Predicting N mineralization from green manure in different soil types during the cold season is instrumental for improving crop management with higher N use efficiency and reduced risks of N losses in a cool and humid climate. The objective of our work was to study the effects of low temperatures and soil type on the net nitrogen (N) mineralization and the relationship between N and carbon (C) mineralization from N-rich plant material. A silty clay loam and a sandy loam were incubated with or without clover leaves for 80 days at 0, 4, 8.5 or 15 °C. The results showed a substantial mineralization of N in clover leaves (7% of N added), unaffected by temperature, already on 3rd day. This was followed by net N immobilization for about 4 weeks in the clay soil, with similar tendencies in the sandy soil, and more severely at the higher than the lower temperatures. After 80 days of incubation, net N mineralization was only 13–22% of total N in clover leaves. The ratio of net mineralized N to C was higher at lower temperatures, and higher in the sandy than in the clay soil. After the immobilization period, the N mineralization increased, positively related to temperature, and the ratio of net mineralized N to C became constant. In conclusion, low temperature during the initial phase of mineralization altered the ratio between net N and C mineralization from easily decomposable plant material, and the net N mineralization occurred more rapidly in the sandy soil. The change in stoichiometry at low temperatures, as well as the modifying effect of soil type, should be considered when predicting N mineralization of N-rich plant material.