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 NO3-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 soilderived N to N uptake, and (iii) FNUE. Labeled 15NH415NO3 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 (15N) 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 15N application ranged from 2.2 kg ha-1 with the lowest N rate to 7.8 kg ha-1 with the two highest rates.