Fate of Nitrogen‐15 in a Long‐Term Nitrogen Rate Study: II. Nitrogen Uptake Efficiency

Increased fertilizer N uptake efficiency (FNUE) leads to more economical corn ( Zea mays L.) production and lower environmental impact. Excessive N application reduces FNUE and may affect subsequent crop response through its influence on NO 3 –N carryover and the amount of readily mineralizable organic N in the soil. Our objective was to determine how prior fertilizer N application rate affects (i) grain yield and agronomic optimum N rate, (ii) contributions of fertilizer‐ and soil‐derived N to N uptake, and (iii) FNUE. Labeled 15 NH 4 15 NO 3 was applied at 0, 67, 134, 201, or 268 kg N ha −1 to subplots within a continuous corn long‐term N rate study. Estimates of FNUE were higher by the difference method (49–69%) than with the isotope ( 15 N) method (31–37%), and different trends were observed with each method as N application rate increased. The disparity between methods is consistent with a differential effect of long‐term N application rate on mineralization–immobilization. Recovery of labeled N from the plant–soil system ranged from 71% at the 67 kg ha −1 N application rate to 64% at the 201 kg ha −1 application rate. Fertilizer N accounted for an increasing proportion of crop N uptake as the N rate was increased, but soil N uptake was always more extensive, accounting for 54 to 83% of total plant N. Crop uptake of labeled N during the second growing season after 15 N application ranged from 2.2 kg ha −1 with the lowest N rate to 7.8 kg ha −1 with the two highest rates.