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Transformations of Fall-Applied Nitrogen-15-Labelled Fertilizers¹

ABSTRACT

In conjunction with field studies on gaseous losses of nitrogen, the transformations of fall-applied ¹⁵N-labelled KNO₃ and urea (with and without nitropyrin) were studied in black and dark brown Chernozemic (Typic Udics and Typic Haploborolls) soils during 1980–81 and 1981–82. The amount of fall-applied N which could not be accounted for in the following spring ranged from 25 to 33% (2 sites) for KNO₃ and only 10 and 5% from urea without and with nitropyrin, respectively. These losses occurred during the fall (before the soil was frozen) and in the following spring (after soil thaw) and were primarily due to denitrification. Nitrification of NH⁺₄-N from applied urea occurred at a slow rate during the cool late fall until freeze-up and commenced again in early spring when the soil warmed up. Application of nitrapyrin in conjunction with urea delayed nitrification as well as downward movement of fertilizer N in the soil profile and was effective in decreasing the denitrification losses. Immobilization of fertilizer N was minimum under fallow conditions (7%) but ranged from 15 to 21% and 24 to 26% of the applied N as KNO₃ and urea, respectively, in wheat stubble fields. Nitrapyrin did not affect the immobilization of fertilizer N or the amount of fertilizer N found in the microbial-biomass.

¹ Contribution from the Saskatchewan Institute of Pedology, Saskatoon, Canada; Publication no. R364.

² Commonwealth Scholar and Professor, respectively, Dep. of Soil Science, University of Saskatchewan, Saskatoon, Canada S7N 0W0.

Received for publication November 14, 1983. Accepted for publication February 28, 1984.

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