EFFECT OF THE RATE OF N-FERTILIZER ON APPARENT NET MINERALIZATION OF N DURING AND AFTER CULTIVATION OF CEREAL AND SUGAR-BEET CROPS

Apparent net N mineralization (mineralization minus immobilization) in fertilized and unfertilized treatments was determined in 133 fertiliz er trials with cereals and sugar beet over 3 years (1988-90). Apparent net mineralization was defined as follows: Apparent net N mineralizat ion = (crop N at harvest - crop N in spring) - (N(min) in spring -N(mi n) at harvest) - N fertilizer applied. Results can be summarised as fo llows: 1. For both crop species, apparent net N mineralization decreas ed in the following order: unfertilized > optimally fertilized > overf ertilized 2. The decrease in apparent net mineralization of N with inc reasing rate of N fertilizer was attributed to immobilization. This wa s confirmed by measurements of increased remineralization during the f ollowing autumn, winter and during the growing season in the following year. 3. Both the soil N(min) at harvest and fertilizer N which was i mmobilized and remineralized during autumn and winter, is at risk of b eing leached. At optimal fertilizer doses 30 kg N/ha and 74 kg N/ha we re leached on average over winter from loamy and sandy soils respectiv ely. 4. Apparent net mineralization was not important for optimally fe rtilized cereals and therefore does not need to be considered for fert ilizer recommendations for winter cereals. This does not apply to land receiving slurry applications before or during the growth period. 5. In contrast to cereals, apparent net mineralization contributed consid erably to the nutrition of sugar beet. Approximately 140 kg N/ha were mineralized at the optimum rate of N fertilizer application. However, the EUF- and CaCl2-methods were unable to predict N mineralization and were therefore unable to improve the prediction of fertilizer require ment even in combination with the NO3 soil N fraction.