PARTITIONING LOSSES OF NITROGEN-FERTILIZER BETWEEN LEACHING AND DENITRIFICATION

When N-15 is used to trace the fate of N fertilizer applied in spring to winter wheat crops, some is not recovered in the crop or the soil and has to be presumed lost. In 13 experiments made from 1980 to 1983 on three widely differing soils, these losses ranged from 1 to 35%. We partitioned them between leaching and denitrification by using models to estimate the loss by leaching, taking into account the N absorbed by the crops, and subtracting this loss from the total loss to obtain the apparent percentage loss by denitrification, L(DN). An analysis of variance showed that L(DN) increased significantly with the quantity of N applied, so the study considered L(DN) values for a standard N application of 150 kg/ha subsequently. Regressions showed that L(DN) was better related to the wetness of the soil during the 3 weeks after fertilizer application than to the corresponding amount of rain, as would be expected for denitrification. Values of L(DN) could not, however, be satisfactorily related to soil temperature, probably because the range of temperatures was too narrow. The apparent losses by denitrification were, on average, nearly twice as large as those by leaching, but the ratio varied greatly between experiments.