantified. To evaluate the interactive effects of soil identity and salt stress on inorganic N concentrations, a 56-day column incubation experiment was conducted using three typical soil textures (loamy sand, silt loam, and clay loam) with a salinity gradient (0, 3, and 6 g kg−1). Uniform urea application (215 mg N kg−1 soil) was performed at 25 ± 1 °C and approximately 60% water-holding capacity, and dynamic variations in soil inorganic N fractions and urease activity were monitored throughout incubation. The results revealed distinct soil-specific inorganic N concentration patterns under salt perturbation. Loamy sand soil exhibited prominent NO3--N accumulation, while clay loam soil showed stronger NH4+-N retention and more significant transient NO2--N accumulation. In the non-saline control treatment, the final concentration of NO3--N reached 202.14 mg kg−1 in loamy sand, which was 48.4% higher than that in clay loam soil. Increasing soil salinity generally prolonged NH4+-N retention and transient NO2--N accumulation while limiting NO3--N formation, with the magnitude of such salt-induced changes highly dependent on soil texture and incubation stage. Specifically, soil salinity at 3 and 6 g kg−1 reduced the final nitrate content of clay loam soil by 14.3% and 15.4%, respectively. Urease activity consistently ranked as clay loam > silt loam > loamy sand across treatments, and this enzymatic difference was decoupled from soil nitrate accumulation capacity. RDA further indicated date-dependent associations of EC with NH4+-N and NO2--N retention, while pH was more closely related to NO3--N accumulation. The results indicate that soil texture is associated with heterogeneous inorganic N responses to salt stress, although initial chemical differences preclude attribution to texture alone. These findings describe measured inorganic N pools and urease activity rather than directly measured nitrification rates or microbial mechanisms. Validation with plants, mixed salts, and field conditions is required before management recommendations can be established.