Denitrification losses from 15 N‐labelled calcium nitrate fertilizer in a clay soil in the field

SUMMARY Calcium nitrate fertilizer containing 92.3 atoms % excess nitrogen‐15 was applied on 5 May 1981 at a rate equivalent to 100 kg N ha −1 to a clay soil in southern England cropped to winter wheat. Samples of the soil gases were collected frequently during the following 3 weeks. The soil oxygen concentration declined to 5% after 60 mm rain. A maximum of 1.5 ± 0.5 atom % N‐15 enrichment in labelled N 2 gas ( 29 N 2 ) was detected in the soil atmosphere on 28 May. 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 greater than 30 kg N ha −1 , 93% in the form N 2 , but the estimation depends on several assumptions about the amount of double labelled gas ( 30 N 2 ), rates of gas diffusion and flux.