Fate of 15N-labelled fertilizer applied to spring barley grown on soils of contrasting nutrient status

An experiment with 15N-labelled fertilizer was superimposed on the Rothamsted Hoosfield Spring Barley Experiment, started in 1852. Labelled 15NH415NO3 was applied in spring at (nominal) rates of 0, 48, 96 and 144 kg N ha-1. The labelled fertilizer was applied to microplots located within four treatments of the original experiment: that receiving farmyard manure (FYM) annually, that receiving inorganic nutrients (PK) annually and to two that were deficient in nutrients: applications were made in two successive years, but to different areas within these original treatments. Maximum yields in 1986 (7.1 t grain ha-1) were a little greater than in 1987. In 1987, microplots on the FYM and PK treatments gave similar yields, provided enough fertilizer N was applied, but in 1986 yields on the PK treatment were always less than those on the FYM treatment, no matter how much fertilizer N was applied. In plots with adequate crop nutrients, about 51% of the labelled N was present in above-ground crop and weed at harvest, about 30% remained in the top 70 cm of soil (mostly in the 0–23 cm layer) and about 19% was unaccounted for, all irrespective of the rate of N application and of the quantity of inorganic N in the soil at the time of application. Less than 4% of the added fertilizer N was present in inorganic form in the soil at harvest, confirming results from comparable experiments with autumn-sown cereals in south-east England. Thus, in this experiment there is no evidence that a spring-sown cereal is more likely to leave unused fertilizer in the soil than an autumn-sown one. With trace applications (ca. 2 kg N ha-1) more labelled N was retained in the soil and less was in the above-ground crop. Where P and K were deficient, yields were depressed, a smaller proportion of the labelled fertilizer N was present in the above-ground crop at harvest and more remained in the soil.