NITROGEN MINERALIZATION DURING THE GROWING-SEASON .2. INFLUENCE OF SOIL ORGANIC-MATTER CONTENT, AND EFFECT ON OPTIMUM NITROGEN-FERTILIZATION OF SPRING BARLEY

Net N mineralization during the growing season of spring barley was estimated on the basis of field measurements in two 3-year experiments (1984-86) in each of the countries Denmark, Finland, Norway and Sweden. The preceding crops were cereals. It was found that a 1 % increase in the soil organic matter content of the 0-20 cm soil layer increased, on average, the net N release by 5 kg N ha-1 from early spring to yellow ripeness. However, this effect seemed to be largely overshadowed by variations in mineralization related to differences in climatic conditions, subsoil organic matter contents and probably also in N mineralization capacities at similar soil organic matter contents. N mineralization mainly took place within 0-20 cm soil depth and, to a less extent, in the 20-40 cm layer. In annual trials with increasing rates of fertilizer nitrogen, carried out at each experimental site, the average amount of residual mineral nitrogen within 0-100 cm depth at the end of the N uptake period was 5 kg N ha-1 larger after N fertilization at economically optimum rates than without N fertilization. This increase may be regarded as a factor contributing less to the risk of increased nitrogen leaching in the following autumn and winter than the N mineralization after crop uptake of N has ceased in late summer or early autumn. The results indicate ways to attain more precise nitrogen fertilization recommendations. For this, mineral nitrogen already present in soil in early spring, net N mineralization during the growing sea son, the expected grain yield level and the total nitrogen content of the grain should be taken into consideration at the time of fertilization. Multiple regression analysis showed that as much as 70 % of the variation in optimum rates of fertilizer nitrogen was explained by variations in these four factors.