FATE OF N-15-LABELED FERTILIZER APPLIED TO SPRING BARLEY GROWN ON SOILS OF CONTRASTING NUTRIENT

An experiment with N-15-labelled fertilizer was superimposed on the Ro thamsted Hoosfield Spring Barley Experiment, started in 1852. Labelled (NH4)-N-15 (NO3)-N-15 was applied in spring at (nominal) rates of 0, 48, 96 and 144 kg N ha(-1). The labelled fertilizer was applied to mic roplots located within four treatments of the original experiment: tha t receiving farmyard manure (FYM) annually, that receiving inorganic n utrients (PK) annually and to two that were deficient in nutrients: ap plications were made in two successive years, but to different areas w ithin these original treatments. Maximum yields in 1986 (7.1 t grain h a(-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 label led 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 ab out 19% was unaccounted for, all irrespective of the rate of N applica tion and of the quantity of inorganic N in the soil at the time of app lication. Less than 4% of the added fertilizer N was present in inorga nic form in the soil at harvest, confirming results from comparable ex periments with autumn-sown cereals in south-east England. Thus, in thi s experiment there is no evidence that a spring-sown cereal is more li kely to leave unused fertilizer in the soil than an autumn-sown one. W ith trace applications (ca. 2 kg N ha(-1)) more labelled N was retaine d in the soil and less was in the above-ground crop. Where P and K wer e deficient, yields were depressed, a smaller proportion of the labell ed fertilizer N was present in the above-ground crop at harvest and mo re remained in the soil. Although the percentage uptake of labelled N was similar across the range of fertilizer N applications, the uptake of total N fell off at the higher N rates, particularly on the FYM tre atment. This was reflected in the appearance of a negative Added Nitro gen Interaction (ANI) at the highest rate of application. Fertilizer N blocked the uptake of soil N, particularly from below 23 cm, once the capacity of the crop to take up N was exceeded. Denitrification and l eaching were almost certainly insufficient to account for the 19% loss of spring-added N across the whole range of N applications and other loss processes must also have contributed.