Immobilization of Nitrogen‐15‐Labelled Urea in a Jack Pine Forest Floor

Abstract

Abstract Urea labelled with 15 N was applied at 200 kg N ha −1 in the spring to a 45‐yr‐old natural jack pine ( Pinus banksiana Lamb.) forest near Chapleau, ON. Fertilizer recovery in the L and F horizons was determined 32, 64 and 96 d after fertilization. Three months after fertilization, 25% of the added N had been immobilized in the forest floor and was recovered as organic N. The possibility that chemical reactions contribute to fertilizer N immobilization was investigated by applying urea to sterilized, mixed L and F horizon materials in an 8‐d laboratory incubation study. The amount of N fixed chemically in urea‐treated forest floor materials exceeded the amount of N immobilized in microbial tissue. The nature of the chemical reaction was investigated in a second incubation study. Urea did not react with L + F horizon materials either at the natural soil pH (4.5) or at the higher pH (8.0) resulting from the addition of KOH. Since urea was applied at a rate sufficient to raise soil pH to the alkaline range, immobilization may be explained by reaction between NH 3 and soil organic matter.