| Of applied N fertilizer and mineralized organic N in calcareous soil in the agro-ecosystem of semi-arid areas, apart from plant uptake, microbial-fixed, clay mineral-fixed, volatilization loss and denitrification loss, significant amount of these N remains in the soil as NO-3-N. Residual NO-3-N in soil profile moves to depth by convection and diffusion, leaving the root zone step by step. The main factors that affect the accumulation and leaching of NO-3includes: N input, plant take up, plant residual after pro-harvest, N fertilizer kinds, N fertilizer applying manner and rainfall et al. Residual NO-3-N itself has high bioavailability. Whether it can be taken up or not depends on the depth of NO-3-N and the distance to the root zone. During the heavy rainfall season in semiarid areas, NO-3-N moves to depth easily; but in dry condition, even NO-3-N in deep soil can move up with water to the root zone and be taken up. As a result, residual NO-3-N accumulated in soil profile impacts N fertilizer use efficiency remarkably. In many regions, residual NO-3-N at certain depth is used as the index of N supplied N to soil to reduce the NO-3-N leaching and improve N fertilizer use efficiency. According to past researches of NO-3-N in soil profile in semiarid areas, three aspects are concerned: first, accumulation of residual NO-3-N in soil profile and the factors that affects it; second, residual NO-3-N that coincides with root depth contributes to N fertilizer use efficiency; third, the determination of rational N input according to residual NO-3-N in soil profile. On the basis of the analysis above, a few scientific problems that should be solved in future research are emphasized. |