Straw incorporation and immobilization of spring-applied nitrogen

The effect of rate and timing of autumn/winter straw incorporation on the immobilization of spring-applied N-fertilizer are studied. The immobilization of N-15-labelled fertilizer by straw was determined in pot trials with a sandy loam soil (Wick series) collected from two field experiments. In experiment 1, straw was incorporated at four different rates in the autumn; experiment 2 had a fixed rate of straw (7.5 t/ha) incorporated at monthly intervals from September to March. Immobilization in spring was not enhanced when only stubble was incorporated in autumn. However, autumn incorporation of 7.5 t/ha straw resulted in a significant increase in the immobilization of spring-applied N, equivalent to 10 kg/ha; when 15 t/ha straw was incorporated, immobilization increased to 18 kg/ha. The enhancement of immobilization, immediately following fertilizer application, was dependent on the extent of straw decomposition prior to N application. Thus immobilization,vas related to cumulative thermal time (day degrees C above 0 degrees C; Tsum between the date of straw incorporation and fertilizer application). Straw which had been incorporated for Tsum > 1200 no longer caused appreciable immobilization of spring fertilizer.