Fate of Tagged Nitrogen Fertilizer Applied to Irrigated Corn

A field experiment was conducted for 2 years with ammonium sulfate tagged with 5.93 atom % ¹⁵N to determine the fate of N fertilizer applied to sprinkler-irrigated corn (Zea mays L.). All areas of triplicate, 356-cm square plots were treated with 50 or 150 kg tagged N/ha. N fertilizer used by the crop and that remaining in the top 240 cm of soil were measured. NH₄⁺-N and NO₃⁻-N in the 0- to 10-cm layers after the second harvest also were determined. Grain yields in 1976 did not differ significantly. In 1977 response to N was significant, but responses to two N rates did not differ significantly. Only about one-fourth of the N fertilizer at either rate was removed by the grain. Amounts in the soil were in proportion to amounts applied and about three times more after 2 years than 1 year, reflecting the N in crop residue from the first crop. The N balance indicated losses from the system of 17 to 18% of the N applied during the 2 years, after correcting for apparent errors. Fertilizer did not significantly alter amounts of soil N used by the crop but total N uptake increased with each fertilizer increment. There was no priming effect on mineralization of indigenous soil N. Comparng the ¹⁵N and difference methods for calculating fertilizer N uptake showed that the difference method varied more and showed significantly less uptake in one case. From 65 to 73% of the N fertilizer remaining in the soil was in the 0- to 10-cm layer. Depth of movement was related to amount applied, varying from 20 cm with 50 kg after 1 year to 240 cm with 150 kg applied each year for 2 years. Some of the N with the 150-kg rate may have leached below 240 cm after 2 years, but there appeared to be no leaching from the measured zone in other cases so losses from the system must have been by denitrification. Most of the N fertilizer in the 0- to 10 cm-layer after 2 years was immobilized with only 4.4 to 4.7% remaining in the NH₄⁺ and NO₃⁻ forms. However, there was about seven times more inorganic N in the soil originating from fertilizer than came from indigenous soil N, which confirms that residual fertilizer N is more likely than indigenous N to be present in inorganic forms. Amounts of fertilizer N found in grain and lost were proportional to amounts applied. The 150 kg N/ha rate gave no higher grain yields than 50 kg N/ha rate, but resulted in much greater crop removal, gaseous loss, and downward movement into the soil profile.