The increasing deposition of nitrogen into the atmosphere has affected the stability and productivity of ecosystem. The effects of nitrogen deposition vary in regards to soil water content. There is uneven precipitation distribution and significant water-nitrogen coupling, which directly affects the growth and development of plants. In order to better understand the responses of plant roots to soil water-nitrogen coupling, we exposed seedlings of Betula albosinensis to five different water regiments of field water holding capacity (FC) (40%[W₁], 50%[W₂], 60%[W₃], 80%[W₄], and 100%[W₅]). As well as three different nitrogen (N) regiments (control, 0 [N₀], 20 [N₁], and 40 [N₂] gN m^-2a^-1 with NH₄NO₃). These experimental conditions allowed us to determine the root activity and physiology of the seedlings when exposed to various water and nitrogen supplies. The results indicated that a decrease of soil water content prompted the root vigor and root respiration rate to decrease while membrane lipid peroxide(MDA), osmotic adjustment substances (i.e. proline, soluble protein, and soluble sugar) content, and the activity of antioxidant enzymes (i.e. superoxide dismutase, peroxidase and catalase) all increased. Moreover, root physiological characteristics were significantly affected by the water-nitrogen coupling effect. When soil moisture was deficient (W₁ and W₂), the addition of nitrogen markedly reduced root activity. However, when adequate soil water (W₄ and W₅) was present, root activity was significantly enhanced. Additionally, the greater the concentration of nitrogen applied, the greater the effects were upon root activity and the contents of proline and soluble MDA. Only the lower concentration of nitrogen addition (N₁) significantly improved root physiological functions under 60%FC. Finally, both root vigor and nitrate reductase activity had a significant negative correlation with membrane lipid peroxide level (MDA content). In conclusion, atmospheric nitrogen deposition could significantly promote root physiological characteristics of B. albosinensis seedlings when soil moisture is plentiful. However, if soil water is deficient, the deposition of atmospheric nitrogen could cause serious damage to the cell membrane systems and consequently restrain the activity of the root system. Therefore, plant roots could defend themselves against these stresses by increasing the osmotic regulation substances content and antioxidant enzyme activity within a certain range.