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Nitrogen-15 Recovery for Five Years after Application of Ammonium Nitrate to Crested Wheatgrass¹

ABSTRACT

Although perennial grasses usually respond to N fertilization if there is adequate water, fertilizer N recovery is usually relatively low. This research was conducted to understand better the fate of fertilizer N applied to grasslands. Twenty open-ended cylinders (102-mm diam by 637-mm length) were pressed into each of two main plots containing 4-year-old stands of crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult.]. One plot had received no fertilizer N, and the other plot had received 22.4 g N/m² each year since seeding. Enriched ¹⁵N was applied at the rate of 8.4 g N/m² to a different replicated set of cylinders each of the 5 years of this study. Remaining cylinders received 8.4 g nonenriched N/m² each year. Aboveground vegetation was harvested, and, after the fifth year, cylinders were removed from the soil. Roots were screened dry, and all plant material, roots, and soil were analyzed for total N and N-isotope ratio.

Average annual top growth varied fivefold as a result of highly variable precipitation and evaporation rate. Plant N concentration and uptake the first several seasons were about 25% greater from previously fertilized than from previously unfertilized soil. Cumulative recovery of ¹⁵N in tops increased with time, but considerable variation existed because of varied growing conditions between years. From 12 to 52% of the ¹⁵N applied was recovered in top growth in the season of application, representing about 75% of total isotope recovery in tops after four additional growing seasons. About 15% of the ¹⁵N was in roots after the first growing season, and about two-thirds of this N appeared in plant tops within 3 y. Soil ¹⁵N varied from 29 to 49% of that applied, present mostly as organic N after the first season. About 70 to 95% of the ¹⁵N applied was accounted for in tops, roots, and soil, with little apparent relationship between ¹⁵N accounted for and either dry-matter production or seasonal precipitation. Although uptake of soil N was initially 20 to 25% greater from previously fertilized than from unfertilized soil, this difference disappeared after the third year of fertilization, suggesting that at least 3 y of fertilization were required for the N cycle to readjust to a new steady state. Growing conditions the first few months after fertilizer application appear to have a major, but largely undefined, effect upon the fate of fertilizer N.

¹ Contribution from the Agricultural Research Service, USDA. Location of research was Mandan, N. Dak. and Beltsville, Md.

² Research Soil Scientist, USDA-ARS, Lincoln, Neb. (formerly Mandan, N. Dak.), and Professor, Agronomy Department, Univ. of Ark., Fayetteville (formerly Research Soil Scientist, USDA-ARS, Beltsville, Md.).

Received for publication June 27, 1983. Accepted for publication September 27, 1983.

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