An important part of the nitrogen available to plant comes from the microbial activity resulting in N mineralization of the N bound to soil organic matter or crop residues. N losses to the environment will also come from N mineralization and not only for N oversupply to the plant-soil system. N mineralization will vary with agricultural practices such as N fertilization, tillage, crop rotation and manure application. An emphasis is put on the method used to evaluate the N mineralization as in situ measurements are relatively new.