Microbial N immobilization is of great importance in acidified mountain spruce forest soils

► Microbial N immobilization was found to be a primary process of soil N retention. ► Total N flux to the microbial pool always exceeded N flux into mineral N pools. ► Organic N was immobilized in preference over mineral N. ► Nitrification was favoured when enough N-NH4 was available in C limited soil.