In order to investigate the high efficient utilization model of N in leaf vegetables substrate
cultivation at northern China in winter，this paper took pakchoi as material and total NO₃^--N as the contrast；
conducted pot experiment and designed 25%，40%，60% of 3 N increasing level treatments；observed the
growth and activity of root system and N metabolism key enzyme activity during the growth process of pakchoi；and
measured its nutrient absorption，yield and quality．The results showed that the treatment of increasing 25%-40%
NH₄⁺-N could increase the root length，root surface area and root volume，promote root system development；
enhance its absorbtion of N and P．Compared with total NO₃^--N treatment（CK）this treatment of increasing
NH₄⁺-N ratio could significantly decrease the nitrate content，increase the content of soluble sugar，and yet had
no significant effect on the single plant mass．The nitrogen partial factor productivity of increasing 40% NH₄⁺-N
treatment was respectively 3.7% and 3.8% higher than that of increasing 25%，60% NH₄⁺-N treatments，and
the above-ground dry matter of the pakchoi was significantly increased either．To sum up，in northern China
for protected vegetable cultivation of pakchoi in winter，the suitable N application（17.33 mmol·L^-1），plus
increasing 40% NH₄⁺-N can achieve the best effect．