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

Abstract

Previous work in north-central Alberta showed large losses of fall-applied 15N-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 15N loss during both the non-cropped winter season and during the following cropped season. Field experiments were conducted at two sites with 15N-labelled urea and potassium nitrate (KNO3) applied in early October, late October, late winter and in the spring. The 15N-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 15N in soil samples taken before sowing in spring indicated over-winter N losses from October-applied N at both locations and especially with KNO3. At the Breton site spring recovery of 15N in soil from the October application was 69% with urea and only 30% with KNO3. The mechanism of N loss was primarily denitrification. The amount of 15N immobilized in the soil was greater with urea than KNO3 for both sites. The total recovery of October- or late winter-applied 15N 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 KNO3 (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 15N 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, 15N, 15N balance, N fertilizer, N losses, winter application of N