It is difficult to illustrate the nitrogen-balanced mechanism of farmlands on regional level through pot-cultivation and
field trials.Based on the analysis of output and input factors,6 inflows and 5 outflows were chosen to establish the nitrogen-balanced model of farmlands on
county level.The surplus and deficit of nitrogen in
Gansu province in 2000 were calculated and the attributes of its spatial distribution were explored.The results indicated:(1) The nitrogen input and output of farmlands in Gansu province in 2000 were 48.52×10~4 t and 59.87×10~4 t respectively,so the deficit of nitrogen in farmlands was 11×10~4 t and the deficit ratio was 23.4%,showing that the excessive depletion of nitrogen was evident.N surplus was widespread in Qilian mountain,Hexi corridor and its northern regions,and N deficit was prevalent in the east and middle regions of Gansu.(2) In the structure of N input in Gansu province in 2000,chemical fertilizer took the highest proportion, and next was organic fertilizer.The ratio of chemical fertilizer,organic fertilizer and other nitrogen sources was 65∶30∶5.(3) In the structure of N output,the proportion of soil erosion was the largest,and the biomass output took the second place.The effective output rate of biomass N in farmlands was 34.15%.