|Hoeft, Robert - UNIVERSITY OF ILLINOIS|
|Mulvaney, Richard - UNIVERSITY OF ILLINOIS|
Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 15, 2004
Publication Date: June 17, 2005
Citation: Stevens, W.B., Hoeft, R.G., Mulvaney, R.L. 2005. Fate of nitrogen-15 in a long-term nitrogen rate study: II. nitrogen uptake efficiency. Agronomy Journal. Vol. 97(4):1046-1053. Interpretive Summary: Monoculture hybrid corn yield response to N fertilization varied greatly based on weather conditions during the growing season. The long-term average agronomic optimum N rate was 181 kg N ha-1. Corn plants recovered from 28% to 40% of the fertilizer N, depending on weather conditions and harvested grain yield. During the second growing season after application of 15N-labeled fertilizer, plant recoveries of labeled N ranged from 6 to 9% of the fertilizer N originally applied and were most extensive with the two highest N rates used. These data suggest that mineralization of residual N is increased when fertilizer N has been applied previously at a high rate, and that a series of such applications may lead to a substantial decrease in crop response to subsequent N fertilization, while promoting N losses. Our study provides further evidence of the need to avoid excessive N fertilization.
Technical Abstract: Increased fertilizer N-uptake efficiency (FNUE) will allow producers to grow corn (Zea mays L.) more economically while reducing the potential for groundwater contamination by NO3. Previous research has suggested that application of N fertilizer at excessive rates can affect subsequent crop response, and thus may have a significant effect on NO3 loss due to leaching. Our research objective was to determine the effect of prior fertilizer N application rate on (i) grain yield and agronomic optimum N rate, (ii) contributions of fertilizer- and soil-derived N to plant N uptake through two growing seasons, and (iii) FNUE. Labeled 15NH415NO3 was applied at rates of 0, 67, 134, 201, and 268 kg N ha-1 to subplots of the N treatment plots within a long-term N rate study under continuous corn production. Estimates of FNUE were higher by the difference method (49-69%) than when calculated from 15N recovery data (31-37%), and different trends were observed in comparing N application rates, depending on which method was employed. The disparity between methods is consistent with a differential effect of these rates on mineralization-immobilization. Over the 3-yr study, FNUE15N averaged 34% of the applied N, and increased from 28 to 39% as growing conditions improved. Nitrogen application rate history did not affect FNUE15N, but FNUEdiff decreased from 69 to 49% as the annual N application rate increased from 67 to 268 kg N ha-1. Recovery of labeled N from the entire 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. As expected, 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, which was attributed to mineralization of crop residues and recently formed soil organic N.