Determination of threshold soil salinity with consideration of salinity stress alleviation by applying nitrogen in the arid region

Soil salinization is a limiting factor for irrigated agriculture, and inappropriate irrigation and fertigation continues to worsen the problem, especially in arid regions. Determination of threshold soil salinity is of great environmental importance for salinity control in irrigation areas. A two consecutive year’s cotton field experiment was conducted to develop the dynamic threshold soil salinity (Sth) and quantitatively evaluate the effects of applying nitrogen on salinity stress alleviation. Four irrigation amounts (75%, 100%, 125%, and 150% of crop water requirement) and four nitrogen application rates (195, 255, 315, and 375 kg ha−1) were applied with groundwater and brackish water irrigation to produce the dynamic soil salinity (EC1:5) and mineral nitrogen (N) content pools with a wide range of values. Results showed that the EC1:5 and soil N pools were built-up in a range of 0.16−1.68 dS m−1 and 0.3–17.9 mg kg−1. The Sth for cotton at the seedling, squaring, flower boll, and mature stages were 0.69, 0.74, 1.02, and 1.02 dS m−1, respectively. Cotton crops presented the highest nitrogen uptake at optimal soil mineral nitrogen values of 12.7 and 16.2 mg kg−1 during the reproductive growth stage, and an appropriate nitrogen application rate increased the Sth by 19−126%. In addition, the linear equations of salt–nitrogen relation were determined and used to calculate the soil N under higher soil salinity (above the Sth) for maintaining cotton production. These findings produced a new perspective to determine threshold soil salinity and mitigate soil salt suppression by nitrogen management.