Residual Effects of N Fertilization on Dryland Spring Wheat in the Northern Plains. II. Fate of Fertilizer N1

Abstract

AbstractResidual effects of fertilizer application on the fate of fertilizer N applied to crops in semiarid regions are largely unknown. Objectives of this study were to provide information on the recovery of fertilizer N during the fertilization and residual periods and to develop an N balance sheet for a continuous spring wheat (Triticum aestivum L.) cropping system.Ammonium nitrate was applied to spring wheat at 0 to 540 kg N/ha rate either (i) all in 1 year, (ii) one‐third in each of 3 years, or (iii) one‐sixth in each of 6 years to evaluate the availability and fate of fertilizer N in a dryland cropping system. Only grain was removed during harvest, and straw was plowed under each spring. Grain N uptake accounted for 31 to 46% of the total fertilizer N applied at 270 and 540 kg N/ha rate for various timing frequency. Fertilizer N uptake in straw and root biomass at the end of the experiment averaged 5 and 26 kg/ha, respectively. Movement of NO3‐N below the 90‐ cm depth (leaching) was detected for the 270 kg N/ha treatment, whether this total amount was applied all in 1 year, or 90 kg N/ha was applied in each for 3 years. However, with an annual application of 45 kg N/ha for 6 continuous years very little leaching of NO3‐N occurred. The amount of fertilizer N that moved beyond the root zone was greater at 540 than at 270 kg N/ha. Leaching of fertilizer N below 90 cm accounted for 4 to 37% of the total fertilizer applied, with greatest leaching occurring For the 1‐year treatment and least for the 6‐year treatment. Loss of N by runoff was negligible. The percent fertilizer N accounted for [(total fertilizer accountable ÷ total fertilizer applied) × 100] averaged 62, 64, and 66 for 1, 3, and 6‐year treatments, respectively. Unaccounted for fertilizer N possibly consisted of that portion of fertilizer N immobilized in soil organic fractions resistant to decomposition and that lost by volatilization. Results showed that if the N rate is adequate for maximum crop production, fertilizer efficiency is greatest when N is applied annually rather than at higher rates less frequently.