Soil organic nitrogen (SON) mineralization is a function of soil properties, land use and climate, which led to monitor a network of 137 cultivated fields covering the wide diversity of soils, crop rotations and cropping practices throughout Brittany (France). SON mineralization was quantified by the mineral N balance calculated for a maize crop not fertilized with N; it was determined by measuring soil mineral N (SMN) in the 0-90 cm soil profile in March (Ni) and October (Nf) and N uptake by the maize crop, and predicting nitrate leaching (Nleached) using the STICS model. The experimental design was based on estimating SON for three consecutive years (2012-2014) to improve the accuracy of measuring mineralization.