FATE OF N-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 (NH4-N)-N-15 or (NO3-N)-N-15 and applied at 50, 100, or 200 kg N ha-1 was evaluated. Microplots (1.06 m2) were established in the main N plots for N-15 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 (NH4)-N-15 than from (NO3)-N-15 (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 (NO3)-N-15 than (NH4)-N-15. 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.