Abstract: We investigated the effects and mechanisms of nitrogen additions (0, 1, 2, 4, 8, 16, 24, 32 g N·m^-2·a^-1) on contents of anion and cation in rhizosphere soil, bulk soil, and mixed rhizosphere and bulk soil in the heavily salinized grassland in the agro-pastoral ecotone of North China. The results showed that pH of rhizosphere, mixed and bulk soils decreased significantly with the increases of nitrogen addition levels. Moreover, pH of three soil types under the 32 g N·m^-2·a^-1 treatment decreased by 1.2, 0.9, and 0.6, respectively, while pH of rhizosphere soil decreased by 0.44 compared with the bulk soil. Na⁺ content of rhizosphere, mixed and bulk soils significantly decreased, while the NO₃^- content significantly increased. The proportion of Na⁺ content in total soluble salt content in rhizosphere soil decreased by 14% and that in bulk soil decreased by 12% after the 32 g N·m^-2·a^-1 addition. NO₃^- content increased by 29% in rhizosphere soil and by 26% in bulk soil. There was significant negative correlation between pH and NO₃^- content, and significant positive correlation between pH and Na⁺ content. The total soluble salt content of rhizosphere soil under the 32 g N·m^-2·a^-1 treatment was significantly reduced by 31.5%. Collectedly, nitrogen deposition could reduce soil pH and total soluble salt content of rhizosphere soil and alleviate saline-alkali stress.

Key words: nitrogen deposition, rhizosphere soil, alkaline soil, Leymus secalinus, base cation