This habilitation thesis is focusing on improving site- and year-specific modelling of net N mineralization in agricultural soils of Northern Germany for online N fertilizer recommendation purposes. The overall goal of the studies were to experimentally parameterize and evaluate a net N mineralization model allowing site-specific online parameterization. Futhermore, substances and processes possibly involved in unusually low net N mineralization in soils with specific former land uses, that are relatively rich in total organic C and N, were elucidated in order to adequately account for it in simulations. The synopsis is dedicated to merge the results from the six included papers plus a description of the achieved model applications, potential model limitations as well as the current working hypothesis on the mode of action of the inhibiting soil organic substances.