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DENITRIFICATION FROM NITROGEN FERTILIZER PLACED AT VARIOUS DEPTHS IN THE SOIL‐PLANT SYSTEM

Denitrification was measured from 30 cm reconstituted profiles of the Urrbrae fine sandy loam in sealed growth chambers. The fertilizer, as NO3−-N, was banded at 5, 15, and 25 cm depths while profiles were taken from two areas, one with a long history of cropping and the other with a long history of pasture. The reconstituted profiles were compared to chambers containing only the surface soil, with the same treatments of fertilizer applications and plant growth. Fertilizer placement did not affect the volatilization losses (3–5 μg N g−1 soil week−1) from the crop soil. However, plant growth of wheat (Triticum aestivum) in this soil doubled these losses, irrespective of the placement of the fertilizer-N and the soil layers in the pot. Three to eight μg N g−1 soil week−1 were immobilized in the control treatments of this soil. The plant competed successfully for the inorganic-N and reduced the immobilization of nitrogen. By contrast, plant growth generally had little effect on denitrification from the pasture soil. The high level of organic carbon in the surface soil from the pasture area favored the reduction of NO3−-N to the extent of 7 μg N g−1 soil week−1 as N2 as, when the surface soil was placed at the bottom of the pot in the region of restricted oxygen flux. In the reconstituted profile, denitrification was decreased as the fertilizer-N was placed deeper in the pot. Plants accumulated one-third to one-half of the applied-N and a net mineralization was found in most treatments.