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Overwinter Transformations of Nitrate Derived from Soil and ¹⁵N-Labeled Potassium Nitrate

Department of Soil Science, Univ. of Alberta, Edmonton, AB, Canada, T6G 2E1

*Corresponding author.

ABSTRACT

Transformations of NO₃-N from fall through spring were examined in a Typic Cryoboralf and a Typic Argialboll in northcentral Alberta. Nitrogen-15 labeled KNO₃ (96% atom abundance) was applied at the rate of 2.4 kg N ha^–1 to fallow and stubble plots in October and incorporated to a depth of 0.13 m. Isotopic analyses of NH₄-N, NO₃-N, and total-N were performed on soil samples taken to a depth 0.60 m in November, January, March, and May. Analysis of November samples indicated apparent denitrification, immobilization, and release of ¹⁵N as NH₄-N occurred in both soils. Losses of NO₃-N occurred simultaneously with additions between November and January in 3 of 4 fallow-stubble/soil combinations. Migration of soil water towards ice lenses in the surface 0.30 m suggested upward movement of NO₃-N took place as the soil froze. Between January and March, NO₃-N levels increased an average of 6.4 mg N kg^–1 in the Cryoboralf while recovery of ¹⁵N was unchanged. In the Argialboll, NO₃-N levels remained constant from January to March. Lower recovery of ¹⁵N under fallow indicated that the NO₃-N pool remained active and losses were balanced by gains. In may recovery as NO₃-N accounted for only 1 to 24% of labeled N added in October. Immobilization accounted for 4 to 30%. Trace amounts of ¹⁵N were detectable in the 0.13 to 0.30-m depth, but not in the 0.30 to 0.60-m depth. Overwinter loss of labeled N from 0 to 0.60 m ranged from 55 to 86%. Processes affecting NO₃-N remained active between October and May, including periods during which soils were frozen throughout the 0 to 0.60-m depth.

Contribution from the Dep. of Soil Science, Univ. of Alberta, Edmonton, AB, Canada, T6G 2E1.

Received for publication December 15, 1986.