In a microplot ¹⁵N-labelled fertilizer N experiment, where measures were taken to inhibit bio-immobilization of nitrogen, application of ¹⁵N-labelled KNO₃ to wheat prior to its shooting stage，and the microplot experiment being carried out in Treatment CK (a treatment of another field experiment that has never been fertilized) successfully exposed another process that affected uptake of soil N by crops, i.e. dilution of soil N pool or substitution of N in the pool by fertilizer N. The crops applied with N fertilizer absorbed less soil N than those applied with nothing, showing negative ANI. Biological N immobilization and dilution of (fertilizer N on) soil N pool are two opposite and intermingled actions. Counteraction and balance of the two actions might lead to positive ANI or negative ANI depending on which one is stronger. As is affected by the abovediscussed process, it is almost impossible to assess N fertilizer recovery rate with reliable result by means of field experiment using either ¹⁵N-labelling method or traditional difference method. As an alternative, the proportion method could help skid over the obstacle and has been verified in this experiment with better results.