Fate of Nitrogen‐15 Enriched Ammonium Nitrate Applied to Corn

Nitrogen utilization by corn ( Zea mays L.) is influenced by the form of inorganic N present in the root zone. A field experiment was conducted on Enon sandy loam (fine, mixed, thermic Ultic Hapludalf) to determine N use efficiency and its partitioning in various plant parts of corn. Ammonium nitrate labeled either as 15 NH 4 ‐N or 15 NO 3 ‐N and applied at 50, 100, or 200 kg N ha −1 was evaluated. Microplots (1.06 m 2 ) were established in the main N plots for 15 N fertilizer application, and main plots were used to determine grain yield. After the crop harvest, soil N alone, fertilizer N used by the crop, and fertilizer N remaining in inorganic and organic forms in the top 75 cm of the soil were measured. Grain yield response to N application was significant ( P < 0.01). Recovery of applied fertilizer N in corn and weeds ranged from 43 to 57% and 3 to 5%, respectively. Nitrogen sources showed no significant differences with respect to N accumulation in corn. Only 17 to 20% of the fertilizer N was recovered in the grain. The amounts of soil N utilized by the corn was about three‐to sixfold higher than the fertilizer N, suggesting extensive turnover of soil and fertilizer N through immobilization and mineralization. More N was recovered (21–63% of added N) in the soil from 15 NH 4 than from 15 NO 3 (6–38%). Loss of N (unaccounted for) ranged from 11 to 18% at 100 kg N ha −1 and from 34 to 48% at 200 kg N ha −1 . Nitrogen loss was higher in the plots receiving 15 NO 3 than 15 NH 4 . Most of the fertilizer N remaining in the soil at the end of the growing season was in the organic fraction, suggesting immobilization into microbial and root biomass.