Field Study of the Fate of Fall‐Applied 15N‐Labelled Fertilizers in Three Alberta Soils1

Abstract

AbstractFall application of N fertilizers is often less effective than spring application in increasing yields of cereal crops in portions of the Prairie Provinces of Canada. This paper reports on five field experiments (and involving three soils) using 15N‐labelled fertilizers to determine the amounts of mineral N lost over the winter by denitrification, leaching, and immobilization. The labelled N fertilizers were applied at 112 kg N ha−1in October in 1974–1975 experiments and in December in 1975–1976 experiments. The plots were sampled to a depth of 120 or 150 cm in late May. In the two 1974–1975 experiments (Malmo sicl soil), KNO3 and urea were mixed into the 0 to 12 cm depth of soil, and (NH4)2SO4 was placed in a band. Recovery of the labelled fertilizer N found in the total soil N was lowest for KNO3 (60 %), next for urea (71 %), the highest for (NH4)2SO4 (84%). The addition of 50 mm of water to the soil, which reduced the soil moisture tension of the 0 to 15 cm depth to less than 33 KPa on fall freezing, decreased the recoveries of KNO3 and urea in the total soil N to 26 and 54%, respectively. There was no leaching below the 60 cm depth. For the three fertilizers, from a 10th to a third of applied N was immobilized. In the three 1975–1976 experiments, the three soil types were, Demay 1, Malmo sicl, and Falun 1. The fertilizers were placed in bands 5 cm deep into the frozen soil. There was little loss of the labelled N from the total soil N for (NH4)2SO4, but very large losses for KNO3. As in the experiments from 1974–1975, the experiments for 1975–1976 showed no 15N below 60 cm depth for KNO3, and below 30 cm depth for (NH4)2SO4. The percent of applied N which became immobilized range from 7 to 49%, with the highest value for (NH4)3SO4 on a Malmo sicl soil. In summary, substantial losses of late fall‐applied l5N‐labelled fertilizers occurred over the winter, with field experiments that were started in 2 different years. These losses from the soil averaged 67% for KNO3 (five experiments, range of 40 to 87%), 38% for urea (two experiments, range of 29 to 46%), and 10% for (NH4)2SO4 (five experiments, range of 1 to 18%). The overwinter loss of fall‐ or winterapplied N was not through leaching, but through denitrification which apparently took place in early spring. The amount of immobilization of the labelled fertilizers varied from experiment to experiment.