EFFECT OF DATE OF APPLICATION ON THE FATE OF 15 N-LABELLED UREA AND POTASSIUM NITRATE

Abstract

Previous work in north-central Alberta showed large losses of fall-applied 15 N-labelled N fertilizers over the winter, but determination was not made for the summer season. The objective of the present study was to discover the amount of 15 N loss during both the non-cropped winter season and during the following cropped season. Field experiments were conducted at two sites with 15 N-labelled urea and potassium nitrate (KNO 3 ) applied in early October, late October, late winter and in the spring. The 15 N-labelled fertilizers at 50 kg N ha −1 were incorporated into the soil. Plots were sown to barley in spring and harvested when mature. Recovery of 15 N in soil samples taken before sowing in spring indicated over-winter N losses from October-applied N at both locations and especially with KNO 3 . At the Breton site spring recovery of 15 N in soil from the October application was 69% with urea and only 30% with KNO 3 . The mechanism of N loss was primarily denitrification. The amount of 15 N immobilized in the soil was greater with urea than KNO 3 for both sites. The total recovery of October- or late winter-applied 15 N fertilizer at harvest (plants plus soil) was low, with a range of 7–71%. The recovery from spring application was near-complete at the Innisfail site (≥ 84%) but at Breton, which had heavy rain and saturated soil in late June, recovery was low with urea (56%) and especially low with KNO 3 (10%). It was estimated that 8 of 45 site-years had sufficient precipitation during June to cause prolonged soil saturation and consequent N loss. In all, major losses of 15 N occurred in the non-cropped over-winter period at both sites, and occurred in the cropped season at one site. Key words: Denitrification, fall application of N, leaching, 15 N, 15 N balance, N fertilizer, N losses, winter application of N