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Nitrogen Management

Fate of Nitrogen-15 in a Long-Term Nitrogen Rate Study

II. Nitrogen Uptake Efficiency

^a USDA-ARS, Northern Plains Agric. Res. Lab., 1500 N. Central Ave., Sidney, MT 59270
^b Dep. of Crop Sci., 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL 61801
^c Dep. of Nat. Resour. and Environ. Sci., 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL 61801

^* Corresponding author (bstevens{at}sidney.ars.usda.gov)

Received for publication December 15, 2003. 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 soil-derived 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.

Abbreviations: AE, agronomic nitrogen use efficiency • ANI, added nitrogen interaction • FNUE, fertilizer nitrogen uptake efficiency • FNUE_diff, fertilizer nitrogen uptake efficiency calculated using the difference method • FNUE_15N, fertilizer nitrogen uptake efficiency calculated using the ¹⁵N isotope method • N_Y, yield-based optimum N application rate • UMRB, Upper Mississippi River Basin • Y_MAX, predicted maximum yield • Y₀, grain yield produced on plots where no N was applied

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