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Ammonia Loss from Applications of Urea-Ammonium Nitrate Solution to Straw Residue¹

ABSTRACT

The increased use of urea and urea-based N fertilizers along with an increase in crops under no- or reduced-tillage practices may require changes in fertilizer management practices. Surface application of urea-based fertilizer risks the loss of some N by volatilization of NH₃. Our objectives were to assess the loss of NH₃ and to simultaneously observe the associated N transformations and movements in the field following broadcast application of urea-ammonium nitrate solution at a rate of 200 kg N ha^–1 to soil with wheat (Triticum aestivum L.) straw residue cover. Also, we wanted to characterize the factors associated with the residue that might affect loss. A micrometeorological mass-balance method was used to measure NH₃ loss during two sequential field studies with different amounts of precipitation and irrigation. Volatilization losses showed diurnal patterns coinciding with fluctuations in residue temperature, residue water content, and wind speed. Cumulative NH₃-N losses were 7.6 and 16.6% of the N applied, respectively. The rate of urea hydrolysis and NH₃ loss were quite sensitive to the availability of water. Simulated rainfalls with 2.5-mm sprinkler irrigation increased NH₃ loss. In both studies, peak volatilization rates came the 1st d after irrigation. The maximum measured rate of NH₃ loss was 51 µg m^–2 s^–1 (1.5 kg N ha^–1 h^–1). Rainfalls of near 30 mm in both studies moved much of the remaining fertilizer below the 40-mm soil depth. Wheat straw was found to have a urease activity of about 1830 mg urea kg^–1 h^–1, a pH near 8, and a H-ion buffering capacity of 53 mmol kg^–1 (pH unit)^–1. These factors undoubtedly contributed to NH₃ loss.

¹ Contribution 85-327-J, Dep. of Agronomy, Kansas Agricultural Experiment Station. Part of a dissertation submitted by the senior author in partial fulfillment of the requirements for the Ph.D. degree at Kansas State Univ. The research was supported in part by grants from USDA-ARS, and Farmland Industries, Inc.

² Graduate Research Assistant, Research Assistant, and Professors, Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506.

Received for publication July 15, 1985.

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