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Immobilization, Nitrification, and Losses of Fall-applied, Labeled Ammonium-Nitrogen during Growth of Winter Wheat¹

A field experiment was conducted to follow immobilization, nitrification, movement, and loss of fall-applied N during growth of winter wheat (Triticum aestivum L.). On 4 Oct. 1979, small plots on a Pachic Argiustoll soil received 80 kg N/ha broadcast as (NH₄)₂SO₄ containing 5.97 atom % ¹⁵N. Plants and soil samples (to a depth of 150 cm) were removed from four randonly selected plots at each of four samplings. Leaching to depths below 150 cm was calculated from N contents of soil water and drainage values derived from an evapotranspiration equation.

Of the applied N, 26% was immobilized by 6 Dec. 1979 and 40% by 21 May 1980. Amounts of inorganic N from fertilizer declined steadily as a result of immobilization, plant uptake, and losses. At maturity, 81.9% of the fertilizer N remaining in the 150-cm profile was immobilized. The surface 10-cm layer contained more than 70% of the fertilizer N in the soil profile at each sampling time. Amounts of fertilizer N as NO^–₃ were greatest 6 December and were always less than amounts as NH⁺₄ –N.

Concentrations of fertilizer N in inorganic forms decreased with time in the 0- to 10-cm and 10- to 20-cm layers. Between 20 and 110 cm, concentrations peaked in April, whereas amounts below 110 cm peaked in May. Unusually high precipitation for this location in late March resulted in the leaching of 6.5% of the fertilizer N below 150 cm. Losses of 10 to 18%, apparently from denitrification, occurred soon after the fertilizer was applied. The crop used 41.6% of the applied N. At maturity, more than half of the applied N remained in the soil and crop residue. Because that was largely in organic forms, only slight additional loss would he expected as the N was slowly mineralized.

Key Words: Leaching • Denitrification • N balance

² Professor of agronomy.

Received for publication April 5, 1982.