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Recovery of Fertilizer Nitrogen by Wheat as Affected by Fallow Method¹

ABSTRACT

The fate of fertilizer N applied to crops is of both environmental and economic concern. To follow fertilizer N recovery in western Nebraska, depleted-¹⁵N NH₄NO₃ was surface broadcast in April on winter wheat (Triticum aestivum L.) growing on plots fallowed the previous year by plowing, subtillage, and no-till. Quantities of labeled N taken up by the growing crop, in the upper 100 mm of soil as inorganic N, and in both visible and partially decomposed crop residues, were followed on two sets of plots through the crop-fallow-crop sequence (approximately 117 weeks). About 16 to 18% of the labeled N was removed in the grain of the first wheat crop, with an additional 3 to 5% in the second crop. Labeled inorganic soil N decreased to <5% within 1 yr, with little effect of fallow method. No more than 4% was in visible residues at any time for all except no-till fallow, for which 7 to 13% of the labeled N was found in visible residues from heading of the first crop through the entire fallow period until the second crop was seeded. Likewise, 4 to 9% of the labeled N applied was found in partially decomposed residues of no-till during this period, compared to no more than 2% for the plow and subtill treatments. Total N in both residue pools combined decreased 50% from October to April of the fallow year for N-fertilized wheat and 25% for unfertilized wheat. Most of the labeled N in the straw found in the visible residue pool by October was transferred over winter to either the partially decomposed pool or to other undetermined pools (including soil organic matter, microbial biomass, and losses to the atmosphere). Results show that, compared to plow and subtill, no-till fallow enhanced retention of labeled N in the several crop residue pools and increased N uptake by the two wheat crops.

¹ Contribution from USDA-ARS, in cooperation with the Nebr. Agric. Res. Div. Published as Paper no. 7937, Journal Series, Nebr. Agric. Res. Div.

² Soil Scientist and Research Leader, Plant Physiologist, and Soil Scientist, USDA-ARS, Univ. of Nebraska-Lincoln, Lincoln, NE 68583-0915.

Received for publication October 28, 1985.