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Transformation and Losses of Applied Nitrogen-15 Labeled Ammonium in a Flooded Organic Soil

Dep. of Soil Science, 439 Borlaug Hall, Univ. of Minnesota, St. Paul, MN 55108

* Corresponding author.

ABSTRACT

Ammonia volatilization, denitrification, and mineralization-immobilization turnover rates (MIT) in a flooded organic soil were estimated using ¹⁵N-labeled NH₄Cl fertilizer. The soil was placed in 15-cm columns either under dark or illuminated conditions. Volatilization of NH₃-¹⁵N was 8 and 4% of the initial fertilizer N added to the dark and illuminated columns, respectively. The rates of fertilizer N and total N denitrification in the dark columns were constant over the experiment at 1.2 and 2.1 mg N kg^–1 d^–1, respectively. After 74 d 22% of the initial amended fertilizer N had been denitrified. The rate of net mineralization of N in the anoxic zone was constant with time at 1.5 mg N kg^–1 d^–1. Mineralization and immobilization turnover fit the model y = y_max (1 – e^-kt), where y is the quantity of N mineralized or immobilized and k is the rate coefficient. For gross mineralization y_max = 480 mg N kg^–1 d^–1 and k = 0.00914 d^–1. For immobilization y_max = 160 mg N kg^–1 d^–1 and k = 0.025 d^–1. Mineralization and immobilization were not affected by the concentration of extractable NH₄-N. Algae increased immobilization in illuminated columns. No priming effect of added NH⁺₄ on N mineralization was found. The impact of MIT on calculation of fertilizer efficiency is illustrated.

Scientific Journal Series, Paper 15249, Minn. Agric. Exp. Stn.

Received for publication January 16, 1987.