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 fertilizer trials with cereals and sugar beet over 3 years (1988-90). Apparent net mineralization was defined as follows: Apparent net N mineralization = (crop N at harvest - crop N in spring) - (N(min) in spring -N(min) at harvest) - N fertilizer applied. Results can be summarised as follows: 1. For both crop species, apparent net N mineralization decreased in the following order: unfertilized > optimally fertilized > overfertilized 2. The decrease in apparent net mineralization of N with increasing rate of N fertilizer was attributed to immobilization. This was confirmed by measurements of increased remineralization during the following autumn, winter and during the growing season in the following year. 3. Both the soil N(min) at harvest and fertilizer N which was immobilized and remineralized during autumn and winter, is at risk of being leached. At optimal fertilizer doses 30 kg N/ha and 74 kg N/ha were leached on average over winter from loamy and sandy soils respectively. 4. Apparent net mineralization was not important for optimally fertilized cereals and therefore does not need to be considered for fertilizer 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 considerably 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 requirement even in combination with the NO3 soil N fraction.