DENITRIFICATION LOSSES FROM N-15-LABELLED CALCIUM NITRATE FERTILIZER IN A CLAY SOIL IN THE FIELD

Calcium nitrate fertilizer containing 92.3 atoms percentage excess 15N was applied on May 5, 1981 at a rate equivalent to 100 kg N ha-1 to a clay soil in southern England [UK] cropped to winter wheat. Samples of the soil gases were collected frequently during the following 3 wk. The soil O2 concentration declined to 5% after 60 mm rain. A maximum of 1.5 .+-. 0.5 atom percentage 15N enrichment in labeled N2 gas (29N2) was detected in the soil atmosphere on May 28. Total denitrification losses, calculated from air-filled pore space and rates of gas loss from the soil estimated using a Fick''s law approximation, were 9.5 kg N ha-1 with a daily rate of 0.30 .+-. 0.07 kg N ha-1. Estimated total losses were > 30 kg N ha-1, 93% in the form N2, but the estimation depends on several assumptions about the amount of double labeled gas (30N2), rates of gas diffusion and flux.